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Phosphorus is a chemical element found on Earth in numerous compound forms, such as the phosphate ion (PO ₄ ³⁻), located in water, soil and sediments. The quantities of phosphorus in soil are generally small, and this often limits plant growth. That is why people often apply phosphate fertilisers on farmland. Animals absorb phosphates by eating plants or plant-eating animals.

The role of phosphorus in animals and plants

Phosphorus is an essential nutrient for animals and plants. It plays a critical role in cell development and is a key component of molecules that store energy, such as ATP (adenosine triphosphate), DNA and lipids (fats and oils). Insufficient phosphorus in the soil can result in a decreased crop yield.

The phosphorus cycle

Phosphorus moves in a cycle through rocks, water, soil and sediments and organisms.

Here are the key steps of the phosphorus cycle

Over time, rain and weathering cause rocks to release phosphate ions and other minerals. This inorganic phosphate is then distributed in soils and water.

Plants take up inorganic phosphate from the soil. The plants may then be consumed by animals. Once in the plant or animal, the phosphate is incorporated into organic molecules such as DNA. When the plant or animal dies, it decays, and the organic phosphate is returned to the soil.

Within the soil, organic forms of phosphate can be made available to plants by bacteria that break down organic matter to inorganic forms of phosphorus. This process is known as mineralisation.

Phosphorus in soil can end up in waterways and eventually oceans. Once there, it can be incorporated into sediments over time.

Most phosphorus is unavailable to plants

Since most of our phosphorus is locked up in sediments and rocks, it's not available for plants to use. A lot of the phosphorus in soils is also not available to plants.

The availability of phosphorus in soil to plants depends of several reversible pathways:

Bacteria: Bacteria convert plant-available phosphate into organic forms that are then not available to plants. Although other bacteria make phosphate available by mineralisation, the contribution of this is small.

Adsorption: Inorganic (and available) phosphorus can be chemically bound (adsorbed) to soil particles, making it unavailable to plants. Desorption is the release of adsorbed phosphorus from its bound state into soil solution.

pH: Inorganic phosphorus compounds need to be soluble to be taken up by plants. This depends on the acidity (pH) of the soil. If soils are less than pH 4 or greater than pH 8, the phosphorus starts to become tied up with other compounds, making it less available to plants.

Many plant crops need more phosphorus than is dissolved in the soil to grow optimally. In addition, crops are usually harvested and removed - leaving no decaying vegetation to replace phosphorus. Therefore, farmers replenish the phosphorus 'pool' by adding fertilisers or effluent to replace the phosphorus taken up by plants.

Phosphate fertilisers replenish soil phosphorus

Many farmers replenish phosphorus through the use of phosphate fertilisers. The phosphorus is obtained by mining deposits of rock phosphate. Locally produced sulfuric acid is used to convert the insoluble rock phosphate into a more soluble and usable form - a fertiliser product called superphosphate.

In New Zealand, superphosphate is made using rock imported mainly from Morocco.

Adjusting the pH of the soil for efficient plant uptake of phosphate should be done prior to fertilisation. For example, adding lime reduces soil acidity, which provides an environment where phosphate becomes more available to plants.

Water pollution by fertilisers

When fields are overfertilised (through commercial fertilisers or manure), phosphate not utilised by plants can be lost from the soil through leaching and water run-off. This phosphate ends up in waterways, lakes and estuaries. Excess phosphate causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication.

Steps are being taken in agriculture to reduce phosphate losses in order to maximise the efficiency of fertiliser and effluent applications.

Nature of science

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Phosphorus is a chemical element found on Earth in numerous compound forms, such as the phosphate ion (PO43-), located in water, soil and sediments. The quantities of phosphorus in soil are generally small, and this often limits plant growth. That is why people often apply phosphate fertilisers on farmland.

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How are fertilizers contributing to dead zones?

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Brent Hughes

Nutrient runoff and the "dead zones" they create are a growing problem around the world. But what's their impact on fish?

A new study published in the journal Proceedings of the National Academy of Sciences suggests that this pollution reduces fish populations by disrupting the near-shore areas where the animals spend their fishy youths.

Marine ecologist Brent Hughes at the University of California-Santa Cruz and colleagues spent years tracking the health of a fish called English sole, in a salt marsh called Elkhorn Slough. It's found in Monterey Bay, which lies on the Pacific Coast, south of Santa Cruz. He discovered that for the most part, populations of these (and other fish) suffered when nutrient levels were high. This was accompanied by declines in the number of fish that grew large enough to move offshore and into the open ocean.

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When nutrients found in fertilizers and sewage, specifically nitrogen and phosphorous, accumulate in the water, this leads to the development of algae blooms, which turn the water green and die shortly thereafter. As this material decomposes, it consumes oxygen, creating a so-called " dead zone " in the water, Hughes explains. This can kill fish or force them to move elsewhere, although the extent of this effect isn't always easy to quantify, he says.

Perhaps counterintuitively, large rainfalls can help reduce the problems caused by nutrient runoff, by flushing all the material further out to sea, Hughes notes. For that reason, El Niño may be good for young fish in California's estuaries. When El Niño conditions are in place, water temperatures in the tropical Pacific are above average, which has wide-ranging effects on climate around the globe, including more precipitation in parts of the Golden State.

But continuing agricultural land development and urbanization creates more nutrient pollution, leading to dead zones that are becoming more common every year, which is bad news for fish, Hughes says. There are steps humans could take to limit nutrient runoff -such as using less fertilizer in agriculture, developing better sewage treatment plants and planting trees along drainage ditches and water bodies-and, thereby help fish populations, the study suggests.

See all of the best photos of the week in these slideshows

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Fertilizer 'Dead Zones' Are Killing Fish

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Nutrient runoff and the "dead zones" they create are a growing problem around the world. But what's their impact on fish? A new study published in the journal Proceedings of the National Academy of Sciences suggests that this pollution reduces fish populations by disrupting the near-shore areas where the animals spend their fishy youths.

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Is Huntington Beach going to be a dead zone?

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Is Huntington Beach going to be a dead zone?

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Why we need carbon in our bodies!

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If you said The Carbon Cycle, then you are on the right track. If you came up with something more creative, then you are doing better than I am today! Your human body contains 18 percent carbon. For a plant to form simple sugars (carbohydrates) it needs carbon dioxide and water to do so.

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Fossil fuels, such as coal, oil and natural gas, are created when organic matter decays and becomes compressed beneath layer upon layer of sand, earth, rock and ocean. Interestingly, the name "fossil fuel" is actually derived from the word "fossil": the mineralised remains of ancient creatures which once populated the earth. Burning fossil fuels yields carbon dioxide, water and energy; the process of burning fossil fuels is known as "combustion".

The combustion reaction

During chemical reactions, energy is either absorbed into the environment (endothermic reaction) or released into the environment (exothermic reaction), and chemical bonds are broken and formed. Bond making is an exothermic process, while bond breaking is an endothermic process. Combustion reactions require oxygen. Fossil fuels are composed primarily of hydrocarbons, which are converted into carbon dioxide and water during a combustion reaction.

Of the three fossil fuels known to us, natural gas is the cleanest burning. Coal and oil are more chemically complicated, and release a cocktail of potentially harmful substances into the atmosphere when combusted. In order to detect SO2 in combustion gases, florescence - induced by ultra violet light - is used. We explore further in this article: Interference-free Sulphur Analysis in Fuels.

The energy content of coal

Coal is composed primarily of carbon, water, some hydrogen and oxygen (although certain types of coal also contain small quantities of nitrogen, sulphur and a handful of other substances). The majority of the carbon found in coal is bound, meaning there is one C-C bond to every C atom. Energy release occurs when a C-C bond is broken. Roughly 43g of carbon dioxide is produced to every 23g of C burnt. If insufficient amounts of oxygen are available during combustion, carbon monoxide - a colourless, odourless and extremely dangerous gas - is produced.

Energy use, efficiency and the future

Fossil fuels represent an increasingly scarce commodity. Recently, the UN-backed Intergovernmental Panel on Climate Change (IPCC) stated that the unrestricted use of fossil fuels must be phased out in order to avoid catastrophic climate change. You can read more about this event in: How Much Longer Can We Depend on Fossil Fuels?

In addition to this, the burning of fossil fuels has been linked to global warming - a rise in the average temperature of the earth's climate system. While historical, political and geographical contexts have led to the adoption of individual fuels and technologies across the globe, it is thought that 95% of the world's energy requirement is currently met by fossil fuels.

As prices and temperatures rise, investment in greener energy sources is increasing. However, even renewable energy sources have environmental costs. Wind power, for example, requires a great deal of land, creates a great deal of noise, and can impact upon birdlife in the surrounding areas, while hydroelectricity has been linked to the destruction of farmland, dislocation of people and loss of habitat.

In the future, new technologies may provide us with answers to the questions we face currently. In the meantime, we must begin to think hard about how the energy we use every day is produced and whether we value it highly enough.

