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Climate change and ecosystems: threats, opportunities and solutions

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Climate change and ecosystems: threats, opportunities and solutions

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https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0104

Climate change is impacting ecosystems because of temperature changes and changes in the atmosphere.

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Climate change and ecosystems: threats, opportunities and solutions

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What Are the Effects of Climate Change?

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What Are the Effects of Climate Change?

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If greenhouse gas emissions aren't being reduced, global temperatures will rise.

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Higher average temperatures

This change in global average temperature-seemingly small but consequential and climbing-means that, each summer, we are likely to experience increasingly sweltering heat waves. Even local news meteorologists are starting to connect strings of record-breaking days to new long-term trends, which are especially problematic in regions where infrastructure and housing have not been built with intensifying heat in mind. And heat waves aren't just uncomfortable-they're the leading cause of weather-related fatalities in the United States.

Longer-lasting droughts

Hotter temperatures increase the rate at which water evaporates from the air, leading to more severe and pervasive droughts. Already, climate change has pushed the American West into a severe "megadrought"-the driest 22-year stretch recorded in at least 1,200 years-shrinking drinking water supplies, withering crops, and making forests more susceptible to insect infestations. Drought can also create a positive feedback loop in which drier soil and less plant cover cause even faster evaporation.

More intense wildfires

This drier, hotter climate also creates conditions that fuel more vicious wildfire seasons-with fires that spread faster and burn longer-putting millions of additional lives and homes at risk. The number of large wildfires doubled between 1984 and 2015 in the western United States. And in California alone, the annual area burned by wildfires increased 500 percent between 1972 and 2018.

From the poles to the tropics, climate change is disrupting ecosystems. Even a seemingly slight shift in temperature can cause dramatic changes that ripple through food webs and the environment.

Melting sea ice

The effects of climate change are most apparent in the world's coldest regions-the poles. The Arctic is heating up twice as fast as anywhere else on earth, leading to the rapid melting of glaciers and polar ice sheets, where a massive amount of water is stored. As sea ice melts, darker ocean waters that absorb more sunlight become exposed, creating a positive feedback loop that speeds up the melting process. In just 15 years, the Arctic could be entirely ice-free in the summer.

Sea level rise

Scientists predict that melting sea ice and glaciers, as well as the fact that warmer water expands in volume, could cause sea levels to rise as much as 3.61 feet by the end of the century, should we fail to curb emissions. The extent (and pace) of this change would devastate low-lying regions, including island nations and densely populated coastal cities like New York City and Mumbai.

But sea level rise at far lower levels is still costly, dangerous, and disruptive. Scientists predict that the United States will see a foot of sea level rise by 2050, which will regularly damage infrastructure, like roads, sewage treatment plants, and even power plants. Beaches that families have grown up visiting may be gone by the end of the century. Sea level rise also harms the environment, as encroaching seawater can both erode coastal ecosystems and invade freshwater inland aquifers, which we rely on for agriculture and drinking water. Saltwater incursion is already reshaping life in nations like Bangladesh, where one-quarter of the lands lie less than 7 feet above sea level.

Flooding

In addition to coastal flooding caused by sea level rise, climate change influences the factors that result in inland and urban flooding: snowmelt and heavy rain. As global warming continues to both exacerbate sea level rise and extreme weather, our nation's floodplains are expected to grow by approximately 45 percent by 2100. In 2022, deadly flooding in Pakistan-which inundated as much as a third of the country-resulted from torrential rains mixed with melting glaciers and snow.

Warmer ocean waters and marine heat waves

Temperature-sensitive fish and other marine life are already changing migration patterns toward cooler and deeper waters to survive, sending food webs and important commercial fisheries into disarray. And the frequency of marine heat waves has increased by more than a third. These spikes have led to mass die-offs of plankton and marine mammals.

To make matters worse, the elevated absorption of carbon dioxide by the ocean leads to its gradual acidification, which alters the fundamental chemical makeup of the water and threatens marine life that has evolved to live in a narrow pH band. Animals like corals, oysters, and mussels will likely feel these effects first, as acidification disrupts the calcification process required to build their shells.

Ecosystem stressors

Land-based ecosystems-from old-growth forests to savannahs to tropical rainforests-are faring no better. Climate change is likely to increase outbreaks of pests, invasive species, and pathogen infections in forests. It's changing the kinds of vegetation that can thrive in a given region and disrupting the life cycles of wildlife, all of which is changing the composition of ecosystems and making them less resilient to stressors. While ecosystems have the capacity to adapt, many are reaching the hard limits of that natural capacity. More repercussions will follow as temperatures rise.

Climate change appears to be triggering a series of cascading ecological changes that we can neither fully predict nor, once they have enough momentum, fully stop. This ecosystem destabilization may be most apparent when it comes to keystone species that have an outsize- role in holding up an ecosystem's structure.

Less predictable growing seasons

In a warming world, farming crops is more unpredictable-and livestock, which are sensitive to extreme weather, become harder to raise. Climate change shifts precipitation patterns, causing unpredictable floods and longer-lasting droughts. More frequent and severe hurricanes can devastate an entire season's worth of crops. Meanwhile, the dynamics of pests, pathogens, and invasive species-all of which are costly for farmers to manage-are also expected to become harder to predict. This is bad news, given that most of the world's farms are small and family-run. One bad drought or flood could decimate an entire season's crop or herd. For example, in June 2022, a triple-digit heat wave in Kansas wiped out thousands of cows. While the regenerative agriculture movement is empowering rural communities to make their lands more resilient to climate change, unfortunately, not all communities can equitably access the support services that can help them embrace these more sustainable farming tactics.

Reduced soil health

Healthy soil has good moisture and mineral content and is teeming with bugs, bacteria, fungi, and microbes that in turn contribute to healthy crops. But climate change, particularly extreme heat and changes in precipitation, can degrade soil quality. These impacts are exacerbated in areas where industrial, chemical-dependent monoculture farming has made soil and crops less able to withstand environmental changes.

Food shortages

Ultimately, impacts to our agricultural systems pose a direct threat to the global food supply. And food shortages and price hikes driven by climate change will not affect everyone equally: Wealthier people will continue to have more options for accessing food, while potentially billions of others will be plummeted into food insecurity-adding to the billions that already have moderate or severe difficulty getting enough to eat.

Many species' behaviors-mating, feeding, migration-are closely tied to subtle seasonal shifts, as in temperature, precipitation level, and foliage. In some cases, changes to the environment are happening quicker than species are able to adapt. When the types and quantity of plant life change across a region, or when certain species bloom or hatch earlier or later than in the past, it impacts food and water supplies and reverberates up food chains.

Ultimately, the way climate change impacts weather, the environment, animals, and agriculture affects humanity as well. But there's more. Around the world, our ways of life-from how we get our food to the industries around which our economies are based-have all developed in the context of relatively stable climates. As global warming shakes this foundation, it promises to alter the very fabric of society. At worst, this could lead to widespread famine, disease, war, displacement, injury, and death. For many around the world, this grim forecast is already their reality. In this way, climate change poses an existential threat to all human life.

Human health

Insect-borne diseases like malaria and Zika become more prevalent in a warming world as their carriers are able to exist in more regions or thrive for longer seasons. In the past 30 years, the incidence of Lyme disease from ticks has nearly doubled in the United States. Thousands of people face injury, illness, and death every year from more frequent or more intense extreme weather events. At a 2-degree Celsius rise in global average temperature, an estimated one billion people will face heat stress risk. In the summer of 2022 alone, thousands died in record-shattering heat waves across Europe. Weeks later, dozens were killed by record-breaking urban flooding in the United States and Korea-and more than 1,500 people perished in the flooding in Pakistan, where resulting stagnant water and unsanitary conditions threaten even more.

The effects of climate change-and the looming threat of what's yet to come-take a significant toll on mental health too. One recent study ( the largest of its kind to date) surveyed 10,000 young people from 10 different countries. Forty-five percent of respondents said that their feelings about climate change, varying from anxiety to powerlessness to anger, impacted their daily lives.

Worsening inequity

The climate crisis exacerbates existing inequities. Though wealthy nations, such as the United States, have emitted the lion's share of historical greenhouse gas emissions, it's developing countries that may lack the resources to adapt and will now bear the brunt of the climate crisis. In some cases, low-lying island nations-like many in the Pacific -may cease to exist before developed economies make meaningful reductions to their carbon emissions.

Even within wealthier nations, disparities will continue to grow between those rich enough to shield themselves from the realities of climate change and those who cannot. Those with ample resources will not be displaced from their homes by wars over food or water-at least not right away. They will have homes with cool air during heat waves and be able to easily evacuate when a hurricane is headed their way. They will be able to buy increasingly expensive food and access treatment for respiratory illness caused by wildfire smoke. Billions of others can't-and are paying the highest price for climate pollution they did not produce.

Hurricane Katrina, for example, displaced more than one million people around the Gulf Coast. But in New Orleans, where redlining practices promoted racial and economic segregation, the city's more affluent areas tended to be located on higher ground-and those residents were able to return and rebuild much faster than others.

Displacement

Climate change will drive displacement due to impacts like food and water scarcities, sea level rise, and economic instability. It's already happening. The United Nations Global Compact on Refugees recognizes that "climate, environmental degradation and disasters increasingly interact with the drivers of refugee movements." Again, communities with the fewest resources-including those facing political instability and poverty-will feel the effects first and most devastatingly.

Economic impacts

Recovering from the destruction wrought by extreme weather like hurricanes, flash floods, and wildfires is also getting more expensive every year. In 2021, the price tag of weather disasters in the United States totaled $145 billion-the third-costliest year on record, including a number of billion-dollar weather events.

Future Effects of Climate Change

The first wave of impacts can already be felt in our communities and seen on the nightly news. In the near future, between 2030 and 2050, climate change is expected to cause an additional 250,000 deaths per year from things like malnutrition, insect-borne diseases, and heat stress. And the World Bank estimates that climate change could displace more than 140 million people within their home countries in sub-Saharan Africa, South Asia, and Latin America by 2050.

But the degree to which the climate crisis upends our lives depends on whether global leaders decide to chart a different course. If we fail to curb greenhouse gas emissions, scientists predict a catastrophic 4.3 degrees Celsius, (or around 8 degrees Fahrenheit) of warming by the end of the century. What would a world that warm look like? Wars over water. Crowded hospitals to contend with spreading disease. Collapsed fisheries. Dead coral reefs. Even more lethal heat waves. These are just some of the impacts predicted by climate scientists.

Climate mitigation, or our ability to reverse climate change and undo its widespread effects, hinges on the successful enactment of policies that yield deep cuts to carbon pollution, end our dependence on dangerous fossil fuels and the deadly air pollution they generate, and prioritize the people and ecosystems on the frontlines. And these actions must be taken quickly in order to ensure a healthier present day and future. In one of its latest reports, the IPCC presented its most optimistic emissions scenario, in which the world only briefly surpasses 1.5 degrees of warming but sequestration measures cause it to dip back below by 2100. Climate adaptation, a term that refers to coping with climate impacts, is no longer optional; it's necessary, particularly for the world's most vulnerable populations.

By following the urgent warnings of the IPCC and limiting warming, we may be able to avoid passing some of the critical thresholds that, once crossed, can lead to potentially irreversible, catastrophic impacts for the planet, including more warming. These thresholds are known as climate tipping points and refer to when a natural system "tips" into an entirely different state. One example would be Arctic permafrost, which stores carbon like a freezer: As the permafrost melts from warming temperatures, it releases carbon dioxide into the atmosphere.

Importantly, climate action is not a binary pass-fail test. Every fraction of a degree of warming that we prevent will reduce human suffering and death, and keep more of the planet's natural systems intact. The good news is that a wide range of solutions exist to sharply reduce emissions, slow the pace of warming, and protect communities on the frontlines of climate impacts. Climate leaders the world over-those on major political stages as well as grassroots community activists-are offering up alternative models to systems that prioritize polluters over people. Many of these solutions are rooted in ancestral and Indigenous understandings of the natural world and have existed for millennia. Some solutions require major investments into clean, renewable energy and sustainable technologies. To be successful, climate solutions must also address intersecting crises-like poverty, racism, and gender inequality -that compound and drive the causes and impacts of the climate crisis. A combination of human ingenuity and immense political will can help us get there.

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A rapidly warming planet poses an existential threat to all life on earth. Just how bad it gets depends on how quickly we act.

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Talking about climate change and human health

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Talking about climate change and human health

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Climate change is impacting human health globally and affects social, physical, and mental health.

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The climate crisis is also an urgent and ongoing health crisis with diverse human impacts leading to physical, mental and cultural losses. Translating knowledge into action involves broad collaboration, which relies heavily on careful communication of a personal and politicized issue.

Against a backdrop of reported historical peaks in monthly temperatures, the past months have seen increasing visibility of the issue of climate change health, as several institutions have pushed awareness, research and action to limit the effects that warming has on human health. These include the inaugural Declaration on Climate and Health at COP28, the publication of the report Quantifying the Impact of Climate Change on Human Health by the World Economic Forum, and a pledge of £23 million by the Wellcome Trust to support transdisciplinary research to protect human health from climate change.

In this issue of Nature Climate Change and an associated online Focus, we highlight research and other content at the climate-health intersection.

Across these works, a clear and familiar theme that arises is the world's current lack of preparation to deal with the ongoing crisis. This is exemplified in an Analysis by Braithwaite and colleagues of the ability of healthcare systems to cope with climate change. In line with the World Economic Forum's finding that climate impacts will cost healthcare systems a further US$1.1 trillion globally by 2050 (ref. ), Braithwaite and colleagues highlight the need for multi-pronged plans to future-proof these at-risk systems. They also demonstrate the heavy bias of current research on acute disaster events and in the Global North.

The second conspicuous theme is that responding to the climate-health crisis will involve diverse actors. In a Viewpoint article, six researchers highlight key issues in their fields, which include mental health, labour, disease spread, maternal and neonatal health, air quality and nutrition, while advocating the need for collaboration across disciplines, sectors and geography. Echoing this need for collaboration, a Feature article by Yessenia Funes on the public drive to seek climate action through the courts focuses on the varied yet complementary roles the public, research scientists, healthcare professionals and lawyers have to play.