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What Happens During the Combustion of Fossil Fuels?

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Fossil fuels, such as coal, oil and natural gas, are created when organic matter decays and becomes compressed beneath layer upon layer of sand, earth, rock and ocean. Interestingly, the name "fossil fuel" is actually derived from the word "fossil": the mineralised remains of ancient creatures which once populated the earth.

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What is acid rain? Click here to see the link.

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Sources of Acid Rain
Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain. Sulfur dioxide and nitrogen oxides dissolve very easily in water and can be carried very far by the wind. As a result, the two compounds can travel long distances where they become part of the rain, sleet, snow, and fog that we experience on certain days.

Human activities are the main cause of acid rain. Over the past few decades, humans have released so many different chemicals into the air that they have changed the mix of gases in the atmosphere. Power plants release the majority of sulfur dioxide and much of the nitrogen oxides when they burn fossil fuels, such as coal, to produce electricity. In addition, the exhaust from cars, trucks, and buses releases nitrogen oxides and sulfur dioxide into the air. These pollutants cause acid rain.

Acid Rain is Caused by Reactions in the Environment
Nature depends on balance, and although some rain is naturally acidic, with a pH level of around 5.0, human activities have made it worse. Normal precipitation-such as rain, sleet, or snow-reacts with alkaline chemicals, or non-acidic materials, that can be found in air, soils, bedrock, lakes, and streams. These reactions usually neutralize natural acids. However, if precipitation becomes too acidic, these materials may not be able to neutralize all of the acids. Over time, these neutralizing materials can be washed away by acid rain. Damage to crops, trees, lakes, rivers, and animals can result.

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Acid Rain Students Site: What causes acid rain?

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Sources of Acid Rain Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

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You may have heard the words "phosphorus" and "phosphate." But you may be wondering what these terms mean and what they have to do with your diet.

Phosphorus is a mineral found in many foods like beer, cheese, beans, and fish. It is also one of the most common substances in your everyday environment and in your body. It plays an important role in the health of your kidneys, bones, muscles, and blood vessels, as well as each cell in your body.

Phosphates are a form of phosphorus. You can find phosphates in dish detergents, baking ingredients, and processed cheeses. It's also the drug form of phosphorus, and can be taken as a dietary supplement when you can't naturally get enough phosphorus. Certain illnesses and diseases can prevent you from getting enough phosphorus.

Phosphorus works with calcium to help build bones. You need the right amount of both calcium and phosphorus for bone health. Phosphorus also plays an important structural role in nucleic acids and cell membranes. And it's involved in the body's energy production.

Your body absorbs less phosphorus when calcium levels are too high, and vice versa. You also need vitamin D to absorb phosphorus properly.

Insufficient phosphorus is referred to as hypophosphatemia. Hypophosphatemia occurs when phosphorus levels in your blood get too low. This causes your energy levels to drop. It can also cause muscle weakness, fatigue, and a low tolerance for exercise.

Insufficient phosphorus coinciding with low levels of calcium and vitamin D can lead to weaker, softer bones over long periods of time. This causes joint and muscle pain.

Phosphorus levels are tightly controlled in the body. And low levels can be a sign of another disorder.

A high level of phosphates in the blood is called hyperphosphatemia. Some of this condition's symptoms overlap with those of low phosphorus. These symptoms include joint pain, muscle pain, and muscle weakness. People with high phosphorus levels can also experience itching and red eyes. Symptoms of more severe cases of high phosphorus may include severe constipation, nausea, vomiting, and diarrhea.

Your doctor can help determine the cause of high or low phosphorus levels in your body, as well as your best course of treatment.

Doctors may prescribe phosphate binders in order to lower phosphorus levels. These are taken with meals and snacks and limit the amount of phosphorus that your body can absorb.

Lifestyle choices can help balance out high levels of phosphorus. Be sure to get enough exercise, stay well-hydrated, and eat a balanced, low-phosphorus diet.

Finding foods with high phosphorus levels isn't hard. Pork, cod, salmon, and tuna are all high in phosphorus. Good dairy sources include:

Bran cereal, blueberry muffins, and nachos are also high in phosphorus.

Almost all fresh fruits and vegetables are low in phosphorus. These foods are also an important part of your daily diet. Other foods that have very low levels of phosphorus include:

Your kidneys help remove excess phosphorus from your blood, keeping your levels balanced and normal. Your phosphorus levels can get too high when your kidneys aren't working correctly. This can also cause your calcium levels to rise.

Dangerous calcium deposits can potentially form in your heart, blood vessels, eyes, and lungs. People with kidney disease may need dialysis to help their kidneys clean out the excess phosphorus.

Phosphorus supplements and medications can be used in many ways. They have been proven to work as laxatives, as a treatment for urinary tract infections and urinary stones, and for treating low levels of the mineral in people with advanced malnutrition.

Phosphorus supplements might also help with bone restoration and treating excessive levels of calcium in the body due to overactive parathyroid glands.

Phosphorus is integral to healthy bones and a healthy body. Sometimes health conditions such as kidney disease can affect the levels of phosphorus in the body. Talk to your healthcare provider if you feel any symptoms of high or low phosphorus.

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You may have heard the words "phosphorus" and "phosphate." But you may be wondering what these terms mean and what they have to do with your diet. Phosphorus is a mineral found in many foods like beer, cheese, beans, and fish. It is also one of the most common substances in your everyday environment and in your body.

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An article that explains why the body needs Nitrogen.

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Lexa W. Lee

Lexa W. Lee is a New Orleans-based writer with more than 20 years of experience. She has contributed to "Central Nervous System News" and the "Journal of Naturopathic Medicine," as well as several online publications. Lee holds a Bachelor of Science in biology from Reed College, a naturopathic medical degree from the National College of Naturopathic Medicine and served as a postdoctoral researcher in immunology.

Your body needs nitrogen to make proteins in your muscles, skin, blood, hair, nails and DNA. You obtain nitrogen from protein-containing foods in your diet, according to the Royal Society of Chemistry. These foods include meat, fish, legumes, nuts, eggs, milk and other dairy products.

Video of the Day

Your body needs the nitrogen in amino acids from protein foods to make other amino acids it uses to synthesize human proteins, according to Virtual Chembook at Elmhurst College. Not only do your various tissues contain protein, your metabolic processes depend upon enzymes, all of which consist of various kinds of proteins. The nucleic acid DNA, which makes up your genes, and RNA, which is involved in protein synthesis, also contain nitrogen.

Normal growth, cell replacement and tissue repair all require nitrogen for production of new cells. Although nitrogen is abundant in the environment, humans cannot directly use it from the air or soil, but instead depend on microbes and green plants to convert it into form our bodies can use. Your body is constantly recycling nitrogen from amino acids. If amino acids are not used for protein synthesis, they can be broken into components, including nitrogen, to produce energy. Nitrogen can also be used to make other types of compounds that aren't proteins, such as the heme in hemoglobin, which carries oxygen in red blood cells.

A healthy adult male needs about 105 milligrams of nitrogen per kilogram, or per 2.2 pounds per day. About 0.83 gram of protein per kilogram per day is considered sufficient to cover nitrogen requirements, according to the International Dairy Foundation. That means a 220-pound man would need 83 grams, or about 3 ounces, of protein a day to supply his nitrogen needs. The breakdown of protein results in ammonia, a nitrogen-containing byproduct that your body eliminates.

Your body eliminates ammonia by converting it to urea, which your kidneys then excrete in urine. In this way, nitrogen is returned to the environment. Unlike populations in underdeveloped countries, Americans do not commonly suffer from a lack of dietary nitrogen unless they are on extreme diets that do not contain sufficient protein. Symptoms of deficiency include hair loss, delayed wound healing, muscle weakness and wasting, brittle hair and hair loss.

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Your body needs nitrogen to make proteins in your muscles, skin, blood, hair, nails and DNA. You obtain nitrogen from protein-containing foods in your diet, according to the Royal Society of Chemistry. These foods include meat, fish, legumes, nuts, eggs, milk and other dairy products.

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What is a carbon sink?

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Trees are excellent carbon sinks

Credit: Stefan Schnitzer.

You won't find it in your kitchen or bathroom: Carbon sinks are natural systems that suck up and store carbon dioxide from the atmosphere.

The main natural carbon sinks are plants, the ocean and soil. Plants grab carbon dioxide from the atmosphere to use in photosynthesis; some of this carbon is transferred to soil as plants die and decompose. The oceans are a major carbon storage system for carbon dioxide. Marine animals also take up the gas for photosynthesis, while some carbon dioxide simply dissolves in the seawater.

"Combined, the Earth's land and ocean sinks absorb about half of all carbon dioxide emissions from human activities," said Paul Fraser of the Commonwealth Scientific and Industrial Research Organization.

But these sinks, critical in the effort to soak up some of our greenhouse gas emissions, may be stopping up, thanks to deforestation, and human-induced weather changes that are causing the oceanic carbon dioxide "sponge" to weaken, a new study led by Fraser and detailed in the May 18 issue of the journal Science found.

Scientists are looking for ways to help nature along by devising ways to artificially sequester, or store, carbon dioxide underground.

Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.

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You won't find it in your kitchen or bathroom: Carbon sinks are natural systems that suck up and store carbon dioxide from the atmosphere. The main natural carbon sinks are plants, the ocean and soil. Plants grab carbon dioxide from the atmosphere to use in photosynthesis; some of this carbon is transferred to soil as plants die and decompose.

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What is it?

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The Nitrogen Cycle | Earth Science | Visionlearning

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This module provides an overview of the nitrogen cycle and the chemical changes that govern the cycle.

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This may only work on chromebooks but you cant try it on your phone!

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The Nitrogen Cycle

This interactive activity adapted from the University of Alberta illustrates how, through a process called fixation, nitrogen flows from the atmosphere, into the soil, through various organisms, and back to the atmosphere in a continuous cycle.

This media asset was adapted from the University of Alberta.

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This interactive activity adapted from the University of Alberta illustrates how, through a process called fixation, nitrogen flows from the atmosphere, into the soil, through various organisms, and back to the atmosphere in a continuous cycle. This media asset was adapted from the University of Alberta.

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What is it?

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Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both.

Eutrophication can be a natural process in lakes, occurring as they age through geological time.

Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the mid-20th century.

Human activities can accelerate the rate at which nutrients enter ecosystems.

Runoff from agriculture and development, pollution from septic systems and sewers, and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems (including coral reefs).

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ecology; ecosystems; pollution; sustainability; acid rain; air quality

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Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both. Eutrophication can be a natural process in lakes, occurring as they age through geological time. Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the mid-20th century.

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More on algal blooms

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Algae are photosynthetic microorganisms that are found in most habitats. Algae vary from small, single-celled forms to complex multi-cellular forms.

An algal bloom is a rapid increase in the density of algae in an aquatic system. Algal blooms sometimes are natural phenomena, but their frequency, duration and intensity are increased by nutrient pollution. Algae can multiply quickly in waterways with an overabundance of nitrogen and phosphorus, particularly when the water is warm and the weather is calm. This proliferation causes blooms of algae that turn the water noticeably green, although other colors can occur. Some species of algae grow in clumps covered in a gelatinous coating and have the capability to float, allowing cells to stick together into large surface scums in calm weather.

Other algae form thick mats that float on or just below the surface along the shoreline. In the St. Johns River, blooms most often are composed of cyanobacteria, also known as blue-green algae. Cyanobacteria are bacteria that, like plants, use solar energy and carbon dioxide to grow (photosynthesis). Cyanobacteria occur naturally in both freshwater and marine (salt) water bodies. Blooms also can be caused by dinoflagellates (single cell microorganisms [phytoflagellates] that include luminescent forms) in marine or estuarine water bodies (red tides are an example of a dinoflagellate bloom).

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Algae are photosynthetic microorganisms that are found in most habitats. Algae vary from small, single-celled forms to complex multi-cellular forms. An algal bloom is a rapid increase in the density of algae in an aquatic system. Algal blooms sometimes are natural phenomena, but their frequency, duration and intensity are increased by nutrient pollution.

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What are they?

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harmful algal bloom and hypoxia amendments act of 2004

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national centers for coastal ocean science

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national oceanic and atmospheric administration

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Ranging from microscopic, single-celled organisms to large seaweeds, algae are simple plants that form the base of food webs. Sometimes, however, their roles are much more sinister. A small percentage of algal species produce toxins that can kill fish, mammals, and birds, and may cause human illness.

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Congratulate yourself for wanting to learn more about the main cause of global warming. Not only does our species create huge problems that are beyond the ability of other animals to create, but we have the fortunate ability to analyze them and work to bring an end to their consequences. You are now actively engaged in that process!

Humans cause global warming with our unique ability to capture and use energy from stored energy sources. No other animal can start fires or build gasoline engines.

Consider your home and the environment in which it is situated. The very fact that you live in one place, something that was quite uncommon before the discovery of stored energy sources like fossil fuels, speaks to how your life is facilitated by these unique energy sources.

Even your basic needs are dependent on fossil fuels. Your water, food, and fuel for heating and cooking likely travel hundreds if not thousands of miles to your home.

This pattern couldn't be sustained if humans didn't have the ability to exploit dense energy sources such as coal, oil, and natural gas. However, their discovery transformed the face of the earth and the nature of humans.

The effects on earth are dangerous. In the last 150 years, the human population has grown by more than five times and the average person uses more energy than ever. This energy comes largely from fossil fuels, the use of which is a direct and important cause of global warming.

Global Warming is a Growing Problem

The growth of the human population and the growth of consumption adds up to an ever-escalating carbon output. You've probably heard a lot of talk about global warming, but if we want to decrease global warming, then the amount of carbon released each year by humans must be drastically reduced.

Human Consumption or Human Population?

Two factors are acting as catalysts of increased carbon levels in our atmosphere:

Increasing consumption among the wealthy (that means you!)

Increasing population among the poor

You may wonder which is the primary culprit. The answer is clear - both! Consider the following:

Americans consume 300 - 600 times as much as people in very poor nations, in some cases.

70 million people are added to the world's population each year, many of whom aspire to live "better" lives, to consume like wealthy people.

From this it becomes clear why humans are the main cause of global warming. However, it is even more important for you to understand that you have the power to help prevent global warming from getting worse - live The Green Life.

Related Stories:
1. 300 Years of Fossil Fuels in 300 Seconds [VIDEO]
2. Humans Cause Global Warming: 10 Indicators
3. 119 One-Liners to Respond to Climate Science Myths
4. Going Green Tips

Image Credit: AZRainman

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Congratulate yourself for wanting to learn more about the main cause of global warming. Not only does our species create huge problems that are beyond the ability of other animals to create, but we have the fortunate ability to analyze them and work to bring an end to their consequences.

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Calculate your carbon footprint

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What's your carbon footprint? Use The Nature Conservancy's carbon footprint calculator to measure your impact on our climate.

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This is a video from 2011.

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a repost - original at http://climate.nasa.gov/warmingworld/

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What happens if we (The US ) begin using nuclear weapons?

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A nuclear weapon is a certain thing-atomic or hydrogen, fission or fusion, bomb or missile, so many megatons-but nothing could be more uncertain than the consequences of using one. Nine nations have nuclear weapons; only the United States has ever used one, and that was in 1945. Our nuclear-weapons policy rests on a seven-decade-long history of events that have never happened: acts of aggression that were not committed, wars that were not waged, an apocalypse that has not come to pass. Strategists attribute the non-occurrence-the deterrence-of these events to the weapons themselves, to bombs on airplanes, missiles in silos, launchers on submarines. The power of deterrence, however, is a claim that cannot be proved. If, while a police car is parked in front of your house, your house is not robbed, you might suspect that a robbery would have taken place had the police not been there, but you can't know that for sure. Nuclear-weapons policy is a body of speculation that relies on fearful acts of faith. Doctrinally, it has something in common with a belief in Hell.

This belief is about to be tested. The United States and its only nuclear rival have been reducing their arsenals since the end of the Cold War. In 1985, the United States and the Soviet Union held a combined stockpile of more than sixty thousand warheads; today, the U.S. and Russia have fewer than fifteen thousand between them. Dangers remain: a computer error, a malfunction in a silo, a rogue state, nuclear terrorism. In 2007, the former Secretaries of State Henry Kissinger and George P. Shultz, the former Defense Secretary William J. Perry, and the former senator Sam Nunn warned in an essay published in the Wall Street Journal that "the world is now on the precipice of a new and dangerous nuclear era . . . that will be more precarious, psychologically disorienting, and economically even more costly than was Cold War deterrence." The time had come, they argued, for the eradication of nuclear weapons. Global Zero, an international nuclear-abolition organization, was formed the next year. In 2009, in a speech in Prague, Barack Obama pledged "America's commitment to seek the peace and security of a world without nuclear weapons." That speech helped earn him the Nobel Peace Prize. The reality did not match the rhetoric. Congress exempted nuclear weapons from mandatory cuts in the military budget, and, in exchange for support for the New START treaty, which reduced deployed warheads by two-thirds, the President pledged eighty-five billion dollars to modernize the nation's aging nuclear arsenal. "I think we can safely say the President's Prague vision is dead," the Alabama congressman Mike Rogers said in 2015, at a weapons conference sponsored by Lockheed Martin, General Dynamics, and Northrop Grumman. "And I'll leave it to the Nobel committee to ask for its prize back."