Thirdly, and critically linked to the previous points, is that improved communication is key for translating research into action. Part of this involves learning the languages of different fields or sectors: in a Q&A article, Maria Neira, director of the Department of Environment, Climate Change and Health at the World Health Organization (WHO), describes how understanding that the climate terms 'adaptation' and 'mitigation' corresponded to the terms 'primary intervention' and 'secondary intervention' in public health helped to align communication between the two fields. Another part also involves ensuring that language is used carefully to support positive action. Psychologist Elizabeth Marks (writing in the Viewpoint article) discusses the importance of identifying eco-anxiety without pathologizing it, while Neira discusses the impact of communicating a negative message (that is, climate change is harming human health) with or without actionable plans, underscoring the difference between problem solving and panic. Funes also highlights the important role of health practitioners, whose personal relationships with patients makes them 'trusted messengers' to discuss climate change health information. In line with this, the WHO has just released a new toolkit to support healthcare professionals to effectively communicate about climate change and health. A Comment from Noa Heiman in this issue also discusses the best ways for therapists to support their clients who experience climate distress.

Overall, talking about climate change health is not just a question of slipping from the technical jargon of climate models to that of healthcare or legal systems. It is also about communicating with an increasingly engaged public on a deeply personal and politicized issue. Finding the right wording is therefore extremely important. But the personal part of health is also what makes discussing climate change from a health point of view such a powerful tool to move forward.

An ongoing global crisis lacking preparedness that requires multiple actors to move forward can leave a lot of room for debate. But as Neira suggests, if instead of talking only about reducing emissions or limiting the amount of degrees warming, we discuss the number of lives that can be saved, there is less room for discussion, and more room to translate words into action.

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Talking about climate change and health - Nature Climate Change

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The climate crisis is also an urgent and ongoing health crisis with diverse human impacts leading to physical, mental and cultural losses. Translating knowledge into action involves broad collaboration, which relies heavily on careful communication of a personal and politicized issue.

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How global warming is disrupting life on Earth

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How global warming is disrupting life on Earth

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Climate change is causing not only rising temperatures, but also more natural disasters, rising seas, and shifting wildlife habits.

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The planet is warming, from North Pole to South Pole. Since 1906, the global average surface temperature has increased by more than 1.6 degrees Fahrenheit (0.9 degrees Celsius)- even more in sensitive polar regions. And the impacts of rising temperatures aren't waiting for some far-flung future-the effects of global warming are appearing right now. The heat is melting glaciers and sea ice, shifting precipitation patterns, and setting animals on the move.

Many people think of global warming and climate change as synonyms, but scientists prefer to use "climate change" when describing the complex shifts now affecting our planet's weather and climate systems. Climate change encompasses not only rising average temperatures but also extreme weather events, shifting wildlife populations and habitats, rising seas, and a range of other impacts. All of these changes are emerging as humans continue to add heat-trapping greenhouse gases to the atmosphere.

Scientists already have documented these impacts of climate change:

Ice is melting worldwide, especially at the Earth's poles. This includes mountain glaciers, ice sheets covering West Antarctica and Greenland, and Arctic sea ice. In Montana's Glacier National Park the number of glaciers has declined to fewer than 30 from more than 150 in 1910.

Much of this melting ice contributes to sea-level rise. Global sea levels are rising 0.13 inches (3.2 millimeters) a year. The rise is occurring at a faster rate in recent years and is predicted to accelerate in the coming decades.

Rising temperatures are affecting wildlife and their habitats. Vanishing ice has challenged species such as the Adélie penguin in Antarctica, where some populations on the western peninsula have collapsed by 90 percent or more.

As temperatures change, many species are on the move. Some butterflies, foxes, and alpine plants have migrated farther north or to higher, cooler areas.

Precipitation (rain and snowfall) has increased across the globe, on average. Yet some regions are experiencing more severe drought, increasing the risk of wildfires, lost crops, and drinking water shortages.

Some species-including mosquitoes, ticks, jellyfish, and crop pests-are thriving. Booming populations of bark beetles that feed on spruce and pine trees, for example, have devastated millions of forested acres in the U.S.

Other effects could take place later this century, if warming continues. These include:

Sea levels are expected to rise between 10 and 32 inches (26 and 82 centimeters) or higher by the end of the century.

Hurricanes and other storms are likely to become stronger. Floods and droughts will become more common. Large parts of the U.S., for example, face a higher risk of decades-long "megadroughts" by 2100.

Less freshwater will be available, since glaciers store about three-quarters of the world's freshwater.

Some diseases will spread, such as mosquito-borne malaria (and the 2016 resurgence of the Zika virus).

Ecosystems will continue to change: Some species will move farther north or become more successful; others, such as polar bears, won't be able to adapt and could become extinct.

Read next: Is Global Warming Real?

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The planet is warming, from North Pole to South Pole, and everywhere in between.

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What is climate change? A really simple guide

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What is climate change? A really simple guide

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Over the last decade, the world was on average around 1.2C warmer than during the late 19th Century.

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What is climate change? A really simple guide

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Human activities are causing world temperatures to rise, posing serious threats to people and nature.

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People are altering decomposition rates in waterways

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People are altering decomposition rates in waterways

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Urbanization is increasing decomposition rates in waterways.

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from research organizations

Faster decomposition could exacerbate greenhouse gas emissions, threaten biodiversity

Date:

Source:

University of Georgia

Summary:

Humans may be accelerating the rate at which organic matter decomposes in rivers and streams on a global scale, according to a new study. That could pose a threat to biodiversity in waterways around the world and increase the amount of carbon in Earth's atmosphere, potentially exacerbating climate change. The study is the first to combine a global experiment and predictive modeling to illustrate how human impacts to waterways may contribute to the global climate crisis.

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Humans may be accelerating the rate at which organic matter decomposes in rivers and streams on a global scale, according to a new study from the University of Georgia, Oakland University and Kent State University.

That could pose a threat to biodiversity in waterways around the world and increase the amount of carbon in Earth's atmosphere, potentially exacerbating climate change.

Published in Science, the study is the first to combine a global experiment and predictive modeling to illustrate how human impacts to waterways may contribute to the global climate crisis.

"Everyone in the world needs water," said Krista Capps, co-author of the study and an associate professor in UGA's Odum School of Ecology and Savannah River Ecology Laboratory. "When human activities change the fundamental ways rivers work, it's concerning. Increases in decomposition rates may be problematic for the global carbon cycle and for animals, like insects and fish, that live in streams because the food resources they need to survive will disappear more quickly, lost to the atmosphere as carbon dioxide."
Global warming, urbanization, increased nutrients altering global carbon cycle
Rivers and streams play a key role in the global carbon cycle by storing and decomposing large amounts of leaves, branches and other plant matter.

Typically, the process would go something like this: Leaf falls into river. Bacteria and fungi colonize the leaf. An insect eats the bacteria and fungi, using the carbon stored in the leaf to grow and make more insects. A fish eats the insect.

The study found that this process is changing in areas of the world impacted by humans.

Rivers impacted by urbanization and agriculture are changing how quickly leaf litter decomposes.

And when the process speeds up, that insect doesn't have a chance to absorb the carbon from the leaf. Instead, the carbon is released into the atmosphere, contributing to greenhouse gas pollution and ultimately disrupting the food chain.

"When we think of greenhouse gas emissions, we tend to think of tailpipes and factories," said Scott Tiegs, co-author of the study and a professor of biological sciences at Oakland. "But a lot of carbon dioxide and methane comes from aquatic ecosystems. This process is natural. But when humans add nutrient pollution like fertilizer to fresh waters and elevate water temperatures, we increase the decomposition rates and direct more CO2 into the atmosphere."
Reducing human impact could improve water quality, help fight climate change
The researchers collected field data from 550 rivers across the globe, collaborating with more than 150 researchers in 40 countries.

Based on that data, the scientists generated one of the first estimates of decomposition rates in rivers and streams throughout the world, including understudied areas such as the tropics.

The authors compiled the data into a free online mapping tool that shows how fast different kinds of leaves decompose in local waterways.

Using predictive modeling, the researchers also identified environmental factors responsible for increased decomposition rates, such as higher temperatures and increased nutrient concentrations.

"Both of these factors are impacted by human activities," said David Costello, co-author of the study and an associate professor at Kent State. "Reducing human impacts on decomposition will keep more carbon in rivers, preventing it from entering the atmosphere as carbon dioxide and contributing to climate change."

The study was co-authored by John Paul Schmidt, from UGA's Odum School of Ecology; Christopher J. Patrick, Virginia Institute of Marine Science; Jennifer J. Follstad Shah, University of Utah; Carrie J. LeRoy, The Evergreen State College; and the CELLDEX Consortium.

Journal Reference:

S. D. Tiegs, K. A. Capps, D. M. Costello, J. P. Schmidt, C. J. Patrick, J. J. Follstad Shah, C. J. LeRoy, Vicenç Acuña, Ricardo Albariño, Daniel C. Allen, Cecilia Alonso, Patricio Andino, Clay Arango, Jukka Aroviita, Marcus V.M. Barbosa, Leon A. Barmuta, Colden Baxter, Brent Bellinger, Luz Boyero, Lyubov Bragina, Lee E. Brown, Andreas Bruder, Denise A. Bruesewitz, Francis Burdon, Marcos Callisto, Antonio Camacho, Cristina Canhoto, María M. Castillo, Eric Chauvet, Joanne Clapcott, Fanny Colas, Checo Colón-Gaud, Julien Cornut, Verónica Crespo-Pérez, Wyatt F. Cross, Joseph Culp, Michael Danger, Olivier Dangles, Elvira de Eyto, Alison M. Derry, Veronica Díaz Villanueva, Michael M. Douglas, Arturo Elosegi, Andrea C. Encalada, Sally Entrekin, Rodrigo Espinosa, Verónica Ferreira, Carmen Ferriol, Kyla M. Flanagan, Alexander S. Flecker, Tadeusz Fleituch, André Frainer, Nikolai Friberg, Paul C. Frost, Erica A. Garcia, Liliana García-Lago, Pavel Ernesto García Soto, Mark O. Gessner, Sudeep Ghate, Darren P. Giling, Alan Gilmer, José Francisco Gonçalves, Rosario Karina Gonzales, Manuel A.S. Graça, Mike Grace, Natalie A. Griffiths, Hans-Peter Grossart, François Guérold, Vlad Gulis, Pablo E. Gutiérrez-Fonseca, Luiz U. Hepp, Scott Higgins, Takuo Hishi, Joseph Huddart, John Hudson, Moss Imberger, Carlos Iñiguez-Armijos, Mark W. Isken, Tomoya Iwata, David J. Janetski, Andrea E. Kirkwood, Aaron A. Koning, Sarian Kosten, Kevin A. Kuehn, Hjalmar Laudon, Peter R. Leavitt, Aurea L Lemes da Silva, Shawn Leroux, Peter J. Lisi, Richard MacKenzie, Amy M. Marcarelli, Frank O. Masese, Peter B. McIntyre, Brendan G. McKie, Adriana Medeiros, Kristian Meissner, Marko Miliša, Shailendra Mishra, Yo Miyake, Ashley Moerke, Shorok Mombrikotb, Rob Mooney, Timothy Moulton, Timo Muotka, Junjiro Negishi, Vinicius Neres-Lima, Mika L. Nieminen, Jorge Nimptsch, Jakub Ondruch, Riku Paavola, Isabel Pardo, Edwin T.H.M. Peeters, Jesus Pozo, Aaron Prussian, Estefania Quenta, Brian Reid, John S. Richardson, Anna Rigosi, José Rincón, Geta Risnoveanu, Christopher T. Robinson, Lorena Rodríguez-Gallego, Todd V. Royer, James A. Rusak, Anna C. Santamans, Géza B. Selmeczy, Gelas Simiyu, Agnija Skuja, Jerzy Smykla, Ryan Sponseller, Kandikere R. Sridhar, Aaron Stoler, Christopher M. Swan, Franco Teixeira de Mello, Jonathan D. Tonkin, Sari Uusheimo, Allison M. Veach, Sirje Vilbaste, Lena B.-M. Vought, Chiao-Ping Wang, Jackson R. Webster, Paul B. Wilson, Stefan Woelfl, Guy Woodward, Marguerite A. Xenopoulos, Adam G. Yates, Chihiro Yoshimura, Catherine M. Yule, Yixin Zhang, Jacob A. Zwart. Human activities shape global patterns of decomposition rates in rivers. Science, 2024; DOI: 10.1126/science.adn1262

Cite This Page:

University of Georgia. "People are altering decomposition rates in waterways." ScienceDaily. ScienceDaily, 30 May 2024. <www.sciencedaily.com/releases/2024/05/240530182119.htm>.

University of Georgia. (2024, May 30). People are altering decomposition rates in waterways. ScienceDaily. Retrieved May 31, 2024 from www.sciencedaily.com/releases/2024/05/240530182119.htm

University of Georgia. "People are altering decomposition rates in waterways." ScienceDaily. www.sciencedaily.com/releases/2024/05/240530182119.htm (accessed May 31, 2024).

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global warming; environmental issues; climate; air quality; environmental policies; resource shortage; world development; land management

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People are altering decomposition rates in waterways

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Humans may be accelerating the rate at which organic matter decomposes in rivers and streams on a global scale, according to a new study. That could pose a threat to biodiversity in waterways around the world and increase the amount of carbon in Earth's atmosphere, potentially exacerbating climate change.

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Linking biodiversity and ecosystems: towards a unifying ecological theory

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Linking biodiversity and ecosystems: towards a unifying ecological theory

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Ecosystems are strongly connected to each other and provide different habitats for many species.

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Abstract

Community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Merging the two perspectives is necessary both to ensure continued scientific progress and to provide society with the scientific means to face growing environmental challenges. Recent research on biodiversity and ecosystem functioning has contributed to this goal in several ways. By addressing a new question of high relevance for both science and society, by challenging existing paradigms, by tightly linking theory and experiments, by building scientific consensus beyond differences in opinion, by integrating fragmented disciplines and research fields, by connecting itself to other disciplines and management issues, it has helped transform ecology not only in content, but also in form. Creating a genuine evolutionary ecosystem ecology that links the evolution of species traits at the individual level, the dynamics of species interactions, and the overall functioning of ecosystems would give new impetus to this much-needed process of unification across ecological disciplines. Recent community evolution models are a promising step in that direction.