The new President's vision is unclear. "We have to be extremely vigilant and extremely careful when it comes to nuclear," Donald Trump said during the campaign. But he also refused to rule out conducting a first strike, even on Europe ("Europe is a big place"); suggested that it might be a good thing for more countries to acquire nuclear weapons; and argued that it was pointless to manufacture weapons that could never be used, asking, "Then why are we making them?" In December, Vladimir Putin told military leaders in Moscow that he intended to bolster Russia's nuclear arsenal. "Let it be an arms race," Trump said in response. "We will outmatch them at every pass and outlast them all."

Trump has often contradicted himself on the subject of nuclear weapons, but one of the more interesting things he's said about them is that they are far more dangerous to the planet than global warming is. It's a revealing comparison. The damage from a nuclear explosion does not respect national boundaries, and this adjustment in scale, from the national to the global, was the key argument put forward by advocates of disarmament. That argument has been won: Trump's tweets aside, there is a bipartisan consensus in favor of significant arms reductions. Bipartisan agreement about the future of the planet falls apart not over the bomb but over the climate. Historically, though, they're inseparable: the weapons and the weather are twisted together, a wire across time, the long fuse to an ongoing debate about the credibility of science, the fate of the Earth, and the nature of uncertainty.

In 1981, when Jimmy Carter delivered his farewell address, part of it was written by Carl Sagan. The Senate had proved unwilling to ratify a treaty that had come out of a second round of Strategic Arms Limitation Talks; Carter wanted to take a moment to reckon with that loss, for the sake of the planet. He turned to Sagan, whose thirteen-part documentary, "Cosmos," first broadcast in 1980, was the most-watched PBS series ever. "Nuclear weapons are an expression of one side of our human character," Carter said, in words written by Sagan. "But there's another side. The same rocket technology that delivers nuclear warheads has also taken us peacefully into space. From that perspective, we see our Earth as it really is-a small and fragile and beautiful blue globe, the only home we have."

Sagan was an astronomer, but early in his career he'd worked* on a classified nuclear-weapons project. This was not unusual. Since the Second World War, the military has funded the preponderance of research in the field of physics, and, as historians have now established, a close second was its funding of the earth sciences. Although the environmental movement may not have started until the nineteen-sixties, the research that lies behind it began in the fifties, in the U.S. military. Indeed, the very term "environmental science" was coined in the fifties by military scientists; it was another decade before civilian scientists used the term.

Beginning on the day black rain fell on Hiroshima, nuclear weapons shaped environmental science. In 1949, the U.S. Weather Bureau launched Project Gabriel, a classified meteorological study of weapons and weather. The next year, the Department of Defense, in a study titled "The Effects of Atomic Weapons," coined the word "fallout." Researchers considered making the quantity, spread, and duration of fallout the standard measure of the force of a nuclear explosion, but found that approach to be too dependent on the weather. (Instead, they chose blast radius.) They measured and modelled the best weather conditions for explosions and the effects of those explosions on the natural world; they invented and refined tools to detect atmospheric weapons tests conducted by the Soviets; and they investigated the possibility of using nuclear weapons to alter the weather and even the climate of adversaries. Sagan, after finishing his Ph.D. at the University of Chicago, in 1960, worked on a secret military project code-named A119, which had begun in 1958, a year after Sputnik. Sagan was charged with calculating "the expansion of an exploding gas/dust cloud rarifying into the space around the Moon." The idea was to assess whether a mushroom cloud would be visible from Earth, and therefore able to serve as an illustration of the United States' military might.

Government-funded environmental scientists began noticing something curious: nuclear explosions deplete the ozone layer, which protects the Earth's atmosphere. This finding related to observations made by scientists who were not working for the military. In the wake of Rachel Carson's "Silent Spring," published in 1962, the U.S. government formed a number of advisory and oversight organizations, including the Environmental Pollution Panel of the President's Science Advisory Committee. The panel's 1965 report, "Restoring the Quality of Our Environment," included an appendix on "Atmospheric Carbon Dioxide," laying out, with much alarm, the consequences of "the invisible pollutant" for the planet as a whole. In 1968, S. Fred Singer, an atmospheric physicist who had worked on satellites and was now a Deputy Assistant Secretary of the Interior, organized a symposium on "Global Effects of Environmental Pollution." Four papers were presented at a panel on "Effects of Atmospheric Pollution on Climate."

Changing weapons policy opened new avenues of research. In 1963, the U.S. and the U.S.S.R. signed the Limited Test Ban Treaty, an agreement to stop testing nuclear weapons in the atmosphere. Using longitudinal data to study the ozone both before and after the test ban, the Berkeley chemist Harold Johnston found that stopping the testing had slowed the depletion. Research into the environmental consequences of nuclear explosions and of other kinds of pollution shared a planetary perspective, a vantage greatly enhanced by the space program; gradually, the meaning of the word "environment" changed from "habitat" to "planet." The first photograph of the whole Earth was taken in 1972, by the crew of Apollo 17. It became an icon of the environmental movement. It also shaped arguments about arms control.

Nuclear-weapons research was usually classified; other environmental research was not. During the nineteen-seventies, military-funded environmental scientists continued their top-secret research into the environmental effects of nuclear weapons. Given the test ban, these studies relied less on experiments on Earth than on computer models and on empirical findings involving dust on other planets, most notably Mars. Meanwhile, some environmental scientists pursued-and published-research on how chlorofluorocarbons, the exhaust from jet engines, and fossil-fuel consumption affected the ozone layer; this research demonstrated, crucially, that even tiny amounts of certain chemicals could catalyze dramatic changes, with planetary consequences. In 1974, the director of the U.S. Arms Control and Disarmament Agency asked the National Academy of Sciences to prepare a report on the effects of nuclear explosions on the ozone. That report, "Long-Term Worldwide Effects of Multiple Nuclear Weapons Detonations," married the logic of nuclear deterrence to the logic of environmental protection, or what might have been called pollution deterrence. Stephen Schneider, a climatologist at the National Center for Atmospheric Research, speculated that the fallout from a nuclear war might make the world colder by blocking sunlight, and that the diminished industrial production in a postwar world could change the climate, too.

Sagan had by this time become an advocate of nuclear disarmament, a cause that gained a great deal of momentum early in 1982, when The New Yorker published a four-part series by Jonathan Schell called "The Fate of the Earth," which did for nuclear weapons what Carson had done for chemical pollution: freaked everyone out. That fall, Representative Al Gore, the chair of the Subcommittee on Investigations and Oversight of the House Committee on Science and Technology, convened hearings into "The Consequences of Nuclear War on the Global Environment." The consequences of nuclear war on the environment, like its consequences on the balance of power, were difficult to prove; most data came from computer models, and from research on other planets. A battle began between those who were willing to place their faith in the speculations of military strategists and those who were willing to place their faith in the speculations of environmental scientists.

At the center of that battle was a plan to build a defensive missile shield: weapons that would orbit the planet. On March 23, 1983, President Ronald Reagan announced the Strategic Defense Initiative, in what came to be called his "Star Wars" speech: "I call upon the scientific community in our country, those who gave us nuclear weapons, to turn their great talents now to the cause of mankind and world peace: to give us the means of rendering these nuclear weapons impotent and obsolete." To its many critics, S.D.I. undermined the nuclear paralysis that had set in with the idea of mutual assured destruction: neither the U.S. nor the U.S.S.R. would launch a missile, the theory went, since everyone would end up dead. But, if the U.S. could defend itself against a missile attack, M.A.D. no longer applied. The Union of Concerned Scientists prepared a hundred-and-six-page report opposing the project. Sagan, who was in intensive care after a ten-hour esophageal procedure, dictated a letter of objection from his hospital bed.*

What Sagan did next is recounted in a new book by Paul Rubinson, "Redefining Science," a history of science in a national-security state. The story of Sagan's campaign against S.D.I., though little remembered, has been told before, in Lawrence Badash's 2009 book, "A Nuclear Winter's Tale"; in "Merchants of Doubt," by Naomi Oreskes and Erik M. Conway, in 2009; and, most richly, in a terrific 2011 journal article by Matthias Dorries. Sagan launched a campaign to warn the world about "nuclear winter"; the very term, as Dorries points out, brought together the weapons and the weather.

Sagan collaborated with four scientists. James Pollack, his first graduate student, had written a dissertation, in 1965, on the greenhouse effect on Venus. Thomas Ackerman had attended Calvin College, a Christian school, as an undergraduate, earned a Ph.D. at the University of Washington, and then worked with Pollack at NASA. (Later, he declined an invitation to work on S.D.I., citing his moral objections as a Christian.) Richard Turco, an atmospheric scientist, was the co-author of a study called "Possible Ozone Depletions Following Nuclear Explosions," which was published in Nature in 1975, the year that Owen Brian Toon, a graduate student at Cornell, submitted to Sagan a dissertation called "Climatic Change on Mars and Earth." Sagan decided to use his celebrity to bring the research of these scientists to the broadest possible public audience, as fast as the scientific method allowed-or maybe faster.

"To the village square we must carry the facts of atomic energy," Albert Einstein said in 1946. "From there must come America's voice." Sagan, in his understanding of the role of science in a democracy, had Einstein behind him, but, more, he had John Dewey, along with a generation of Progressive engineers, New Deal reformers, and Manhattan Project-era atomic scientists. In 1946, the Federation of Atomic Scientists, which had been founded to advocate for international, civilian control of atomic energy, had established the National Committee on Atomic Information. Atomic scientists organized a speakers' bureau: they spoke at Kiwanis clubs, at churches and synagogues, at schools and libraries. In Kansas alone, eight Atomic Age Conferences were held. The F.B.I. launched an investigation. In 1948, the head of the National Committee on Atomic Information, suspected of being a Communist, was fired. His entire staff resigned in protest, and the committee disbanded. The next year, the Soviet Union tested its first nuclear weapon. Public-spirited science yielded to the demands of a national-security state.

Sagan received his training in that world. But that world did not survive Vietnam, or the Love Canal disaster. This much Sagan understood. But what he could not have fully understood were two forces that had gained strength in the nineteen-seventies, both of which were at odds with his neo-Deweyism: a postmodern critique of objectivity, fashionable among intellectuals, artists, and writers; and a conservative movement determined to expose the liberal bent of the academy and of the press. Sagan waded into these waters early in 1983, with a paper he prepared with Turco, Toon, Ackerman, and Pollack. The paper, known by its authorial acronym, TTAPS, used meteorological models derived from the study of volcanoes to calculate the effects on light and temperature of different kinds and numbers of nuclear explosions, factoring in the dust, smoke, and soot produced by the burning of cities; some of the data came from Mars. Moving beyond Schell, whose essays had predicted the end of humanity, TTAPS forecast a nuclear winter that might result in the end of all life on the planet.

Sagan circulated a draft to fifty scientists, then convened a conference at the American Academy of Arts and Sciences in April, 1983, which was attended by twice as many. Meanwhile, he sent George F. Kennan a draft of an essay he'd written for Foreign Affairs. "I cannot tell you what a great thing I feel you have done," Kennan wrote in reply, thanking him for providing a "clear and irrefutable demonstration of the enormity of the danger presented by these vast nuclear arsenals." Next, Sagan and an assortment of colleagues submitted two papers to the peer-reviewed journal Science and planned another conference, to be held at a Sheraton in Washington, with five hundred participants, a hundred members of the press, and a live "Moscow Link." The day before the conference, Sagan published an article about nuclear winter in Parade. Using only the worst-case numbers, he admitted no room for doubt about what was, after all, a theory, presenting nuclear winter as the consensus of more than a hundred scientists from around the world.

Two charges were levelled at Sagan: that he shouldn't be writing for a Sunday newspaper supplement, and that he'd exaggerated the certainty of an untested theory. "In the scientific community you don't publish first results in Parade magazine," George Rathjens, of M.I.T., wrote. (Unknown to Sagan's critics, the two Science papers had already been accepted for publication when the Parade article appeared.) In the Wall Street Journal, S. Fred Singer, at that time a fellow at the Heritage Foundation and later a consultant for the tobacco industry, argued that the theory relied too heavily on predictive models. The physicist and S.D.I. enthusiast Edward Teller wrote Sagan, privately, "My concern is that many uncertainties remain and that these uncertainties are sufficiently large as to cast doubt on whether the nuclear winter will actually occur." He added, "I can compliment you on being, indeed, an excellent propagandist-remembering that a propagandist is the better the less he appears to be one." In Nature, Teller attacked the theory ("A severe climatic change must be considered dubious rather than robust"), and offered this sermon: "Highly speculative theories of worldwide destruction-even the end of life on Earth-used as a call for a particular kind of political action serve neither the good reputation of science nor dispassionate political thought." That S.D.I. rested on highly speculative theories of worldwide destruction was not lost on Teller's many critics.

Sagan's grandiosity helped him gain a vast popular audience; it also hurt his cause. So did his partisanship: he declined an invitation to dine with the Reagans at the White House. His celebrity knew no bounds. The Pope asked him for an audience. Talking about warheads seemed like a fabulous way to be famous. "This morning, Trump has a new idea," Lois Romano wrote in a Washington Post profile of Donald Trump in November, 1984, the week after Reagan defeated Mondale. "He wants to talk about the threat of nuclear war. He wants to talk about how the United States should negotiate with the Soviets. He wants to be the negotiator." He knew just how to do it. "It would take an hour-and-a-half to learn everything there is to learn about missiles," Trump told Romano. "I think I know most of it anyway."

Sagan was widely resented, and he made some poor decisions, but he was a serious scientist. Despite a number of adjustments-Stephen Schneider ran his own numbers and determined that the likeliest consequence of nuclear war was something more like a nuclear autumn-the theory gained widespread scientific acceptance. Declassified documents demonstrate that Navy scientists, for instance, were persuaded by TTAPS. In May, 1984, William Cohen, a Republican senator from Maine, wrote to Reagan about the paper and suggested that the Administration conduct a study. The House Republicans Jim Leach and Newt Gingrich joined their Democratic colleagues Tim Wirth, Buddy Roemer, and Al Gore in proposing a budget amendment mandating a "comprehensive study of the atmospheric, climatic, environmental, and biological consequences of nuclear war and the implications that such consequences have for the nuclear weapons strategy and policy, the arms control policy, and the civil defense policy of the United States."

Hearings on nuclear winter were held that summer, before a House subcommittee that, Paul Rubinson says, "essentially put deterrence on trial." If a misfire or an accident would mean the end of all life on the planet, could there really be any strategic argument in favor of a nuclear stockpile? During another round of congressional hearings, Sagan said that he'd give a recent and inconclusive Department of Defense report on nuclear winter a D or "maybe a C-minus if I was in a friendly mood." Assistant Secretary of Defense Richard Perle said that he'd give Sagan an F. "I didn't hear a word of science this morning," Perle declared. "I heard a shallow, demagogic, rambling policy pronouncement." William Buckley observed, "Carl Sagan gave a half-hour's performance so arrogant he might have been confused with, well, me."

In December, 1984, Reagan's National Security Council presented the President with a summary of a report on nuclear winter prepared by the National Academy of Sciences. While allowing that the model's quantitative risk assessment involved uncertainties, the report argued that the model's calculations suggested "temperature changes of a size that could have devastating consequences" and, with urgency, called on all available agencies and scientists to conduct investigations to narrow the range of uncertainty. Nuclear winter could be debated, but it couldn't be dismissed.

Nuclear winter did not end the Cold War, but it did weaken the logic of deterrence, and not merely by undermining the idea of a winnable nuclear war. Nuclear winter relied on computer models and projections; its predictions were uncertain. Deterrence relies on computer models and projections; its predictions are uncertain. At one point, Richard Perle said that he wished Sagan would go back to his laboratory and stop "playing political scientist." And that, inadvertently, got to the heart of the matter. However much Sagan might have overreached, his intellectual extravagance was nothing compared with the entirely hypothetical musings and game-theory models of the political scientists and strategists on the basis of whose speculations the United States government spent more than five trillion dollars between 1940 and 1996.

Reagan was himself persuaded by nuclear winter; a nuclear war, he said, "could just end up in no victory for anyone because we would wipe out the earth as we know it." In the U.S.S.R., nuclear winter energized dissidents. In 1985, when the Soviet physician Vladimir Brodsky was arrested, one of the charges was "transmitting a letter to the Soviet Academy of Sciences requesting greater publicity about the nuclear winter." Protesters in Moscow's Gagarin Square chanted, "Tell the truth about the nuclear-winter phenomenon to our people." Eduard Shevardnadze, the Soviet foreign minister, talked about nuclear winter in a speech at the U.N., and Mikhail Gorbachev alluded to it on another occasion. In 1985, the Federation of American Scientists presented Sagan with an award honoring him as the "Most Visible Member of the Scientific Community on the Planet Earth." In 1986, Turco won a MacArthur prize. After that year, the number of nuclear weapons in the world began to decline.

Since the end of the Cold War and the collapse of the Soviet Union, most nuclear-weapons talk has been about non-proliferation and coercion. Under the terms of the Non-Proliferation Treaty, signed in 1968, non-nuclear powers agreed to forgo nuclear weapons in exchange for the assurance that they could develop nuclear energy, and for a promise from nuclear powers to pursue disarmament in good faith. Since then, no nuclear nation except South Africa has dismantled its arsenal, which is why non-nuclear states continue to press nuclear states to make good on the promise they made in 1968.