Keywords: biodiversity, community, ecosystem, ecology, evolution, theory

1. Strengths and weaknesses of community ecology and ecosystem ecology

One of the distinctive and fascinating features of ecological systems is their extraordinary complexity. An ecosystem is often composed of thousands of different species that interact in myriad different ways at the scale of a single hectare. These complex local systems are strongly connected to each other and aggregate into larger and larger entities from the landscape scale to that of the entire biosphere, where it becomes evident that they exert a major influence on the physical and chemical properties of our planet. How can such enormously complex systems be studied?

During the second half of the twentieth century, two increasingly divergent approaches to ecological systems developed within ecology, which have gradually led to two distinct disciplines, community ecology and ecosystem ecology. A community is defined broadly as a set of species that live together in some place. The focus in community ecology has traditionally been on species diversity: what exogenous and endogenous forces lead to more or less diverse communities? How do species interactions constrain the number of species that can coexist? What patterns emerge from these species interactions? An ecosystem is the entire system of biotic and abiotic components that interact in some place. The ecosystem concept is broader than the community concept because it includes a wide range of biological, physical, and chemical processes that connect organisms and their environment. But the focus on ecosystem ecology has traditionally been on the overall functioning of ecosystems as distinct entities: how is energy captured, transferred, and ultimately dissipated in different ecosystems? How are limiting nutrients recycled, thereby ensuring the renewal of the material elements necessary for growth? What factors and processes control energy and material flows, from local to global scales?

In a sense, community ecology provides a microscopic perspective on ecosystems because it analyses their parts, while ecosystem ecology provides a macroscopic perspective on the same systems because it studies them as a whole. The distinction between micro- and macroscopic, however, does not necessarily apply to the spatial scales considered by the two disciplines. Although much of community ecology does consider species interactions at small scales, a growing fringe, known as macroecology, considers patterns of species diversity and species distributions at vast spatial scales. The focus on species-species distributions, species diversity, species interactions-is more central to the community ecological approach than the spatial scale considered. Similarly, ecosystem ecology studies the fluxes of energy and materials at various spatial scales. What distinguishes the ecosystem ecological approach is its focus on the system as a whole, irrespective of the species that compose it.

At a time when humankind rises to the status of a major global biogeochemical force and raises the prospect of a global ecological crisis, it seems appropriate to step back and ask whether individually studying communities and ecosystems is the best path to follow. Human environmental impacts include the destruction and fragmentation of natural habitats, pollution, climate change, overexploitation of biological resources, homogenization of biota and biodiversity loss. These impacts affect species and ecosystems indistinctly. Moreover, they interact with each other, which may lead to non-additive cumulative effects. For instance, climate change is likely to cause massive additional biodiversity loss. Biodiversity loss in turn is likely to decrease the ability of ecosystems to resist the effects of climate change, with possible feedbacks on the climate system itself. Species, communities and ecosystems have always been inextricably linked, but the major disruptions generated by humans in the current period make this reality plainly obvious. A synthetic approach to ecology, which integrates populations, communities and ecosystems, is required to develop appropriate responses to the global ecological crisis we are entering.

Lawton (1999) recently drew a rather dark picture of the current status of community ecology, 'where there are a large number of case histories, and very little other than weak, fuzzy generalisations'. The community is indeed the hierarchical level where the basic characteristics of life-its diversity, complexity and historical nature-are perhaps most daunting and challenging. Community ecology is a dynamic field of research in which knowledge has accumulated rapidly during the past 50 years or so based largely on a modern hypothetico-deductive approach. But it is replete with poorly tested theories and hypotheses, which hinders steady scientific progress. As a result, it has few laws or robust generalizations, except for some large-scale empirical patterns such as species-area relationships ( Rosenzweig 1999). Despite the proliferation of theories and hypotheses, however, a large amount of robust results have been obtained on a wide range of small-scale ecological processes. What is lacking is a synthetic theoretical framework to organize these results into a coherent set of alternative or complementary hypotheses that yield testable predictions. For instance, there are many theories of species coexistence, each of which invokes a different mechanism. But these various mechanisms can be organized into a limited number of classes based on the way these mechanisms allow competitive exclusion to be avoided, as in Chesson's (2000) distinction between equalizing and stabilizing mechanisms. If distinctions such as this were made operational through the development of appropriate quantitative measures and if their implications for the functioning of entire communities or ecosystems were clearly identified, they would be more readily amenable to empirical tests, they would foster unification of studies of species coexistence and they would yield valuable robust insights that could be used in broader and more applied contexts.

In contrast, ecosystem ecology is an ecological discipline that has traditionally had a strong empirical basis. Its theories are largely based on inductive generalizations from field measurements, with comparatively few theory-driven hypotheses and experimental tests. There is no doubt that the ecosystem approach has been instrumental in developing our understanding of the global biogeochemistry of the Earth system and of current global environmental changes. Yet, despite these successes, a number of authors have questioned its static view of ecological systems and even its scientific relevance, calling for a fundamental rejuvenation of the discipline ( O'Neill 2001). Two key elements that are still insufficiently developed in ecosystem ecology are a strong hypothetico-deductive approach and proper consideration of the internal dynamics and complexity of ecological systems. Strengthening theory, experimental tests and their interaction, and paying due attention to ecological dynamics and complexity would probably contribute to make ecosystem ecology a more dynamic and attractive scientific discipline.

On balance, then, it appears that community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Both disciplines have been called into question, and each would benefit from the perspective developed by the other. Developing theories about interactions between species and between these and their environment, with the ultimate goal of predicting ecosystem functioning and ecosystem services, would help to focus community ecology on issues that are both scientifically important and socially relevant. Incorporating the diversity, complexity and dynamical nature of communities in its view of ecosystem functioning would help ecosystem ecology to be livelier and to provide more reliable, if probably more uncertain, predictions. It is becoming increasingly clear that merging the two perspectives is necessary both to ensure continued scientific progress and to provide society with the scientific means to face growing environmental challenges.

The need for integration of community ecology and ecosystem ecology has been increasingly recognized during the past 20 years. There have been a number of attempts at doing so from a variety of perspectives, such as those provided by hierarchy theory ( O'Neill et al. 1986), linking nutrient cycling and food webs ( DeAngelis 1992), linking species and ecosystems ( Jones & Lawton 1995), complex systems theory ( Levin 1999; Solé & Bascompte 2006), ecological stoichiometry across levels of biological organization ( Sterner & Elser 2002) and the metabolic theory of ecology ( Brown et al. 2004). Each of these perspectives has contributed to addressing part of the problem. But a broader synthesis is still needed ( Loreau in press). Here I will show how the new biodiversity and ecosystem functioning research field has contributed to this goal, before turning to what I see as a major scientific challenge for the future.

2. Biodiversity and ecosystem functioning: an integrative approach

The relationship between biodiversity and ecosystem functioning has emerged as a central issue in ecological and environmental sciences during the past 15 years. The idea that greater plant diversity allows greater plant biomass production dates back to Darwin ( McNaughton 1993; Hector & Hooper 2002), but it was only in the 1990s that the interest in the effects of biodiversity on ecosystem functioning penetrated experimental and theoretical ecology. This interest spread very rapidly, leading to an entire new research field at the interface between community ecology and ecosystem ecology ( Schulze & Mooney 1993; Tilman 1999; Chapin et al. 2000; Loreau 2000; Kinzig et al. 2001; Loreau et al. 2001; Loreau et al. 2002b; Hooper et al. 2005; Balvanera et al. 2006; Cardinale et al. 2006; Cardinale et al. 2007).

Interest in this issue grew largely out of practical concerns about the potential ecological consequences of current biodiversity loss caused by the increased impact of human activities on natural and managed ecosystems. There is growing recognition that the world's ecosystems provide society with a wide range of ecological 'services' that are crucial to human well-being and sustainable development ( Millennium Ecosystem Assessment 2005). These services are derived from the normal functioning of ecosystems, raising the important question of whether impoverished ecosystems may in some way function less efficiently than the more species-rich systems from which they are derived, and hence gradually lose their ability to deliver ecosystem services to human societies. But beyond this eminently practical motivation, the new biodiversity and ecosystem functioning research field has had a much broader and deeper transformative role in ecology.

(a) Fostering integration of community ecology and ecosystem ecology

One of its main benefits has been to foster integration of community ecology and ecosystem ecology. Ecology has traditionally regarded, implicitly or explicitly, species diversity as an epiphenomenon driven by a combination of abiotic environmental factors (such as temperature, rainfall and soil fertility), ecosystem processes that are themselves determined by these abiotic factors (such as productivity, biomass and nutrient cycling) and biotic interactions within communities (such as competition and predation). This tenet is shared by community ecology and ecosystem ecology. Community ecology has been devoted historically to explaining patterns and processes of species coexistence and diversity. Ecosystem ecology has often ignored species diversity as some sort of 'background noise' irrelevant to ecosystem functioning. Thus, the two disciplines have considered species diversity in contrasting ways, which explains their historical divergence. But both have shared the basic assumption that species diversity is an epiphenomenon. The new biodiversity and ecosystem functioning research field has overthrown this central tenet by considering biodiversity-in particular species and genetic diversity-as a driver of ecosystem functioning ( Naeem 2002).

In a way, this change may be viewed as a simple extension of the classical paradigm, in which an additional arrow pointing from biodiversity to ecosystem functioning completes the picture of causalities between abiotic factors, ecosystem functioning and biodiversity ( figure 1 ). This apparently simple addition, however, has far deeper consequences than appears at first sight. The presumed lack of functional consequences of biodiversity is implicitly what justified the separation of community ecology and ecosystem ecology: ecosystem ecologists could study ecosystems as though these were independent of biodiversity and the community processes that maintain it, while community ecologists could study biodiversity and the processes that affect it without explicitly considering the ecosystems in which communities are embedded, by treating ecosystem processes as external constraints. If biodiversity affects ecosystem functioning, ecosystem ecologists can no longer ignore the dynamics of biodiversity within ecosystems. Similarly, community ecologists can no longer ignore the potential feedback that biodiversity has on its own maintenance through ecosystem functioning. The basis for the historical separation of the two disciplines then vanishes, even though it may take some time until both sides recognize the full implications of this change.

(b) The controversy over experimental results

Recent work has now clearly established that biodiversity does indeed affect ecosystem processes. The strongest evidence to date comes from field experimental studies that have manipulated plant species richness in temperate grasslands. Two of the largest such experiments have been the Cedar Creek biodiversity experiment in Minnesota, USA ( Tilman et al. 1997; Tilman et al. 2001) and the BIODEPTH experiment in Europe ( Hector et al. 1999; Spehn et al. 2005). The advantage of BIODEPTH is that it was replicated over eight sites under different biogeographic, climatic and soil conditions across Europe, which allowed testing the generality of biodiversity effects. The advantage of the Cedar Creek experiment is that it was run for more than a decade, which allowed testing the robustness of biodiversity effects through time. The two experiments provided very similar results overall. BIODEPTH showed a log-linear increase in plant above-ground biomass production with species richness across sites ( figure 2 a). The Cedar Creek experiment showed a positive response of total plant biomass production to species richness, which became stronger through time ( figure 2 b). The number of plant functional groups also had positive effects on plant biomass production in both cases.

These experimental results, however, generated a vigorous debate within the scientific community. Although they were consistent with expectations from niche theory, other theories suggested that overyielding (i.e. the fact that mixtures of several species yield more than expected based on the yield of these species in monoculture) and niche differentiation might not be the rule in plant communities. This prompted other hypotheses about the mechanisms that could generate the effects observed in the experiments. In particular, the 'sampling effect' hypothesis proposes that mean biomass production increases with plant species richness in experiments simply because of the increased probability of including a highly productive species in high-diversity plots ( Huston 1997). Though sometimes heated, this debate ultimately revealed two of the main strengths of the new biodiversity and ecosystem functioning research field, i.e. its ability to link theory and experiments, and its ability to reach consensus despite strongly diverging initial views.

(c) Linking theory and experiments

Ecology has had an unfortunate propensity to disconnect empirical and theoretical research, with a profusion of poorly generalized empirical data and an equal profusion of poorly tested theories. As a result of this disconnection, there have been few attempts at resolving controversies through consensus building within the ecological scientific community, leading to periodic shifts in fashionable research topics. In this context, the scientific process through which the biodiversity and ecosystem functioning research field developed is quite remarkable. At first, experiments outpaced theory, but soon theory played a key role in the resolution of the controversy raised by experimental results.

Scientific controversies are often the result of a lack of clarity in the theoretical framework, a lack of appropriate tools or a lack of sufficient empirical evidence to distinguish among clearly identified competing hypotheses. This one was no exception and combined all three problems. Therefore, the first step in its resolution was a theoretical advance in the conceptual framework in which the experiments were being conceived and interpreted. New theoretical work identified two main classes of mechanisms by which biodiversity influences productivity or other ecosystem processes, leading to two types of biodiversity effects: (i) functional complementarity-the complementarity effect, and (ii) selection of particular functional traits that affect species' competitive abilities-the selection effect ( Loreau 2000). This apparently straightforward clarification was instrumental in resolving the debate over biodiversity experiments in several ways. First, it became clear that biodiversity matters only for ecosystem functioning to the extent that it provides phenotypic trait variation, or functional diversity, related to the particular ecosystem process considered. Second, it became also clear that the sampling effect is but a special, extreme case of the more general selection effect. Since selection and complementarity are two processes that operate concomitantly in competitive systems, this clarification contributed to build a consensus conceptual framework on biodiversity and ecosystem functioning ( Loreau et al. 2001). Third, the analogy between the ecological selection effect and the evolutionary process of natural selection helped appreciate the selection effect as a relevant biological effect, not a mere statistical artefact.