In a new book titled "Nuclear Politics: The Strategic Causes of Proliferation," the Yale political-science professors Alexandre Debs and Nuno P. Monteiro struggle with a very small data set. Of the eight nations other than the U.S. that have nuclear weapons, three (the U.K., France, and Israel) are American allies; two (India and Pakistan) are friendly; and three (China, North Korea, and Russia) are adversaries. Two of these countries (North Korea and Pakistan) acquired nuclear weapons since the eighties, which is very worrying, but both acquired them at great cost. (Zulfikar Ali Bhutto, then the Pakistani foreign-affairs minister, said his people could "eat grass" if that's what it took.) The U.S. has stopped several states from developing nuclear weapons, either by threatening to abandon an alliance (in the cases of Taiwan and West Germany), or by threatening, indirectly, to use military force (Libya), or by using it, a perilous course (Iraq). Under what circumstances do states develop nuclear weapons? Debs and Monteiro argue that most states are too weak to do so; most weak states aren't interested; some weak states aren't especially threatened; and most weak states that are threatened are protected by stronger states. These findings question conventional wisdom, which has it that the bomb is a tool of weak states. "No doubt, the atomic bomb would enable a weak state to stand up to more powerful adversaries," they write. "So far, however, no weak unprotected state has ever managed to obtain it."

In "Nuclear Weapons and Coercive Diplomacy," just published, another pair of political scientists, Todd S. Sechser and Matthew Fuhrmann, investigate nuclear coercion, an idea that has all but replaced deterrence in some policy circles. Deterrence involves stopping your enemy from doing what you don't want; coercion involves getting your enemy to do what you want. The theory of deterrence rests on an analysis of the balance between two roughly equal superpowers. Those conditions no longer apply. Coercion is a theory for a single superpower: a new game, requiring a new game theory. Does it work? Not really. As Sechser and Fuhrmann demonstrate, nuclear powers have not generally been able to coerce other nuclear powers: in the sixties, the Soviets' nuclear superiority didn't help solve territorial disputes with China; and, more recently, the United States hasn't been able to coerce North Korea into abandoning its nuclear-weapons development. Nor have nuclear powers been able to alter the behavior of non-nuclear powers, the authors argue, and their list is long: "The shadow of America's nuclear arsenal did not convince Afghan leaders to hand over al Qaeda operatives after the group conducted terrorist attacks against American targets in 1998 or 2001. Great Britain could not coerce Argentine forces to withdraw from the Falkland Islands without a fight in 1982, despite deploying nuclear forces to the South Atlantic. The Soviet Union could not force Iran or Turkey to hand over disputed territory in the early 1950s, after Moscow acquired the bomb. China has similarly been unable to make relatively weak states-including Brunei, Malaysia, Philippines, Taiwan, and Vietnam-abandon their claims to the disputed Spratly Islands in the South China Sea."

These arguments are very interesting, but they are based on the shaky science of very small numbers. An _n_th case could unravel any of them. The most contested number in this debate is zero. In 2009, Thomas Schelling, an economist, national-security expert, and Cold War deterrence theorist, who had won a Nobel Prize for his game-theory analysis of conflict, issued a dire warning:

A "world without nuclear weapons" would be a world in which the United States, Russia, Israel, China, and half a dozen or a dozen other countries would have hair-trigger mobilization plans to rebuild nuclear weapons and mobilize or commandeer delivery systems, and would have prepared targets to preempt other nations' nuclear facilities, all in a high-alert status, with practice drills and secure emergency communications. Every crisis would be a nuclear crisis, any war could become a nuclear war. The urge to preempt would dominate; whoever gets the first few weapons will coerce or preempt. It would be a nervous world.

Schelling's nervous world is the setting for "The Case for U.S. Nuclear Weapons in the 21st Century," a careful and balanced study by Brad Roberts, the director of the Center for Global Security Research at Lawrence Livermore National Laboratory. Lamenting the hardened lines between advocates and abolitionists, Roberts calls for a fresh and broad-minded debate: "Whether nuclear weapons will continue to be effective in preventing limited wars among major powers is an open question." The case for deterrence began to fall apart in the nineteen-nineties and two-thousands, he argues, owing to a lack of leadership. This isn't entirely correct. Presidents in that period did fail to make the case for deterrence, but deterrence had already been dismantled by the spectre of nuclear winter.

The biggest consequence of the nuclear-winter debate, though, has had to do not with nuclear-weapons policy but with the environmental movement. In the short term, the idea of a nuclear winter defeated the idea of deterrence. In the long term, Sagan's haste and exuberance undermined environmental science. More important, the political campaign waged against nuclear winter-against science, and against the press-included erecting a set of structures, arguments, and institutions that have since been repurposed to challenge the science of global warming.

In 1984, in an effort to counter Sagan and to defend the Strategic Defense Initiative, the George C. Marshall Institute was founded by Robert Jastrow, a NASA physicist; Frederick Seitz, a former president of the National Academy of Sciences; and William Nierenberg, a past director of the Scripps Institute of Oceanography. Jastrow argued that "the Nuclear Winter scenario could not serve the needs of Soviet leaders better if it had been designed for that purpose." One of the Marshall Institute's first projects was to try to persuade PBS affiliates not to air a documentary critical of S.D.I.; citing the Fairness Doctrine, the institute argued that equal time ought to be given to its own report, in favor of S.D.I. The report represented the views of three scientists-Jastrow, Seitz, and Nierenberg-while a statement expressing concerns about the science behind S.D.I. had been signed by sixty-five hundred scientists. Nevertheless, most PBS stations decided not to broadcast the documentary. With funding from the Marshall Institute, Seitz's cousin Russell Seitz, a physicist at Harvard's Center for International Affairs, published an essay in The National Interest, in the fall of 1986, dismissing the nuclear-winter paper as nothing but "a long series of conjectures" and declaring nuclear winter dead: "Cause of death: notorious lack of scientific integrity." In 1988, funded, in part, by ExxonMobil, the Marshall Institute turned its attention to the science behind global warming.

Another of Sagan's most vociferous critics, S. Fred Singer, had repeatedly challenged nuclear winter on the grounds of its uncertainty. "Sagan's scenario may well be correct," Singer wrote in 1983, "but the range of uncertainty is so great that the prediction is not particularly useful." A longtime consultant to ARCO, Exxon, Shell Oil, and Sun Oil, Singer is currently the director of the Science and Environmental Policy Project at the Heartland Institute, founded in 1984. Its position on global warming: "Most scientists do not believe human greenhouse gas emissions are a proven threat to the environment or to human well-being, despite a barrage of propaganda insisting otherwise coming from the environmental movement and echoed by its sycophants in the mainstream media."

The nuclear-winter debate has long since been forgotten, but you can still spy it behind every cloud and confusion. It holds a lesson or two. A public understanding of science is not well served by shackling science to a national-security state. The public may not naturally have much tolerance for uncertainty, but uncertainty is the best that many scientific arguments can produce. Critics of climate-change science who ground their argument on uncertainty have either got to apply that same standard of evidence to nuclear-weapons strategy or else find a better argument. Because, as Sagan once put it, theories that involve the end of the world are not amenable to experimental verification-at least, not more than once. ♦

* An earlier version of this article erroneously stated that Sagan had begun his career working on a classified nuclear-weapons project.

* It also misidentified the operation Sagan was recovering from when he dictated his letter of objection.

This article appears in the print edition of the January 30, 2017, issue, with the headline "Autumn of the Atom."

Jill Lepore is a staff writer and a professor of history at Harvard University. Her latest book, "These Truths: A History of the United States," will come out in September.
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A nuclear weapon is a certain thing-atomic or hydrogen, fission or fusion, bomb or missile, so many megatons-but nothing could be more uncertain than the consequences of using one. Nine nations have nuclear weapons; only the United States has ever used one, and that was in 1945.

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Article explains anthropogenic climate change and gives examples.

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Anthropogenic climate change refers to the production of greenhouse gases emitted by human activity. By examining the polar ice cores, scientists are convinced that human activity has increased the proportion of greenhouse gases in the atmosphere, which has skyrocketed over the past few hundred years.

The IPCC, Fourth Report released in 2007 stated that, multiple lines of evidence confirms that the post-industrial rise in greenhouse gases does not stem from natural mechanisms. In other words this is anthropogenic climate change, and the significant increases in the atmosphere of these potent greenhouse gases are a result of human activity.

The most potent of the greenhouse gases are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N20). Alarmingly, these are a result of anthropogenic climate change, and the gases are at the highest levels for over 650,000 years.

The IPCC Fourth Report confirms that over the past 8,000 years, and just before Industrialization in 1750 , carbon dioxide concentration in the atmosphere increased by a mere 20 parts per million (ppm). The concentration of atmospheric CO2 in 1750 was 280ppm, and increased to 379 ppm in 2005. That is a whopping increase of 100 ppm in 250 years. For comparison and at the end of the most recent ice age there was approximately an 80ppm rise in CO2 concentration. This rise took over 5,000 years, and higher values than at present have only occurred many millions of years ago.

Since 1750, it is estimated that about two thirds of anthropogenic climate change CO2 emissions have come from fossil fuel burning (coal and petroleum) and about one third from land use change (mainly deforestation and agricultural). About 45% of this CO2 has remained in the atmosphere, while about 30% has been taken up by the oceans and the remainder has been taken up by the trees and plants. About half of a CO2 going into the atmosphere is removed over a time scale of 30 years; a further 30% is removed within a few centuries; and the remaining 20% will typically stay in the atmosphere for many thousands of years.