After clarification of the theoretical framework in which biodiversity experiments can be properly interpreted, the second step in the resolution of the controversy consisted in devising new theoretical tools to analyse their results. Since several experiments were already completed at a considerable cost, how could the data collected in these experiments be used to assess the respective contributions of the selection and complementarity effects? A new additive partition methodology inspired by the Price equation in evolutionary genetics provided a powerful tool to separate the two effects ( Loreau & Hector 2001). A large number of experimental studies have now used this additive partition. These have showed that selection and complementarity often co-occur in biodiversity experiments, but that positive net biodiversity effects are predominantly driven by functional complementarity between species, and further that this complementarity tends to increase through time ( Cardinale et al. 2007).

The tight link between theory and experiments is what made the resolution of the debate possible scientifically. Experiments helped to build a more practically oriented theory, and theory helped to analyse and interpret experiments more effectively. This interactive process allowed clear but balanced conclusions to be drawn, thereby providing healthy ground for future studies.

(d) Building scientific consensus

Resolving a controversy also has a human dimension-as does the scientific process in general. Bringing scientists with different views together with the explicit goal to move beyond existing divergences and build consensus without compromising on scientific rigour played an important part in the progress achieved in the biodiversity and ecosystem functioning research field. When I, Shahid Naeem and Pablo Inchausti convened the Conference 'Biodiversity and ecosystem functioning: Synthesis and perspectives' held in Paris in December 2000, the debate was becoming very acrimonious as a result of growing misunderstandings. At the end of the meeting, this acrimony had largely dissipated, common scientific grounds on a few key issues could be found, and scientific evidence based on rigorous theory and experimental data prevailed again in the subsequent scientific process. Two influential consensus papers came out of this meeting ( Loreau et al. 2001; Hooper et al. 2005). These papers articulate a general framework for biodiversity and ecosystem functioning research, and provide a detailed assessment of scientific certainties and uncertainties in this area. I am convinced that efforts to build consensus and assess current knowledge and future challenges would help make ecology a more scientifically dynamic and socially relevant science. An international mechanism of scientific expertise on biodiversity and ecosystem services could provide a global framework to coordinate these efforts and link them to policy decision, thus playing a role akin to that of the IPCC for the climate scientific community ( Loreau et al. 2006).

(e) Expanding to other issues and research fields

As with many successful new research fields, biodiversity and ecosystem functioning research did not remain confined in its initial scientific boundaries, but expanded its scope to encompass a wide range of fundamental issues in ecology, such as the functioning of food webs ( Thébault & Loreau 2006; Duffy et al. 2007), the spread of diseases ( Keesing et al. 2006), the spatial dynamics of metacommunities ( Loreau et al. 2003), and the relationship between the diversity and stability of ecological systems ( McCann 2000; Loreau et al. 2002a; Ives & Carpenter 2007), where it has provided fresh perspectives.

Progress achieved on the diversity-stability relationship has been particularly significant given the long, controversial history of this issue in ecology. The traditional view that permeated ecology in its early days held that complex, diverse natural ecosystems are inherently more stable than simple or artificially simplified systems ( Odum 1953; MacArthur 1955; Elton 1958). Theoretical work by Levins (1970); Gardner & Ashby (1970); May ( 1972, 1973) and others challenged this traditional view in the early 1970s, and eventually led to an almost diametrically opposite paradigm, i.e. complexity and diversity beget instability, not stability. Although the recent paradigm had rigorous mathematical underpinnings, it also had a number of limitations. In particular, 'stability' is really a meta-concept that covers a wide range of different properties. Furthermore, each of these stability properties can be applied to a number of variables of interest at different hierarchical levels, such as individual species abundance, community species composition and aggregate ecosystem properties. The relationship between diversity and stability may be different for different properties and variables ( Pimm 1984; Loreau et al. 2002a; Ives & Carpenter 2007). The recent paradigm specifically concerned the qualitative stability and resilience of communities as ensembles of populations, not the stability of aggregate ecosystem properties. A second important limitation of this paradigm is that it was based on the formalism of autonomous deterministic dynamical systems, which does not consider changes in the external environment and the response of biological systems to these changes through a variety of mechanisms on different time-scales, such as phenotypic plasticity, population dynamics, species replacement and evolution.

Two decisive features distinguish the new theoretical approaches to the diversity-stability relationship developed within the biodiversity and ecosystem functioning research agenda from earlier ones: first, these new approaches explicitly differentiate, and link, stability properties at the population level and at the aggregate community or ecosystem level, and second, they abandon the implicit assumptions that the environment is constant and that populations and ecosystems reach an equilibrium, to explicitly incorporate population dynamical responses to environmental fluctuations. They have led to two closely related hypotheses known as the 'portfolio effect' ( Doak et al. 1998; Tilman 1999) and the 'insurance hypothesis' ( Yachi & Loreau 1999), which predict a stabilizing effect of species diversity on aggregate ecosystem properties despite or, more exactly, through fluctuations of component species. The general mechanism that makes ecosystem properties less variable in more diverse communities is asynchrony of species' responses to environmental fluctuations driven by niche differences between species ( Yachi & Loreau 1999). This asynchrony ensures that as one species decreases sharply in abundance, biomass or productivity, another species decreases less sharply, or even increases, thus compensating partly or wholly for the decrease of the first species. The more species there are and the more asynchronous their environmental responses, the larger the potential for stabilization of aggregate community or ecosystem properties ( figure 3 ). The realization of this potential, however, requires that population dynamics are not strongly destabilized by species interactions at high diversity ( Ives & Hughes 2002).

These conclusions extend to complex food webs under some conditions; the outcome in food webs, however, depends critically on how interaction strength varies with species diversity ( Thébault & Loreau 2005). Interestingly, both MacArthur's (1955) hypothesis that prey diversity buffers generalist predators against asynchronous fluctuations of their prey and May's ( 1972, 1973) hypothesis that species diversity generates larger population fluctuations, which have been traditionally opposed, can be obtained with the same model, sometimes even under the same conditions ( Thébault & Loreau 2005). This emphasizes that the diversity-stability relationship is a complex, multifaceted one, which does not lend itself to sweeping statements.

Although rigorous empirical support for the new diversity-stability theory is scantier than for the effects of diversity on biomass and production, a few experiments that have manipulated species diversity have provided clear evidence for its stabilizing effect on ecosystem properties in both plant communities ( Tilman et al. 2006) and aquatic food webs ( Steiner et al. 2005). Perhaps more surprisingly, recent studies have shown variable responses of population-level stability to species diversity, ranging from negative ( Gonzalez & Descamps-Julien 2004; Tilman et al. 2006) to positive ( Romanuk & Kolasa 2004; Steiner et al. 2005). This suggests that current theory may still be missing some significant elements.

Because its initial impetus was provided by the societal relevance of the issues it was addressing, biodiversity and ecosystem functioning research also impacted on social sciences and environmental management. The results of this research supported the work of the Millennium Ecosystem Assessment (2005) on the links between biodiversity, ecosystem services and sustainable development. The value of biodiversity as insurance against the uncertain provision of ecosystem services is being incorporated formally in ecological economics ( Armsworth & Roughgarden 2003; Baumgärtner 2007).

(f) Conclusion

In many ways, the new biodiversity and ecosystem functioning research field represents what I see as a model for the future development of ecology. By addressing a new question of high relevance for both science and society, by challenging existing paradigms, by tightly linking theory and experiments, by building scientific consensus beyond differences in opinion, by integrating fragmented disciplines and research fields, and by connecting itself to other disciplines and management issues, it has contributed to transform ecology not only in content, but also in form.

3. Creating an evolutionary ecosystem ecology that integrates community dynamics

I see biodiversity and ecosystem functioning research as it has developed during the past 15 years as the starting point, not the endpoint, of a process of integration of community ecology and ecosystem ecology. This integration still has a long way to go until we can fully understand biodiversity, ecosystem functioning and their multiple linkages. The main obstacle to unification of the two disciplines has been the gap between the macroscopic, holistic perspective of ecosystem ecology and the more microscopic, mechanistic perspective of community ecology. It would be unreasonable to expect this gap to be filled all at once.

One unfortunate consequence of this gap has been the historical divorce between ecosystem ecology and evolutionary biology. The modern theory of evolution sees evolution as a two-step process of mutation and selection that changes gene frequencies within populations. Since genes are carried, expressed and transmitted by individual organisms, the individual organism occupies a central place in this theory, although multilevel selection theory recognizes the potential for higher levels of selection as well ( Wilson 1980; Sober & Wilson 1997). As a discipline that studies the overall functioning of ecosystems, ecosystem ecology has had a natural tendency to view ecosystems as integrated units on their own, and hence to search for laws and principles that govern the development and evolution of these higher level units ( Fath et al. 2001). Although this search has revealed some robust and intriguing patterns, its main weakness lies in the fact that many of the hypotheses proposed to explain these patterns are not explicitly connected to the evolutionary dynamics that takes place at the individual level. As a result, evolutionary biologists have usually disregarded them as wishful thinking. The interplay between the ecological and evolutionary dynamics of communities and ecosystems has received increasing attention recently ( Fussmann et al. 2007). A major progress would be the emergence of a genuine evolutionary ecosystem ecology that connects aggregate properties at the ecosystem level and evolution of species traits at the individual level ( Loreau in press). This endeavour would automatically include community dynamics as a component of this connection.

(a) The surge of community evolution models

Community genetics is a promising new approach that is providing empirical evidence for predictable effects of heritable traits in a single dominant species on community structure and ecosystem processes ( Whitham et al. 2006). Community genetics, however, does not consider the reciprocal effects of community structure and ecosystem processes on evolution of these traits, and hence is not yet equipped to tackle evolution of entire ecosystems. But the recent development of community evolution models contains the promise of a genuine theory of ecosystem evolution. These models have so far mainly explored the evolutionary emergence of complex food webs and the constraints generated by this evolutionary history on existing food webs ( McKane 2004; Loeuille & Loreau in press). In these models, each species is represented by a set of traits that determine its population dynamics and trophic interactions with other species. These traits are subject to mutations, which create the potential for speciation events. Species may also go extinct as a result of species interactions.

There has been a recent surge of such models, many of which have been developed simultaneously and independently ( Caldarelli et al. 1998; Christensen et al. 2002; Loeuille & Loreau 2005; Ito & Ikegami 2006; Rossberg et al. 2006; Bell 2007). The main difference between these models lies in the number and identity of traits they consider. Most of them involve a large number of arbitrary traits ( Caldarelli et al. 1998; Christensen et al. 2002; Loeuille & Loreau 2005; Ito & Ikegami 2006; Rossberg et al. 2006; Bell 2007). This makes them flexible, but also relatively abstract and difficult to test empirically. Ito & Ikegami (2006) built a model that includes only two traits for each species, one that describes it as a prey, and the other that describes it as a predator. Loeuille & Loreau (2005) built an even simpler model in which a single evolving trait, body size, determines each species' population dynamics and interactions. The advantages of the latter approach are that it clearly identifies a measurable trait and the ecological trade-offs it generates, and as a result, it makes empirically testable predictions. Body size is well known to play a key role in the physiological and ecological characteristics of species ( Kleiber 1961; Peters 1983; Brown et al. 2004). Therefore, it is a particularly appropriate trait to consider as a first step towards building a testable theory of food-web evolution.

Our simple model generates complex food webs that emerge by evolution from a single ancestor through a succession of species replacement, coexistence, diversification and divergence processes ( figure 4 ). Diversification is very fast in the beginning, but the food web gradually stabilizes into an evolutionary quasi-equilibrium in which species continue to turn over but food-web structure is relatively stable. These features are found in all existing food-web evolution models. Another intriguing feature of these models is that they are able to generate a wide range of food webs with different structures that strikingly resemble real food webs, from linear food chains with well-defined trophic levels to omnivorous food webs with blurred trophic levels. Even more surprisingly, our simple model is able to fit empirical data on aggregate food-web attributes by varying only two parameters that govern small-scale species interactions, i.e. interference rate and food niche width ( Loeuille & Loreau 2005). This is a significant improvement over static food-web models ( Cohen et al. 1990; Williams & Martinez 2000; Cattin et al. 2004), which use aggregate attributes, such as species diversity and connectance, to predict other aggregate attributes, such as food chain length and proportion of omnivores, but which cannot account for the former attributes in the first place. It is also an improvement over other food-web evolution models, which use a large number of poorly specified traits. Interference and food niche width are two individual-level traits that are easy to define and measure, which makes the model's predictions potentially testable.

(b) Strengths and limitations of community evolution models

Current food-web evolution models may be viewed as extensions of earlier community assembly models ( Post & Pimm 1983; Taylor 1989). But I believe that several differences make them more promising and more apt to foster integration across community and ecosystem perspectives. The first and most important difference is that they are based on evolving traits that are defined at the individual level. In contrast, community assembly models have often used population-level phenomenological descriptions of life-history parameters and species interactions such as those found in classical Lotka-Volterra models. The advantage of the former approach is that population, community and ecosystem processes are all emergent properties of individual-level processes and thus are intimately linked through their common dependence on individual-level traits such as body size. Another important difference is that food-web evolution models let species and species interactions emerge spontaneously from the evolutionary dynamics of the system. Community assembly models are more constrained because the range of possible species and food-web configurations is defined a priori through the choice of the species pool from which the community is assembled.

Because they use a common set of rules to describe all trophic interactions, existing food-web evolution models are best suited to represent the animal consumer part of natural food webs. They do not account for the basic role division between autotrophs, heterotrophic consumers and heterotrophic decomposers that lies at the core of all ecosystems. Recent theoretical studies have shown that material cycling between autotrophs and heterotrophs can strongly affect the evolution of these functional groups, and, reciprocally, this evolution can strongly affect ecosystem processes ( Loreau 1998; de Mazancourt & Loreau 2000; de Mazancourt et al. 2001). A few models have explored the evolutionary emergence of nutrient cycling in microbial systems ( Downing & Zvirinsky 1999; Downing 2002; Williams & Lenton 2007b). These models typically lead to the emergence of a diversity of biochemical guilds with complementary nutrient uptake and release patterns, which collectively recycle nutrients, sustain high biomass and regulate their abiotic environment.