In recent decades, carbon emissions have continued to increase. Global annual fossil emissions increased from an average of 6.4 GtC yr (one giga tonne of carbon per year) in the 1990s to 7.2 GtC yr in the period 2000 to 2005. Fossil fuel use is not the only human contribution to carbon dioxide levels, and at least another 1.6GtC should be added for emissions from land use. To gain some idea of figures we are talking about here, 1 GtC is equal to 1,000,000,000 metric tonnes, that is one billion tonnes.

While you think about these numbers, it should be pointed out that when using the emission factors above, 1 GtC (carbon) corresponds to 3.67 GtCO2 (carbon dioxide). The box above shows why 1GtC = 3.67GCO2, and we will now work out how Many Gt of CO2 are emitted every year (and still rising). Firstly, we can add fossil fuel emissions 7.2 to land use, 1.6 to come up with a total of 8.8GtC annually. Secondly, we multiply 8.8 x 3.67 and so we have a total of 32.29GtCO2 being emitted to the atmosphere each year!

In addition to escalating coal use after the Industrial Revolution, came the widespread use of another fossil fuel; petroleum for transport. At the beginning of the 20th century, annual global oil output was about 150 million barrels of oil, now, that amount is extracted globally in just two days.

Look at the graph at the top of the page again, and see how concentrations of CO2, CH4 and N20 have risen almost vertically in recent years. The vertical pathway is cause for alarm, as CO2 levels correlate highly with an increase in average global surface temperatures.

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Anthropogenic Climate Change | Global Greenhouse Warming

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Anthropogenic climate change refers to the production of greenhouse gases emitted by human activity. By examining the polar ice cores, scientists are convinced that human activity has increased the proportion of greenhouse gases in the atmosphere, which has skyrocketed over the past few hundred years.

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A fight to reverse climate change.

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It got so hot in Australia in January that the weather service had to add two new colors to its charts. A few weeks later, at the other end of the planet, new data from the CryoSat-2 satellite showed 80 percent of Arctic sea ice has disappeared. We're not breaking records anymore; we're breaking the planet. In 50 years, no one will care about the fiscal cliff or the Euro crisis. They'll just ask, "So the Arctic melted, and then what did you do?"

Here's the good news: We'll at least be able to say we fought.

After decades of scant organized response to climate change, a powerful movement is quickly emerging around the country and around the world, building on the work of scattered front-line organizers who've been fighting the fossil-fuel industry for decades. It has no great charismatic leader and no central organization; it battles on a thousand fronts. But taken together, it's now big enough to matter, and it's growing fast.
The Fossil Fuel Resistance: Meet the New Green Heroes
Americans got to see some of this movement spread out across the Mall in Washington, D.C., on a bitter-cold day in February. Press accounts put the crowd upward of 40,000 - by far the largest climate rally in the country's history. They were there to oppose the Keystone XL pipeline, which would run down from Canada's tar sands, south to the Gulf of Mexico, a fight that Time magazine recently referred to as the Selma and the Stonewall of the environmental movement. But there were thousands in the crowd also working to block fracking wells across the Appalachians and proposed Pacific coast deep-water ports that would send coal to China. Students from most of the 323 campuses where the fight for fossil-fuel divestment is under way mingled with veterans of the battles to shut down mountaintop-removal coal mining in West Virginia and Kentucky, and with earnest members of the Citizens Climate Lobby there to demand that Congress enact a serious price on carbon. A few days earlier, 48 leaders had been arrested outside the White House - they included ranchers from Nebraska who didn't want a giant pipeline across their land and leaders from Texas refinery towns who didn't want more crude spilling into their communities. Legendary investor Jeremy Grantham was on hand, urging scientists to accompany their research with civil disobedience, as were solar entrepreneurs quickly figuring out how to deploy panels on rooftops across the country. The original Americans were well-represented; indigenous groups are core leaders of the fight, since their communities have been devastated by mines and cheated by oil companies. The Rev. Lennox Yearwood Jr. of the Hip Hop Caucus was handcuffed next to Julian Bond, former head of the NAACP, who recounted stories of being arrested for integrating Atlanta lunch counters in the Sixties.

It's a sprawling, diverse and remarkably united movement, marked by its active opposition to the richest and most powerful industry on Earth. The Fossil Fuel Resistance has already won some serious victories, blocking dozens of new coal plants and closing down existing ones - ask the folks at Little Village Environmental Justice Organization who helped shutter a pair of coal plants in Chicago, or the Asian Pacific Environmental Network, which fought to stop Chevron from expanding its refinery in Richmond, California. "Up to this point, grassroots organizing has kept more industrial carbon out of the atmosphere than state or federal policy," says Gopal Dayaneni of the Movement Generation Justice and Ecology Project. It's an economic resistance movement, too, one that's well aware renewable energy creates three times as many jobs as coal and gas and oil. Good jobs that can't be outsourced because the sun and the wind are close to home. It creates a future, in other words.

These are serious people: You're not a member of the Resistance just because you drive a Prius. You don't need to go to jail, but you do need to do more than change your light bulbs. You need to try to change the system that is raising the temperature, the sea level, the extinction rate - even raising the question of how well civilization will survive this century.

Soon after the big D.C. rally, the state department issued a report downplaying Keystone XL's environmental impact, thus advancing the pipeline proposal another step. Since then, at the urging of the remarkable cellphone-company-cum-activist-group Credo, nearly 60,000 people have signed a pledge promising to resist, peacefully but firmly, if the pipeline is ever approved. By early March, even establishment commentators like Thomas Friedman had noticed - he used his New York Times column to ask activists to "go crazy" with civil disobedience; 48 hours later, 25 students and clergy were locked down inside a pipeline-company office outside Boston. It's not a one-sided fight anymore.

No movement this diverse is going to agree on a manifesto, but any reckoning begins with the idea that fossil fuel is dirty at every stage, and we need to put it behind us as fast as we can. For those of us in affluent countries, small shifts in lifestyle won't be enough; we'll also need to alter the policies that keep this industry fat and happy. For the poor world, the much harder goal is to leapfrog the fossil-fuel age and go straight to renewables - a task that those of us who prospered by filling the atmosphere with carbon must help with, for reasons both moral and practical. And for all of us, it means standing with communities from the coal fields of Appalachia to the oil-soaked Niger Delta as they fight for their homes. They've fought longest and hardest and too often by themselves. Now that global warming is starting to pour seawater into subways, the front lines are expanding and the reinforcements are finally beginning to arrive.
Climate Change and the End of Australia
Right now, the fossil-fuel industry is mostly winning. In the past few years, they've proved "peak-oil" theorists wrong - as the price rose for hydrocarbons, companies found lots of new sources, though mostly by scraping the bottom of the barrel, spending even more money to get even-cruddier energy. They've learned to frack (in essence, explode a pipe bomb a few thousand feet beneath the surface, fracturing the surrounding rock). They've figured out how to take the sludgy tar sands and heat them with natural gas till the oil flows. They've managed to drill miles beneath the ocean's surface. And the hyperbolic enthusiasm has gushed even higher than the oil. The Wall Street Journal has declared North Dakota a new Saudi Arabia. The New York Times described a new shale-oil find in California as more than four times as large as North Dakota's. "We could make OPEC 'NOPEC' if we really put our minds to it," said Charles Drevna of the American Fuel and Petrochemical Manufacturers. "We're talking decades, if not into the hundreds of years, of supply in North America."

But all that fossil fuel will only get pumped and mined and burned if we decide to ignore the climate issue; were we to ever take it seriously, the math would quickly change. As I pointed out in these pages last summer, the world's fossil-fuel companies, even before these new finds, had five times more carbon in their reserves than we could burn if we hope to stay below a two-degreeCelsius rise in global temperatures. That's the red line almost every government in the world has agreed on, but the coal, oil and natural-gas companies, propelled by record profits, just keep looking for more - and ignore reality. A new report shows that an anonymous group of industry billionaires has secretly poured more than $100 million into anti-environmentalfront groups. Weeks before Election Day, Chevron gave the largest corporate Super PAC contribution of the post-Citizens United era, making sure that Congress stayed in the hands of climate deniers.

But every flood erodes their position, and every heat wave fuels the Resistance. When the Keystone pipeline first became controversial, in 2011, a poll of D.C. "energy insiders" showed that more than 70 percent of them thought they'd have permits to build it by the end of the year. Big Oil, of course, may end up getting its way, but so far its money hasn't overwhelmed the passion, spirit and creativity its foes have brought to the battle. And we're not just playing defense anymore: The rapidly spreading divestment movement may be the single biggest face of the Resistance. It's no longer confined to campuses; city governments and religious denominations have begun to unload their stakes in oil companies, and the movement is even spreading to self-interested investors now that HSBC has calculated that taking climate change seriously could cut share prices of oil companies by up to 60 percent.