Existing food-web evolution models also lack explicit description of the physical and chemical environment that both affects and is affected by species within ecosystems. The ecosystem engineer and niche construction concepts seek to capture the organisms' power to transform their abiotic environment, which has been too often neglected in ecology and evolutionary biology ( Jones et al. 1994; Odling-Smee et al. 2003). Niche construction generates an adaptive feedback between organisms and their environment, which can yield adaptive regulation of the abiotic environment ( Kylafis & Loreau 2008). The interplay between organisms and their physico-chemical environment plays a key role in both evolution and ecosystem functioning. Therefore, an evolutionary ecosystem ecology cannot ignore it.

Although community evolution models are still rudimentary and exploratory, they have the potential to transform ecosystem ecology into a more dynamic discipline that incorporates the perspective of community ecology. Ecosystem evolution models would naturally bridge the gap between the holistic perspective of ecosystem ecology and the mechanistic perspective of community ecology, because both species interactions and ecosystem properties would emerge spontaneously from the interplay between individual-level traits and collective ecosystem-level constraints.

(c) Moving toward a genuine evolutionary ecosystem ecology

To realize their potential and contribute to create a genuine evolutionary ecosystem ecology, however, these models must evolve beyond their current stage of essentially abstract theoretical explorations. We now need more realistic ecosystem evolution models to generate testable predictions, as well as empirical and experimental data to test these predictions. As I mentioned above, some of the existing food-web evolution models are already able to reproduce aggregate food web patterns accurately. This is encouraging but hardly sufficient to make these models truly predictive tools because it is currently impossible to assess whether they fit empirical data for good reason. There are at least two ways in which their predictive power can be enhanced and tested. First, by collecting simultaneous empirical data on aggregate food-web properties and individual-level traits. This would test the mechanistic basis of the aggregate food-web properties predicted by the models. Second, by developing more mechanistic ecosystem evolution models that take into account the main physical, chemical and biological factors that affect, or are affected by, individuals and the ecosystems they constitute. Such models would provide distinct predictions for different types of ecosystems (e.g. terrestrial versus pelagic), which could then be compared with empirical data from these ecosystems. Experiments using fast-evolving microbial systems could also be used to test some of the basic predictions of these models under relatively simple conditions.

Once appropriately tested against empirical data, ecosystem evolution models have huge application potential. These models could be used to resolve some of the oldest and most controversial issues in ecology and evolutionary biology, such as the respective roles of individual-level and ecosystem-level selection in shaping ecosystem properties ( Williams & Lenton 2007a). They could also be used to predict both the short-term ecological responses and the long-term evolutionary responses of natural and managed ecosystems to human interventions and anthropogenic disturbances. Their contribution to predicting the long-term consequences of human actions on biodiversity and the long-term future provision of ecosystem services would be particularly valuable.

4. Conclusion

In a world that is being transformed by humans at an ever-increasing scale and speed, ecology is bound to gain importance during this century. But in order to help human societies to face growing environmental challenges, ecology will have to deliver relevant scientific knowledge on how ecosystems function and change, how they are linked to human well-being and how humankind can use and transform them in a sustainable way. This requires that ecology transform itself into a more integrated, predictive science. The traditional slicing of ecology into autecology, population ecology, community ecology and ecosystem ecology has some value because different organizational levels obey partly different sets of constraints, and hence partly different laws. But it also has strong limitations because it tends to perpetuate arbitrary divisions and hamper the emergence of unifying perspectives. Current environmental changes affect species, communities and ecosystems indiscriminately. Therefore, a unifying ecological theory is needed to address the consequences of these changes. Recent research on biodiversity and ecosystem functioning has contributed to this goal in several different ways. Creating a genuine evolutionary ecosystem ecology that links the evolution of species traits at the individual level, the dynamics of species interactions, and the overall functioning of ecosystems would give new impetus to this much-needed process of unification across ecological disciplines.

Acknowledgements

I thank Andrew Gonzalez and Shawn Leroux for helpful comments on the manuscript. This contribution was supported by the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chair programme.

Articles from Philosophical Transactions of the Royal Society B: Biological Sciences are provided here courtesy of The Royal Society

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Linking biodiversity and ecosystems: towards a unifying ecological theory

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Community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Merging the two perspectives is necessary both to ensure continued scientific progress and to provide ...

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Young fish fuel Coral Reefs and usually stay in this ecosystem instead of migrating.

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Smithsonian's National Museum of Natural History

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Encountering a healthy coral reef while snorkeling or diving is like coming upon a flash mob in the desert. Hordes of flamboyant fishes generate a constantly moving scene against a waving backdrop of sponges, corals, and sea anemones.

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Exploring the Unseen: Human Impact on Ecosystems Revealed

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Exploring the Unseen: Human Impact on Ecosystems Revealed

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Humans alter ecosystems by sometimes changing species permanently.

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When a certain animal or plant is labeled an invasive species, the logical conclusion is to prevent and control it, right?

Well, that's not always the case - especially when human emotions come into play.

Take, for example, domestic cats. In Episode 4, Shane learns that these furry creatures are actually invasive, as their hunting habits contribute to biodiversity loss. However, we continue to protect and love them, sometimes at the expense of native species. In some places, human residents even chase off local neighbors like coyotes - which are indigenous to the area - to protect pets.

And we don't just play favorites when a species is cuddly, either. Later in Episode 4, Shane shares that we use 80 million pounds of fertilizer and trillions of gallons of water on turf grass each year. Despite the strain on just about every resource, we keep replacing native plant species we find less appealing - even if they're better for natural ecosystems, more drought-resistant and easier to care for.

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Exploring the Unseen: Human Impact on Ecosystems Revealed

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An ecosystem is a particular environment where plant and animal species interact. As humans, we all play a role in our ecosystems - and as Biologist Shane Campbell-Staton explores in "Human Footprint" on PBS, that role is often surprising.

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Ecosystem Benefits to Humanity Expected to Decline by 9% by 2100

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Ecosystem Benefits to Humanity Expected to Decline by 9% by 2100

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Humans are causing climate change to increase, which affects ecosystems and natural qualities harms the economy.

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As climate change redistributes terrestrial ecosystems across the globe, the world's natural capital is expected to decrease, causing a 9% loss of ecosystem services by 2100. That's according to a study of natural capital published today in the journal Nature led by scientists at the University of California, Davis, and Scripps Institution of Oceanography at UC San Diego.

"The big question is what do we lose when we lose an ecosystem?" said lead author Bernardo Bastien-Olvera, a doctoral student at UC Davis when the study was conducted and currently a postdoctoral fellow at Scripps. "Flipping the question: What do we gain if we are able to limit climate change and avoid some of its impacts on natural systems? This study helps us better consider damages not usually accounted for. It also reveals an overlooked, yet startling dimension of climate change effects on natural systems - its capacity to exacerbate global economic inequality."

"Our research found that the world's poorest 50% of countries and regions are expected to bear a staggering 90% of the GDP damages," Bastien-Olvera said. "In sharp contrast, the losses for the wealthiest 10% might be limited to just 2%."

"Thanks to the efforts of this research team, we now know that damage to ecosystems impacts human well-being in ways that are both measurable and wildly disproportionate across populations," said Jeffrey Mantz, NSF program officer. "The results will be critical to abating economic losses in the coming decades."
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department of environmental science and policy

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Ecosystem Benefits to Humanity Expected to Decline by 9% by 2100

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A UC Davis study of natural capital finds nature's benefits to humanity are expected to decline as climate change shifts ecosystems across borders.

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Environmental and Health Impacts of Air Pollution: A Review

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Environmental and Health Impacts of Air Pollution: A Review

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Human activities have an adverse affect on the environment because water is being polluted.

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Delphis S.A., Kifisia, Greece

Laboratory of Hygiene and Environmental Protection, Faculty of Medicine, Democritus University of Thrace, Alexandroupolis, Greece

Centre Hospitalier Universitaire Vaudois (CHUV), Service de Médicine Interne, Lausanne, Switzerland

School of Social and Political Sciences, University of Glasgow, Glasgow, United Kingdom

One of our era's greatest scourges is air pollution, on account not only of its impact on climate change but also its impact on public and individual health due to increasing morbidity and mortality. There are many pollutants that are major factors in disease in humans. Among them, Particulate Matter (PM), particles of variable but very small diameter, penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Despite the fact that ozone in the stratosphere plays a protective role against ultraviolet irradiation, it is harmful when in high concentration at ground level, also affecting the respiratory and cardiovascular system. Furthermore, nitrogen oxide, sulfur dioxide, Volatile Organic Compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) are all considered air pollutants that are harmful to humans. Carbon monoxide can even provoke direct poisoning when breathed in at high levels. Heavy metals such as lead, when absorbed into the human body, can lead to direct poisoning or chronic intoxication, depending on exposure. Diseases occurring from the aforementioned substances include principally respiratory problems such as Chronic Obstructive Pulmonary Disease (COPD), asthma, bronchiolitis, and also lung cancer, cardiovascular events, central nervous system dysfunctions, and cutaneous diseases. Last but not least, climate change resulting from environmental pollution affects the geographical distribution of many infectious diseases, as do natural disasters. The only way to tackle this problem is through public awareness coupled with a multidisciplinary approach by scientific experts; national and international organizations must address the emergence of this threat and propose sustainable solutions.

Approach to the Problem

The interactions between humans and their physical surroundings have been extensively studied, as multiple human activities influence the environment. The environment is a coupling of the biotic (living organisms and microorganisms) and the abiotic (hydrosphere, lithosphere, and atmosphere).

Pollution is defined as the introduction into the environment of substances harmful to humans and other living organisms. Pollutants are harmful solids, liquids, or gases produced in higher than usual concentrations that reduce the quality of our environment.

Human activities have an adverse effect on the environment by polluting the water we drink, the air we breathe, and the soil in which plants grow. Although the industrial revolution was a great success in terms of technology, society, and the provision of multiple services, it also introduced the production of huge quantities of pollutants emitted into the air that are harmful to human health. Without any doubt, the global environmental pollution is considered an international public health issue with multiple facets. Social, economic, and legislative concerns and lifestyle habits are related to this major problem. Clearly, urbanization and industrialization are reaching unprecedented and upsetting proportions worldwide in our era. Anthropogenic air pollution is one of the biggest public health hazards worldwide, given that it accounts for about 9 million deaths per year ( 1).

Air pollution has various health effects. The health of susceptible and sensitive individuals can be impacted even on low air pollution days. Short-term exposure to air pollutants is closely related to COPD (Chronic Obstructive Pulmonary Disease), cough, shortness of breath, wheezing, asthma, respiratory disease, and high rates of hospitalization (a measurement of morbidity).

Air pollution mainly affects those living in large urban areas, where road emissions contribute the most to the degradation of air quality. There is also a danger of industrial accidents, where the spread of a toxic fog can be fatal to the populations of the surrounding areas. The dispersion of pollutants is determined by many parameters, most notably atmospheric stability and wind ( 6).

In developing countries ( 7), the problem is more serious due to overpopulation and uncontrolled urbanization along with the development of industrialization. This leads to poor air quality, especially in countries with social disparities and a lack of information on sustainable management of the environment. The use of fuels such as wood fuel or solid fuel for domestic needs due to low incomes exposes people to bad-quality, polluted air at home. It is of note that three billion people around the world are using the above sources of energy for their daily heating and cooking needs ( 8). In developing countries, the women of the household seem to carry the highest risk for disease development due to their longer duration exposure to the indoor air pollution ( 8, 9). Due to its fast industrial development and overpopulation, China is one of the Asian countries confronting serious air pollution problems ( 10, 11). The lung cancer mortality observed in China is associated with fine particles ( 12). As stated already, long-term exposure is associated with deleterious effects on the cardiovascular system ( 3, 5). However, it is interesting to note that cardiovascular diseases have mostly been observed in developed and high-income countries rather than in the developing low-income countries exposed highly to air pollution ( 13). Extreme air pollution is recorded in India, where the air quality reaches hazardous levels. New Delhi is one of the more polluted cities in India. Flights in and out of New Delhi International Airport are often canceled due to the reduced visibility associated with air pollution. Pollution is occurring both in urban and rural areas in India due to the fast industrialization, urbanization, and rise in use of motorcycle transportation. Nevertheless, biomass combustion associated with heating and cooking needs and practices is a major source of household air pollution in India and in Nepal ( 14, 15). There is spatial heterogeneity in India, as areas with diverse climatological conditions and population and education levels generate different indoor air qualities, with higher PM 2.5 observed in North Indian states (557-601 μg/m ³) compared to the Southern States (183-214 μg/m ³) ( 16, 17). The cold climate of the North Indian areas may be the main reason for this, as longer periods at home and more heating are necessary compared to in the tropical climate of Southern India. Household air pollution in India is associated with major health effects, especially in women and young children, who stay indoors for longer periods. Chronic obstructive respiratory disease (CORD) and lung cancer are mostly observed in women, while acute lower respiratory disease is seen in young children under 5 years of age ( 18).

Furthermore, extremely high levels of pollution are reported in Mexico City and Rio de Janeiro, followed by Milan, Ankara, Melbourne, Tokyo, and Moscow ( 19).

Based on the magnitude of the public health impact, it is certain that different kinds of interventions should be taken into account. Success and effectiveness in controlling air pollution, specifically at the local level, have been reported. Adequate technological means are applied considering the source and the nature of the emission as well as its impact on health and the environment. The importance of point sources and non-point sources of air pollution control is reported by Schwela and Köth-Jahr ( 21). Without a doubt, a detailed emission inventory must record all sources in a given area. Beyond considering the above sources and their nature, topography and meteorology should also be considered, as stated previously. Assessment of the control policies and methods is often extrapolated from the local to the regional and then to the global scale. Air pollution may be dispersed and transported from one region to another area located far away. Air pollution management means the reduction to acceptable levels or possible elimination of air pollutants whose presence in the air affects our health or the environmental ecosystem. Private and governmental entities and authorities implement actions to ensure the air quality ( 22). Air quality standards and guidelines were adopted for the different pollutants by the WHO and EPA as a tool for the management of air quality ( 1, 23). These standards have to be compared to the emissions inventory standards by causal analysis and dispersion modeling in order to reveal the problematic areas ( 24). Inventories are generally based on a combination of direct measurements and emissions modeling ( 24).

As an example, we state here the control measures at the source through the use of catalytic converters in cars. These are devices that turn the pollutants and toxic gases produced from combustion engines into less-toxic pollutants by catalysis through redox reactions ( 25). In Greece, the use of private cars was restricted by tracking their license plates in order to reduce traffic congestion during rush hour ( 25).

Concerning industrial emissions, collectors and closed systems can keep the air pollution to the minimal standards imposed by legislation ( 26).

Current strategies to improve air quality require an estimation of the economic value of the benefits gained from proposed programs. These proposed programs by public authorities, and directives are issued with guidelines to be respected.

In this vein, funds have been financing, directly or indirectly, projects related to air quality along with the technical infrastructure to maintain good air quality. These plans focus on an inventory of databases from air quality environmental planning awareness campaigns. Moreover, pollution measures of air emissions may be taken for vehicles, machines, and industries in urban areas.

Technological innovation can only be successful if it is able to meet the needs of society. In this sense, technology must reflect the decision-making practices and procedures of those involved in risk assessment and evaluation and act as a facilitator in providing information and assessments to enable decision makers to make the best decisions possible. Summarizing the aforementioned in order to design an effective air quality control strategy, several aspects must be considered: environmental factors and ambient air quality conditions, engineering factors and air pollutant characteristics, and finally, economic operating costs for technological improvement and administrative and legal costs. Considering the economic factor, competitiveness through neoliberal concepts is offering a solution to environmental problems ( 22).

In the present paper, we focus on the sources of environmental pollution in relation to public health and propose some solutions and interventions that may be of interest to environmental legislators and decision makers.

Sources of Exposure

It is known that the majority of environmental pollutants are emitted through large-scale human activities such as the use of industrial machinery, power-producing stations, combustion engines, and cars. Because these activities are performed at such a large scale, they are by far the major contributors to air pollution, with cars estimated to be responsible for approximately 80% of today's pollution ( 31). Some other human activities are also influencing our environment to a lesser extent, such as field cultivation techniques, gas stations, fuel tanks heaters, and cleaning procedures ( 32), as well as several natural sources, such as volcanic and soil eruptions and forest fires.

The classification of air pollutants is based mainly on the sources producing pollution. Therefore, it is worth mentioning the four main sources, following the classification system: Major sources, Area sources, Mobile sources, and Natural sources.

Major sources include the emission of pollutants from power stations, refineries, and petrochemicals, the chemical and fertilizer industries, metallurgical and other industrial plants, and, finally, municipal incineration.

Indoor area sources include domestic cleaning activities, dry cleaners, printing shops, and petrol stations.

Mobile sources include automobiles, cars, railways, airways, and other types of vehicles.

Finally, natural sources include, as stated previously, physical disasters ( 33) such as forest fire, volcanic erosion, dust storms, and agricultural burning.

However, many classification systems have been proposed. Another type of classification is a grouping according to the recipient of the pollution, as follows:

Air pollution is determined as the presence of pollutants in the air in large quantities for long periods. Air pollutants are dispersed particles, hydrocarbons, CO, CO ₂, NO, NO ₂, SO ₃, etc.

Water pollution is organic and inorganic charge and biological charge ( 10) at high levels that affect the water quality ( 34, 35).

Soil pollution occurs through the release of chemicals or the disposal of wastes, such as heavy metals, hydrocarbons, and pesticides.

Lastly, pollution is classified following type of origin:

Radioactive and nuclear pollution, releasing radioactive and nuclear pollutants into water, air, and soil during nuclear explosions and accidents, from nuclear weapons, and through handling or disposal of radioactive sewage.

Radioactive materials can contaminate surface water bodies and, being noxious to the environment, plants, animals, and humans. It is known that several radioactive substances such as radium and uranium concentrate in the bones and can cause cancers ( 38, 39).

Noise pollution is produced by machines, vehicles, traffic noises, and musical installations that are harmful to our hearing.

The World Health Organization introduced the term DALYs. The DALYs for a disease or health condition is defined as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with the health condition or its consequences ( 39). In Europe, air pollution is the main cause of disability-adjusted life years lost (DALYs), followed by noise pollution. The potential relationships of noise and air pollution with health have been studied ( 40). The study found that DALYs related to noise were more important than those related to air pollution, as the effects of environmental noise on cardiovascular disease were independent of air pollution ( 40). Environmental noise should be counted as an independent public health risk ( 40).

Environmental pollution occurs when changes in the physical, chemical, or biological constituents of the environment (air masses, temperature, climate, etc.) are produced.

Pollutants harm our environment either by increasing levels above normal or by introducing harmful toxic substances. Primary pollutants are directly produced from the above sources, and secondary pollutants are emitted as by-products of the primary ones. Pollutants can be biodegradable or non-biodegradable and of natural origin or anthropogenic, as stated previously. Moreover, their origin can be a unique source (point-source) or dispersed sources.

Climate and Pollution

Air pollution and climate change are closely related. Climate is the other side of the same coin that reduces the quality of our Earth ( 44). Pollutants such as black carbon, methane, tropospheric ozone, and aerosols affect the amount of incoming sunlight. As a result, the temperature of the Earth is increasing, resulting in the melting of ice, icebergs, and glaciers.

In this vein, climatic changes will affect the incidence and prevalence of both residual and imported infections in Europe. Climate and weather affect the duration, timing, and intensity of outbreaks strongly and change the map of infectious diseases in the globe ( 45). Mosquito-transmitted parasitic or viral diseases are extremely climate-sensitive, as warming firstly shortens the pathogen incubation period and secondly shifts the geographic map of the vector. Similarly, water-warming following climate changes leads to a high incidence of waterborne infections. Recently, in Europe, eradicated diseases seem to be emerging due to the migration of population, for example, cholera, poliomyelitis, tick-borne encephalitis, and malaria ( 46).

The spread of epidemics is associated with natural climate disasters and storms, which seem to occur more frequently nowadays ( 47). Malnutrition and disequilibration of the immune system are also associated with the emerging infections affecting public health ( 48).

The Chikungunya virus "took the airplane" from the Indian Ocean to Europe, as outbreaks of the disease were registered in Italy ( 49) as well as autochthonous cases in France ( 50).

An increase in cryptosporidiosis in the United Kingdom and in the Czech Republic seems to have occurred following flooding ( 36, 51).

As stated previously, aerosols compounds are tiny in size and considerably affect the climate. They are able to dissipate sunlight (the albedo phenomenon) by dispersing a quarter of the sun's rays back to space and have cooled the global temperature over the last 30 years ( 52).

Air Pollutants

The World Health Organization (WHO) reports on six major air pollutants, namely particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Air pollution can have a disastrous effect on all components of the environment, including groundwater, soil, and air. Additionally, it poses a serious threat to living organisms. In this vein, our interest is mainly to focus on these pollutants, as they are related to more extensive and severe problems in human health and environmental impact. Acid rain, global warming, the greenhouse effect, and climate changes have an important ecological impact on air pollution ( 53).

Particulate Matter (PM) and Health

Studies have shown a relationship between particulate matter (PM) and adverse health effects, focusing on either short-term (acute) or long-term (chronic) PM exposure.

Particulate matter (PM) is usually formed in the atmosphere as a result of chemical reactions between the different pollutants. The penetration of particles is closely dependent on their size ( 53). Particulate Matter (PM) was defined as a term for particles by the United States Environmental Protection Agency ( 54). Particulate matter (PM) pollution includes particles with diameters of 10 micrometers (μm) or smaller, called PM ₁₀, and extremely fine particles with diameters that are generally 2.5 micrometers (μm) and smaller.

Particulate matter contains tiny liquid or solid droplets that can be inhaled and cause serious health effects ( 55). Particles <10 μm in diameter (PM ₁₀) after inhalation can invade the lungs and even reach the bloodstream. Fine particles, PM 2.5, pose a greater risk to health ( 6, 56) ( Table 1).

Table 1. Penetrability according to particle size.

Multiple epidemiological studies have been performed on the health effects of PM. A positive relation was shown between both short-term and long-term exposures of PM 2.5 and acute nasopharyngitis ( 56). In addition, long-term exposure to PM for years was found to be related to cardiovascular diseases and infant mortality.

Those studies depend on PM 2.5 monitors and are restricted in terms of study area or city area due to a lack of spatially resolved daily PM 2.5 concentration data and, in this way, are not representative of the entire population. Following a recent epidemiological study by the Department of Environmental Health at Harvard School of Public Health (Boston, MA) ( 57), it was reported that, as PM 2.5 concentrations vary spatially, an exposure error (Berkson error) seems to be produced, and the relative magnitudes of the short- and long-term effects are not yet completely elucidated. The team developed a PM 2.5 exposure model based on remote sensing data for assessing short- and long-term human exposures ( 57). This model permits spatial resolution in short-term effects plus the assessment of long-term effects in the whole population.

Moreover, respiratory diseases and affection of the immune system are registered as long-term chronic effects ( 58). It is worth noting that people with asthma, pneumonia, diabetes, and respiratory and cardiovascular diseases are especially susceptible and vulnerable to the effects of PM. PM 2.5, followed by PM ₁₀, are strongly associated with diverse respiratory system diseases ( 59), as their size permits them to pierce interior spaces ( 60). The particles produce toxic effects according to their chemical and physical properties. The components of PM ₁₀ and PM 2.5 can be organic (polycyclic aromatic hydrocarbons, dioxins, benzene, 1-3 butadiene) or inorganic (carbon, chlorides, nitrates, sulfates, metals) in nature ( 55).

Particulate Matter (PM) is divided into four main categories according to type and size ( 61) ( Table 2).

Table 2. Types and sizes of particulate Matter (PM).

Gas contaminants include PM in aerial masses.

Particulate contaminants include contaminants such as smog, soot, tobacco smoke, oil smoke, fly ash, and cement dust.

Biological Contaminants are microorganisms (bacteria, viruses, fungi, mold, and bacterial spores), cat allergens, house dust and allergens, and pollen.

Types of Dust include suspended atmospheric dust, settling dust, and heavy dust.

Finally, another fact is that the half-lives of PM ₁₀ and PM 2.5 particles in the atmosphere is extended due to their tiny dimensions; this permits their long-lasting suspension in the atmosphere and even their transfer and spread to distant destinations where people and the environment may be exposed to the same magnitude of pollution ( 53). They are able to change the nutrient balance in watery ecosystems, damage forests and crops, and acidify water bodies.

As stated, PM 2.5, due to their tiny size, are causing more serious health effects. These aforementioned fine particles are the main cause of the "haze" formation in different metropolitan areas ( 12, 13, 61).

Ozone Impact in the Atmosphere

Ozone (O ₃) is a gas formed from oxygen under high voltage electric discharge ( 62). It is a strong oxidant, 52% stronger than chlorine. It arises in the stratosphere, but it could also arise following chain reactions of photochemical smog in the troposphere ( 63).

Ozone can travel to distant areas from its initial source, moving with air masses ( 64). It is surprising that ozone levels over cities are low in contrast to the increased amounts occuring in urban areas, which could become harmful for cultures, forests, and vegetation ( 65) as it is reducing carbon assimilation ( 66). Ozone reduces growth and yield ( 47, 48) and affects the plant microflora due to its antimicrobial capacity ( 67, 68). In this regard, ozone acts upon other natural ecosystems, with microflora ( 69, 70) and animal species changing their species composition ( 71). Ozone increases DNA damage in epidermal keratinocytes and leads to impaired cellular function ( 72).

Ground-level ozone (GLO) is generated through a chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or following anthropogenic activities.

Ozone uptake usually occurs by inhalation. Ozone affects the upper layers of the skin and the tear ducts ( 73). A study of short-term exposure of mice to high levels of ozone showed malondialdehyde formation in the upper skin (epidermis) but also depletion in vitamins C and E. It is likely that ozone levels are not interfering with the skin barrier function and integrity to predispose to skin disease ( 74).

Due to the low water-solubility of ozone, inhaled ozone has the capacity to penetrate deeply into the lungs ( 75).

Toxic effects induced by ozone are registered in urban areas all over the world, causing biochemical, morphologic, functional, and immunological disorders ( 76).

The European project (APHEA2) focuses on the acute effects of ambient ozone concentrations on mortality ( 77). Daily ozone concentrations compared to the daily number of deaths were reported from different European cities for a 3-year period. During the warm period of the year, an observed increase in ozone concentration was associated with an increase in the daily number of deaths (0.33%), in the number of respiratory deaths (1.13%), and in the number of cardiovascular deaths (0.45%). No effect was observed during wintertime.

Carbon Monoxide (CO)

Carbon monoxide is produced by fossil fuel when combustion is incomplete. The symptoms of poisoning due to inhaling carbon monoxide include headache, dizziness, weakness, nausea, vomiting, and, finally, loss of consciousness.

The affinity of carbon monoxide to hemoglobin is much greater than that of oxygen. In this vein, serious poisoning may occur in people exposed to high levels of carbon monoxide for a long period of time. Due to the loss of oxygen as a result of the competitive binding of carbon monoxide, hypoxia, ischemia, and cardiovascular disease are observed.

Carbon monoxide affects the greenhouses gases that are tightly connected to global warming and climate. This should lead to an increase in soil and water temperatures, and extreme weather conditions or storms may occur ( 68).

However, in laboratory and field experiments, it has been seen to produce increased plant growth ( 78).

Nitrogen Oxide (NO₂)

Nitrogen oxide is a traffic-related pollutant, as it is emitted from automobile motor engines ( 79, 80). It is an irritant of the respiratory system as it penetrates deep in the lung, inducing respiratory diseases, coughing, wheezing, dyspnea, bronchospasm, and even pulmonary edema when inhaled at high levels. It seems that concentrations over 0.2 ppm produce these adverse effects in humans, while concentrations higher than 2.0 ppm affect T-lymphocytes, particularly the CD8+ cells and NK cells that produce our immune response ( 81).It is reported that long-term exposure to high levels of nitrogen dioxide can be responsible for chronic lung disease. Long-term exposure to NO ₂ can impair the sense of smell ( 81).

However, systems other than respiratory ones can be involved, as symptoms such as eye, throat, and nose irritation have been registered ( 81).

High levels of nitrogen dioxide are deleterious to crops and vegetation, as they have been observed to reduce crop yield and plant growth efficiency. Moreover, NO ₂ can reduce visibility and discolor fabrics ( 81).

Sulfur Dioxide (SO₂)

Sulfur dioxide is a harmful gas that is emitted mainly from fossil fuel consumption or industrial activities. The annual standard for SO ₂ is 0.03 ppm ( 82). It affects human, animal, and plant life. Susceptible people as those with lung disease, old people, and children, who present a higher risk of damage. The major health problems associated with sulfur dioxide emissions in industrialized areas are respiratory irritation, bronchitis, mucus production, and bronchospasm, as it is a sensory irritant and penetrates deep into the lung converted into bisulfite and interacting with sensory receptors, causing bronchoconstriction. Moreover, skin redness, damage to the eyes (lacrimation and corneal opacity) and mucous membranes, and worsening of pre-existing cardiovascular disease have been observed ( 81).

Environmental adverse effects, such as acidification of soil and acid rain, seem to be associated with sulfur dioxide emissions ( 83).

Lead

Lead is a heavy metal used in different industrial plants and emitted from some petrol motor engines, batteries, radiators, waste incinerators, and waste waters ( 84).

Moreover, major sources of lead pollution in the air are metals, ore, and piston-engine aircraft. Lead poisoning is a threat to public health due to its deleterious effects upon humans, animals, and the environment, especially in the developing countries.

Exposure to lead can occur through inhalation, ingestion, and dermal absorption. Trans- placental transport of lead was also reported, as lead passes through the placenta unencumbered ( 85). The younger the fetus is, the more harmful the toxic effects. Lead toxicity affects the fetal nervous system; edema or swelling of the brain is observed ( 86). Lead, when inhaled, accumulates in the blood, soft tissue, liver, lung, bones, and cardiovascular, nervous, and reproductive systems. Moreover, loss of concentration and memory, as well as muscle and joint pain, were observed in adults ( 85, 86).

Children and newborns ( 87) are extremely susceptible even to minimal doses of lead, as it is a neurotoxicant and causes learning disabilities, impairment of memory, hyperactivity, and even mental retardation.

Elevated amounts of lead in the environment are harmful to plants and crop growth. Neurological effects are observed in vertebrates and animals in association with high lead levels ( 88).

Polycyclic Aromatic Hydrocarbons(PAHs)

The distribution of PAHs is ubiquitous in the environment, as the atmosphere is the most important means of their dispersal. They are found in coal and in tar sediments. Moreover, they are generated through incomplete combustion of organic matter as in the cases of forest fires, incineration, and engines ( 89). PAH compounds, such as benzopyrene, acenaphthylene, anthracene, and fluoranthene are recognized as toxic, mutagenic, and carcinogenic substances. They are an important risk factor for lung cancer ( 89).

Volatile Organic Compounds(VOCs)

Volatile organic compounds (VOCs), such as toluene, benzene, ethylbenzene, and xylene ( 90), have been found to be associated with cancer in humans ( 91). The use of new products and materials has actually resulted in increased concentrations of VOCs. VOCs pollute indoor air ( 90) and may have adverse effects on human health ( 91). Short-term and long-term adverse effects on human health are observed. VOCs are responsible for indoor air smells. Short-term exposure is found to cause irritation of eyes, nose, throat, and mucosal membranes, while those of long duration exposure include toxic reactions ( 92). Predictable assessment of the toxic effects of complex VOC mixtures is difficult to estimate, as these pollutants can have synergic, antagonistic, or indifferent effects ( 91, 93).

Dioxins

Dioxins originate from industrial processes but also come from natural processes, such as forest fires and volcanic eruptions. They accumulate in foods such as meat and dairy products, fish and shellfish, and especially in the fatty tissue of animals ( 94).

Short-period exhibition to high dioxin concentrations may result in dark spots and lesions on the skin ( 94). Long-term exposure to dioxins can cause developmental problems, impairment of the immune, endocrine and nervous systems, reproductive infertility, and cancer ( 94).

Without any doubt, fossil fuel consumption is responsible for a sizeable part of air contamination. This contamination may be anthropogenic, as in agricultural and industrial processes or transportation, while contamination from natural sources is also possible. Interestingly, it is of note that the air quality standards established through the European Air Quality Directive are somewhat looser than the WHO guidelines, which are stricter ( 95).

Effect of Air Pollution on Health

The most common air pollutants are ground-level ozone and Particulates Matter (PM). Air pollution is distinguished into two main types:

Outdoor pollution is the ambient air pollution.

Indoor pollution is the pollution generated by household combustion of fuels.

People exposed to high concentrations of air pollutants experience disease symptoms and states of greater and lesser seriousness. These effects are grouped into short- and long-term effects affecting health.

Susceptible populations that need to be aware of health protection measures include old people, children, and people with diabetes and predisposing heart or lung disease, especially asthma.

As extensively stated previously, according to a recent epidemiological study from Harvard School of Public Health, the relative magnitudes of the short- and long-term effects have not been completely clarified ( 57) due to the different epidemiological methodologies and to the exposure errors. New models are proposed for assessing short- and long-term human exposure data more successfully ( 57). Thus, in the present section, we report the more common short- and long-term health effects but also general concerns for both types of effects, as these effects are often dependent on environmental conditions, dose, and individual susceptibility.

Short-term effects are temporary and range from simple discomfort, such as irritation of the eyes, nose, skin, throat, wheezing, coughing and chest tightness, and breathing difficulties, to more serious states, such as asthma, pneumonia, bronchitis, and lung and heart problems. Short-term exposure to air pollution can also cause headaches, nausea, and dizziness.

These problems can be aggravated by extended long-term exposure to the pollutants, which is harmful to the neurological, reproductive, and respiratory systems and causes cancer and even, rarely, deaths.

The long-term effects are chronic, lasting for years or the whole life and can even lead to death. Furthermore, the toxicity of several air pollutants may also induce a variety of cancers in the long term ( 96).

As stated already, respiratory disorders are closely associated with the inhalation of air pollutants. These pollutants will invade through the airways and will accumulate at the cells. Damage to target cells should be related to the pollutant component involved and its source and dose. Health effects are also closely dependent on country, area, season, and time. An extended exposure duration to the pollutant should incline to long-term health effects in relation also to the above factors.

Particulate Matter (PMs), dust, benzene, and O ₃ cause serious damage to the respiratory system ( 97). Moreover, there is a supplementary risk in case of existing respiratory disease such as asthma ( 98). Long-term effects are more frequent in people with a predisposing disease state. When the trachea is contaminated by pollutants, voice alterations may be remarked after acute exposure. Chronic obstructive pulmonary disease (COPD) may be induced following air pollution, increasing morbidity and mortality ( 99). Long-term effects from traffic, industrial air pollution, and combustion of fuels are the major factors for COPD risk ( 99).

Multiple cardiovascular effects have been observed after exposure to air pollutants ( 100). Changes occurred in blood cells after long-term exposure may affect cardiac functionality. Coronary arteriosclerosis was reported following long-term exposure to traffic emissions ( 101), while short-term exposure is related to hypertension, stroke, myocardial infracts, and heart insufficiency. Ventricle hypertrophy is reported to occur in humans after long-time exposure to nitrogen oxide (NO ₂) ( 102, 103).

Neurological effects have been observed in adults and children after extended-term exposure to air pollutants.

Figure 1. Impact of air pollutants on the brain.

Brain inflammation was observed in dogs living in a highly polluted area in Mexico for a long period ( 105). In human adults, markers of systemic inflammation (IL-6 and fibrinogen) were found to be increased as an immediate response to PNC on the IL-6 level, possibly leading to the production of acute-phase proteins ( 106). The progression of atherosclerosis and oxidative stress seem to be the mechanisms involved in the neurological disturbances caused by long-term air pollution. Inflammation comes secondary to the oxidative stress and seems to be involved in the impairment of developmental maturation, affecting multiple organs ( 105, 107). Similarly, other factors seem to be involved in the developmental maturation, which define the vulnerability to long-term air pollution. These include birthweight, maternal smoking, genetic background and socioeconomic environment, as well as education level.

However, diet, starting from breast-feeding, is another determinant factor. Diet is the main source of antioxidants, which play a key role in our protection against air pollutants ( 108). Antioxidants are free radical scavengers and limit the interaction of free radicals in the brain ( 108). Similarly, genetic background may result in a differential susceptibility toward the oxidative stress pathway ( 60). For example, antioxidant supplementation with vitamins C and E appears to modulate the effect of ozone in asthmatic children homozygous for the GSTM1 null allele ( 61). Inflammatory cytokines released in the periphery (e.g., respiratory epithelia) upregulate the innate immune Toll-like receptor 2. Such activation and the subsequent events leading to neurodegeneration have recently been observed in lung lavage in mice exposed to ambient Los Angeles (CA, USA) particulate matter ( 61). In children, neurodevelopmental morbidities were observed after lead exposure. These children developed aggressive and delinquent behavior, reduced intelligence, learning difficulties, and hyperactivity ( 109). No level of lead exposure seems to be "safe," and the scientific community has asked the Centers for Disease Control and Prevention (CDC) to reduce the current screening guideline of 10 μg/dl ( 109).

Higher morbidity is reported to fetuses and children when exposed to the above dangers. Impairment in fetal growth, low birth weight, and autism have been reported ( 112).

Environmental Impact of Air Pollution

Air pollution is harming not only human health but also the environment ( 115) in which we live. The most important environmental effects are as follows.

Acid rain is wet (rain, fog, snow) or dry (particulates and gas) precipitation containing toxic amounts of nitric and sulfuric acids. They are able to acidify the water and soil environments, damage trees and plantations, and even damage buildings and outdoor sculptures, constructions, and statues.

Haze is produced when fine particles are dispersed in the air and reduce the transparency of the atmosphere. It is caused by gas emissions in the air coming from industrial facilities, power plants, automobiles, and trucks.

Ozone, as discussed previously, occurs both at ground level and in the upper level (stratosphere) of the Earth's atmosphere. Stratospheric ozone is protecting us from the Sun's harmful ultraviolet (UV) rays. In contrast, ground-level ozone is harmful to human health and is a pollutant. Unfortunately, stratospheric ozone is gradually damaged by ozone-depleting substances (i.e., chemicals, pesticides, and aerosols). If this protecting stratospheric ozone layer is thinned, then UV radiation can reach our Earth, with harmful effects for human life (skin cancer) ( 116) and crops ( 117). In plants, ozone penetrates through the stomata, inducing them to close, which blocks CO ₂ transfer and induces a reduction in photosynthesis ( 118).

Global climate change is an important issue that concerns mankind. As is known, the "greenhouse effect" keeps the Earth's temperature stable. Unhappily, anthropogenic activities have destroyed this protecting temperature effect by producing large amounts of greenhouse gases, and global warming is mounting, with harmful effects on human health, animals, forests, wildlife, agriculture, and the water environment. A report states that global warming is adding to the health risks of poor people ( 119).

People living in poorly constructed buildings in warm-climate countries are at high risk for heat-related health problems as temperatures mount ( 119).

Wildlife is burdened by toxic pollutants coming from the air, soil, or the water ecosystem and, in this way, animals can develop health problems when exposed to high levels of pollutants. Reproductive failure and birth effects have been reported.

Eutrophication is occurring when elevated concentrations of nutrients (especially nitrogen) stimulate the blooming of aquatic algae, which can cause a disequilibration in the diversity of fish and their deaths.

Without a doubt, there is a critical concentration of pollution that an ecosystem can tolerate without being destroyed, which is associated with the ecosystem's capacity to neutralize acidity. The Canada Acid Rain Program established this load at 20 kg/ha/yr ( 120).

Hence, air pollution has deleterious effects on both soil and water ( 121). Concerning PM as an air pollutant, its impact on crop yield and food productivity has been reported. Its impact on watery bodies is associated with the survival of living organisms and fishes and their productivity potential ( 121).

An impairment in photosynthetic rhythm and metabolism is observed in plants exposed to the effects of ozone ( 121).

Sulfur and nitrogen oxides are involved in the formation of acid rain and are harmful to plants and marine organisms.

Last but not least, as mentioned above, the toxicity associated with lead and other metals is the main threat to our ecosystems (air, water, and soil) and living creatures ( 121).

Discussion

In 2018, during the first WHO Global Conference on Air Pollution and Health, the WHO's General Director, Dr. Tedros Adhanom Ghebreyesus, called air pollution a "silent public health emergency" and "the new tobacco" ( 122).

Undoubtedly, children are particularly vulnerable to air pollution, especially during their development. Air pollution has adverse effects on our lives in many different respects.

Diseases associated with air pollution have not only an important economic impact but also a societal impact due to absences from productive work and school.

Despite the difficulty of eradicating the problem of anthropogenic environmental pollution, a successful solution could be envisaged as a tight collaboration of authorities, bodies, and doctors to regularize the situation. Governments should spread sufficient information and educate people and should involve professionals in these issues so as to control the emergence of the problem successfully.

Technologies to reduce air pollution at the source must be established and should be used in all industries and power plants. The Kyoto Protocol of 1997 set as a major target the reduction of GHG emissions to below 5% by 2012 ( 123). This was followed by the Copenhagen summit, 2009 ( 124), and then the Durban summit of 2011 ( 125), where it was decided to keep to the same line of action. The Kyoto protocol and the subsequent ones were ratified by many countries. Among the pioneers who adopted this important protocol for the world's environmental and climate "health" was China ( 3). As is known, China is a fast-developing economy and its GDP (Gross Domestic Product) is expected to be very high by 2050, which is defined as the year of dissolution of the protocol for the decrease in gas emissions.

A more recent international agreement of crucial importance for climate change is the Paris Agreement of 2015, issued by the UNFCCC (United Nations Climate Change Committee). This latest agreement was ratified by a plethora of UN (United Nations) countries as well as the countries of the European Union ( 126). In this vein, parties should promote actions and measures to enhance numerous aspects around the subject. Boosting education, training, public awareness, and public participation are some of the relevant actions for maximizing the opportunities to achieve the targets and goals on the crucial matter of climate change and environmental pollution ( 126). Without any doubt, technological improvements makes our world easier and it seems difficult to reduce the harmful impact caused by gas emissions, we could limit its use by seeking reliable approaches.

Synopsizing, a global prevention policy should be designed in order to combat anthropogenic air pollution as a complement to the correct handling of the adverse health effects associated with air pollution. Sustainable development practices should be applied, together with information coming from research in order to handle the problem effectively.

At this point, international cooperation in terms of research, development, administration policy, monitoring, and politics is vital for effective pollution control. Legislation concerning air pollution must be aligned and updated, and policy makers should propose the design of a powerful tool of environmental and health protection. As a result, the main proposal of this essay is that we should focus on fostering local structures to promote experience and practice and extrapolate these to the international level through developing effective policies for sustainable management of ecosystems.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

IM is employed by the company Delphis S.A.

The remaining authors declare that the present review paper was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Frontiers | Environmental and Health Impacts of Air Pollution: A Review

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Air pollution: The invisible health threat

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Air pollution: The invisible health threat

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Air pollution is a major environmental threat and one of the main cases of death among all risk factors, ranking just below hypertension, tobacco smoking and high glucose. WHO estimates that, globally, air pollution is responsible for about 7 million premature deaths per year from ischemic heart disease, stroke, chronic obstructive pulmonary disease and lung cancer, but also from acute respiratory infections such as pneumonia which mainly affects children in low- and middle-income countries.

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Breaking the Plastic Wave: Top Findings for Preventing Plastic Pollution

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Renewable Energy Is the Key to Building a More Resilient and Reliable Electricity Grid

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Renewable Energy Is the Key to Building a More Resilient and Reliable Electricity Grid

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Renewable energy has seen considerable growth in recent years, but there is a long way to go to achieve a clean energy future that averts the worst effects of the climate crisis

By Andrea Thompson

Humanity is at a crossroads in choosing the way we will power our future. Depending on what electricity infrastructure we build now, we could lock in still more decades of planet-warming emissions, or we could lay a solid foundation for a clean energy future and stave off the climate emergency's worst effects.

The choice is an urgent one because the window is quickly closing on our ability to meet the goal of the 2015 Paris climate agreement: keeping global temperature rise well below two degrees Celsius by the end of the century. The latest report from the Intergovernmental Panel on Climate Change (IPCC) stressed that the world needs fast and deep emissions cuts to meet that goal and eventually reach net zero emissions by 2050. Switching to renewable forms of power generation, such as solar, wind and hydropower, will be a key component of that effort. "The target is very ambitious," says Heymi Bahar, a senior energy analyst at the International Energy Agency. And every year that goes by without major climate action, "we are basically losing the carbon [budget] that is left, and we need to go faster in a more expansive way. In that sense, most of the job, according to our models, needs to be done in the coming seven years," Bahar says.

There are encouraging signs we are making this transition, such as substantial growth in renewable electricity generation in recent years. But there are still hurdles, and the rate of transition must accelerate, according to many experts in the field.

Globally, renewables account for about one third of electricity generation-and that share is rising. In 2022 renewable generation capacity grew by a record 295 gigawatts, according to the International Renewable Energy Agency (IRENA). Further, renewables accounted for more than 80 percent of all added power capacity last year, the agency reported.

Last year renewables produced more electricity than coal-powered plants for the first time in the U.S. Wind and solar now produce about 14 percent of the country's electricity, up from virtually nothing just 25 years ago. The U.S. Energy Information Administration expects that more than half of electric generation capacity added to the nation's grid in 2023 will be from solar energy.

The main reason renewable energy has grown so much in recent years is a dramatic decline in the expense of generating solar and wind power. The cost of solar photovoltaic cells has dropped a stunning 90 percent over the past decade, partly because of ramped-up manufacturing-particularly in China-Bahar says. Government subsidies in countries such as the U.S. also helped renewables grow in the early years, as did policies making commitments to renewable adoption, says Inês Azevedo, an associate professor in the department of energy science and engineering at Stanford University.* For example, many U.S. states set standards for how much of their electricity needs should be met with renewable energy by a particular year.

But the current rate of renewable adoption is still far below what is needed to meet climate goals. Though global renewables capacity grew by 9.6 percent last year, IRENA says the capacity needs to grow at triple that rate to meet the Paris climate goals. And as other sectors such as manufacturing and transportation decarbonize, electricity needs will only increase.

There are obstacles to accelerating the rate of adopting renewables, though. For one, solar and wind are intermittent power sources, meaning they need to be deployed with batteries or other types of energy storage; this can increase costs. Photovoltaic cells, as well as the lithium-ion batteries that are today's key storage technology, also require critical (and sometimes relatively rare) minerals to produce them. Demand for these resources could outpace supply and create a future production bottleneck, Azevedo says.

Another impediment, she adds, is that "we're not starting from scratch. We have this massive existing infrastructure that we spent a bunch of money on. Some of that, globally, has been built fairly recently, and so that creates the risk of stranded assets." IRENA's analysts have said that 41 percent of planned energy investment by 2050 still remains targeted at fossil fuels.

Strong government policies to reduce emissions will be essential to push renewable adoption, Azevedo says: "Regulators need to regulate. This is not going to get there just through market forces."

Financing is also an issue. Even though the overall costs of renewables have come down, "you need to invest everything upfront," Bahar says, "which means that financing cost and risk management is extremely important."

Financing is particularly an issue in the equitable deployment of renewables. The U.S., Europe, India and China account for 80 percent of new renewables capacity. And "85 percent of investments in renewables has benefitted only 50 percent of the global population," primarily in the world's largest economies, says Roland Roesch, acting director of IRENA's Innovation and Technology Center. He adds that multinational development banks (a key funding source for projects in developing countries) must support developing countries in adopting renewable projects-and that this could be done by addressing the upfront costs, as well as the added risk that can come from political instability in some places. "The last mile of net zero is the most difficult one," Bahar says. "So it has to include everybody."
*Editor's Note (4/21/23): This sentence was edited after posting to clarify Inês Azevedo's primary affiliation at Stanford University.

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Renewable energy has seen considerable growth in recent years, but there is a long way to go to achieve a clean energy future that averts the worst effects of the climate crisis

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Solar and wind energy are the best forms of renewable energy for use.

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The wind, the sun, and Earth are sources of renewable energy. These energy sources naturally renew, or replenish themselves.

Wind, sunlight, and the planet have energy that transforms in ways we can see and feel. We can see and feel evidence of the transfer of energy from the sun to Earth in the sunlight shining on the ground and the warmth we feel when sunlight shines on our skin. We can see and feel evidence of the transfer of energy in wind's ability to pull kites higher into the sky and shake the leaves on trees. We can see and feel evidence of the transfer of energy in the geothermal energy of steam vents and geysers.

People have created different ways to capture the energy from these renewable sources.

Solar energy can be captured "actively" or "passively."

Active solar energy uses special technology to capture the sun's rays. The two main types of equipment are photovoltaic cells (also called PV cells or solar cells) and mirrors that focus sunlight in a specific spot. These active solar technologies use sunlight to generate electricity, which we use to power lights, heating systems, computers, and televisions.

Passive solar energy does not use any equipment. Instead, it gets energy from the way sunlight naturally changes throughout the day. For example, people can build houses so their windows face the path of the sun. This means the house will get more heat from the sun. It will take less energy from other sources to heat the house.

Other examples of passive solar technology are green roofs, cool roofs, and radiant barriers. Green roofs are completely covered with plants. Plants can get rid of pollutants in rainwater and air. They help make the local environment cleaner.

Cool roofs are painted white to better reflect sunlight. Radiant barriers are made of a reflective covering, such as aluminum. They both reflect the sun's heat instead of absorbing it. All these types of roofs help lower the amount of energy needed to cool the building.

Advantages and Disadvantages
There are many advantages to using solar energy. PV cells last for a long time, about 20 years.

However, there are reasons why solar power cannot be used as the only power source in a community. It can be expensive to install PV cells or build a building using passive solar technology.

Sunshine can also be hard to predict. It can be blocked by clouds, and the sun doesn't shine at night. Different parts of Earth receive different amounts of sunlight based on location, the time of year, and the time of day.

Wind Energy

People have been harnessing the wind's energy for a long, long time. Five-thousand years ago, ancient Egyptians made boats powered by the wind. In 200 B.C.E., people used windmills to grind grain in the Middle East and pump water in China.

Today, we capture the wind's energy with wind turbines. A turbine is similar to a windmill; it has a very tall tower with two or three propeller-like blades at the top. These blades are turned by the wind. The blades turn a generator (located inside the tower), which creates electricity.

Groups of wind turbines are known as wind farms. Wind farms can be found near farmland, in narrow mountain passes, and even in the ocean, where there are steadier and stronger winds. Wind turbines anchored in the ocean are called "offshore wind farms."

Wind farms create electricity for nearby homes, schools, and other buildings.

Advantages and Disadvantages
Wind energy can be very efficient. In places like the Midwest in the United States and along coasts, steady winds can provide cheap, reliable electricity.

Another great advantage of wind power is that it is a "clean" form of energy. Wind turbines do not burn fuel or emit any pollutants into the air.

Wind is not always a steady source of energy, however. Wind speed changes constantly, depending on the time of day, weather, and geographic location. Currently, it cannot be used to provide electricity for all our power needs.

Wind turbines can also be dangerous for bats and birds. These animals cannot always judge how fast the blades are moving and crash into them.

Geothermal Energy

Deep beneath the surface is Earth's core. The center of Earth is extremely hot-thought to be over 6,000 °C (about 10,800 °F). The heat is constantly moving toward the surface.

We can see some of Earth's heat when it bubbles to the surface. Geothermal energy can melt underground rocks into magma and cause the magma to bubble to the surface as lava. Geothermal energy can also heat underground sources of water and force it to spew out from the surface. This stream of water is called a geyser.

However, most of Earth's heat stays underground and makes its way out very, very slowly.

We can access underground geothermal heat in different ways. One way of using geothermal energy is with "geothermal heat pumps." A pipe of water loops between a building and holes dug deep underground. The water is warmed by the geothermal energy underground and brings the warmth aboveground to the building. Geothermal heat pumps can be used to heat houses, sidewalks, and even parking lots.

Another way to use geothermal energy is with steam. In some areas of the world, there is underground steam that naturally rises to the surface. The steam can be piped straight to a power plant. However, in other parts of the world, the ground is dry. Water must be injected underground to create steam. When the steam comes to the surface, it is used to turn a generator and create electricity.

In Iceland, there are large reservoirs of underground water. Almost 90 percent of people in Iceland use geothermal as an energy source to heat their homes and businesses.

Advantages and Disadvantages
An advantage of geothermal energy is that it is clean. It does not require any fuel or emit any harmful pollutants into the air.

Geothermal energy is only avaiable in certain parts of the world. Another disadvantage of using geothermal energy is that in areas of the world where there is only dry heat underground, large quantities of freshwater are used to make steam. There may not be a lot of freshwater. People need water for drinking, cooking, and bathing.

Biomass is any material that comes from plants or microorganisms that were recently living. Plants create energy from the sun through photosynthesis. This energy is stored in the plants even after they die.

Trees, branches, scraps of bark, and recycled paper are common sources of biomass energy. Manure, garbage, and crops, such as corn, soy, and sugar cane, can also be used as biomass feedstocks.

We get energy from biomass by burning it. Wood chips, manure, and garbage are dried out and compressed into squares called "briquettes." These briquettes are so dry that they do not absorb water. They can be stored and burned to create heat or generate electricity.

Biomass can also be converted into biofuel. Biofuels are mixed with regular gasoline and can be used to power cars and trucks. Biofuels release less harmful pollutants than pure gasoline.

Advantages and Disadvantages
A major advantage of biomass is that it can be stored and then used when it is needed.

Growing crops for biofuels, however, requires large amounts of land and pesticides. Land could be used for food instead of biofuels. Some pesticides could pollute the air and water.

Biomass energy can also be a nonrenewable energy source. Biomass energy relies on biomass feedstocks-plants that are processed and burned to create electricity. Biomass feedstocks can include crops, such as corn or soy, as well as wood. If people do not replant biomass feedstocks as fast as they use them, biomass energy becomes a non-renewable energy source.

Hydroelectric energy is made by flowing water. Most hydroelectric power plants are located on large dams, which control the flow of a river.

Dams block the river and create an artificial lake, or reservoir. A controlled amount of water is forced through tunnels in the dam. As water flows through the tunnels, it turns huge turbines and generates electricity.

Advantages and Disadvantages
Hydroelectric energy is fairly inexpensive to harness. Dams do not need to be complex, and the resources to build them are not difficult to obtain. Rivers flow all over the world, so the energy source is available to millions of people.

Hydroelectric energy is also fairly reliable. Engineers control the flow of water through the dam, so the flow does not depend on the weather (the way solar and wind energies do).

However, hydroelectric power plants are damaging to the environment. When a river is dammed, it creates a large lake behind the dam. This lake (sometimes called a reservoir) drowns the original river habitat deep underwater. Sometimes, people build dams that can drown entire towns underwater. The people who live in the town or village must move to a new area.

Hydroelectric power plants don't work for a very long time: Some can only supply power for 20 or 30 years. Silt, or dirt from a riverbed, builds up behind the dam and slows the flow of water.

Other Renewable Energy Sources

Scientists and engineers are constantly working to harness other renewable energy sources. Three of the most promising are tidal energy, wave energy, and algal (or algae) fuel.

Tidal energy harnesses the power of ocean tides to generate electricity. Some tidal energy projects use the moving tides to turn the blades of a turbine. Other projects use small dams to continually fill reservoirs at high tide and slowly release the water (and turn turbines) at low tide.

Wave energy harnesses waves from the ocean, lakes, or rivers. Some wave energy projects use the same equipment that tidal energy projects do-dams and standing turbines. Other wave energy projects float directly on waves. The water's constant movement over and through these floating pieces of equipment turns turbines and creates electricity.

Algal fuel is a type of biomass energy that uses the unique chemicals in seaweed to create a clean and renewable biofuel. Algal fuel does not need the acres of cropland that other biofuel feedstocks do.

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Renewable energy comes from sources that will not be used up in our lifetimes, such as the sun and wind.