With each passing month, something else weakens the industry's hand: the steady rise of renewable energy, a technology that's gone from pie-in-the-sky to panel-on-the-roof in remarkably short order. In the few countries where governments have really gotten behind renewables, the results are staggering: There were days last spring when Germany (pale, northern Germany) managed to generate half its power from solar panels. Even in this country, much of the generating capacity added last year came from renewables. A December study from the University of Delaware showed that by 2030 we could affordably power the nation 99.9 percent of the time on renewable energy. In other words, logic, physics and technology work against the fossil-fuel industry. For the moment, it has the political power it needs - but political power shifts perhaps more easily than physics.
Global Warming's Terrifying New Math
Which is where the resistance comes in. Forty-three years ago, the first anarchic Earth Day drew 20 million Americans into the streets. That surge helped push through all kinds of legislation - the Clean Air Act, the Endangered Species Act - and spurred the growth of organizations like the Natural Resources Defense Council and the Environmental Defense Fund. As these "green groups" became the face of the environmental movement, they grew adept at playing an inside-the-Beltway lobbying game. But that strategy got harder as the power of the right wing grew; for 25 years, they've been unable to win significant progress on climate change.

Now, energized by the Keystone protests, some strides have been made. The NRDC has done yeoman's work against the pipeline. The Sierra Club, which just a few years ago was taking millions from the fracking industry to shill for natural gas, has been reinvented. In January, the club dropped a 120-year ban on civil disobedience. The following month, its executive director, Michael Brune, was led away from the White House in handcuffs.

But the center of gravity has also shifted from big, established groups to local, distributed efforts. In the Internet age, you don't need direct mail and big headquarters; you need Twitter. In Texas and Oklahoma, hundreds have joined actions led by the Tar Sands Blockade, which has used daredevil tactics and lots of courage to get between the industry and the pipeline it needs to move oil overseas. In Montana, author Rick Bass and others sat-in to stop the export of millions of tons of coal from ports on the West Coast. And all across the Marcellus and Utica shale formations in the Northeast, people have been standing up for their communities, often by sitting down in front of the fracking industry. The Fossil Fuel Resistance looks more and more like Occupy - in fact, they've overlapped from the beginning, since oil companies are the one percent of the one percent. The movements share a political analysis, too: A grid with a million solar rooftops feels more like the Internet than ConEd; it's a farmers market in electrons, with the local control that it implies.

Like Occupy, this new Resistance is not obsessed with winning over Democratic Party leaders. The Keystone arrests in 2011 marked the most militant protests outside the White House during Obama's first term; now Van Jones, who once worked for the president, has taken to calling Keystone the "Obama pipeline." Used to dealing with the established green groups, the administration thinks in terms of deals - "We'll approve the pipeline but give you something else you want" - the kind of logic that gains the approval of op-ed columnists and talking heads. But given that the Arctic has already melted, we don't have room for easy compromises. The president's insistence that he favors an "all of the above" energy system, where oil and gas are as welcome as solar and wind, seems increasingly like a classic political hedge. In fact, if the GOP wasn't in the tank for the oil industry, you couldn't do much worse than Obama's campaigntrail rhetoric. Last year, the president went to Oklahoma, posed in front of a stack of oil pipe and bragged of adding enough new pipelines to encircle Earth. Since the election, the president has started talking green, promising that now climate change would be a priority - but this growing Resistance is, I think, unconvinced. As climate leader Naomi Klein said, "This time, no honeymoon and no hero worship."

Only grit and hard work. We've watched great cultural shifts and organizing successes in recent years, like the marriage-equality and immigration-reform movements. But breaking the power of oil companies may be even harder because the sums of the money on the other side are so fantastic - there are trillions of dollars worth of oil in Canada's tar sands and the North Dakota shale. The men who own the coal mines and the gas wells will spend what they need to assure their victories. Last month, Rex Tillerson, Exxon's $100,000-a-day CEO, said that environmentalists were "obtuse" for opposing new pipelines. He announced the company planned to more than double the acreage on which it was exploring for new hydrocarbons and said he expected that renewables would account for just one percent of our energy in 2040, essentially declaring that the war to save the climate was over before it started. He added, "My philosophy is to make money."

That same day, scientists announced that Earth was now warming 50 times faster than it ever has in human civilization, and that carbon-dioxide levels had set a perilous new record at Mauna Loa's measuring station. Right now, we're losing. But as the planet runs its spiking fever, the antibodies are starting to kick in. We know what the future holds unless we resist. And so resist we will.
This story is from the April 25th, 2013 issue of Rolling Stone.

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It got so hot in Australia in January that the weather service had to add two new colors to its charts. A few weeks later, at the other end of the planet, new data from the CryoSat-2 satellite showed 80 percent of Arctic sea ice has disappeared.

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This article talks about ways to generate energy without fossil fuels.

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Three times now, Mark Jacobson has gone out on the same limb. In 2009 he and co-author Mark Delucchi published a cover story in Scientific American that showed how the entire world could get all of its energy-fuel as well as electricity-from wind, water and solar sources by 2030. No coal or oil, no nuclear or natural gas. The tale sounded infeasible-except that Jacobson, from Stanford University, and Delucchi, from the University of California, Davis, calculated just how many hydroelectric dams, wave-energy systems, wind turbines, solar power plants and rooftop photovoltaic installations the world would need to run itself completely on renewable energy.

The article sparked a spirited debate on our Web site, and it also sparked a larger debate between forward-looking energy planners and those who would rather preserve the status quo. The duo went on to publish a detailed study in the journal Energy Policy that also called out numbers for a U.S. strategy.

Two weeks ago Jacobson and a larger team, including Delucchi, did it again. This time Jacobson showed in much finer detail how New York State's residential, transportation, industrial, and heating and cooling sectors could all be powered by wind, water and sun, or "WWS," as he calls it. His mix: 40 percent offshore wind (12,700 turbines), 10 percent onshore wind (4,020 turbines), 10 percent concentrated solar panels (387 power plants), 10 percent photovoltaic cells (828 facilities), 6 percent residential solar (five million rooftops), 12 percent government and commercial solar (500,000 rooftops), 5 percent geothermal (36 plants), 5.5 percent hydroelectric (6.6 large facilities), 1 percent tidal energy (2,600 turbines) and 0.5 percent wave energy (1,910 devices).

In the process, New York would reduce power demand by 37 percent, largely because the new energy sources are more efficient than the old ones. And because no fossil fuels would have to be purchased or burned, consumer costs would be similar to what they are today, and the state would eliminate a huge portion of its carbon dioxide emissions.

New York State could end fossil fuel use and generate all of its energy from wind, water and solar power, according to Mark Jacobson. Image: Graphic by Karl Burkart

Once again, reaction was swift. The New York Times heralded the study as scientifically groundbreaking and practically impossible. But this time Jacobson, a professor of civil and environmental engineering, is digging in. He took his analysis a step further and found a surprising way to sell his plan. And he's close to finishing a similar study for California, which will lend more depth to his vision. I asked Jacobson why he's out to change the world, how he answers his critics and what it will take for his plans to get traction in government.

[ An edited transcript of the interview follows.]

At first glance, your proposals to convert society wholesale to renewable energy, and relatively soon, sound wild. What kinds of reactions do you get?

Mostly, it's pretty positive. A lot of people say, "Wow, we should really make a huge effort to push this forward." There are always naysayers who think it's pie in the sky, that we'll never get there. And there are people who are tied into a certain industry who push back the most. It's almost like motherhood and apple pie, though; it's hard to say "Oh, I don't like it." The real question is: How do policy makers react? Few of them say they would be against it. It's more that they still want to push other energy sources. You need policy makers behind it, and you also need grassroots efforts.

Why did you dive down into New York State after having done the entire world?

To get any traction I figured we had to do a plan with higher resolution, because otherwise it's just too massive for anyone to actually do anything about it. And some people in New York are really interested in coming up with a new energy plan. At first I was hesitant but then I thought it could be quite a coup, it could be very valuable. I know Governor [Andrew] Cuomo's office is reading it.

After the global plan appeared in Scientific American a lot of people said the exercise was intriguing but it would never be taken seriously as a blueprint. What are the main obstacles to such a sweeping overhaul at a state or national level?

The main obstacles are political and social-getting politicians onboard. There are always local zoning issues. I'm sure there will be a big push by the gas lobby and the oil lobby against this.

So then how do you sell the plan?

There is a huge savings in lives. The New York plan would prevent 4,000 mortalities a year in the state due to less air pollution, and a related savings of $33 billion-about 3 percent of the GDP of the state. That resonates more with people than climate change issues. We also looked at job creation; more jobs would be created than lost.

The main criticism about heavy reliance on wind and solar power is that the sources are intermittent: the wind doesn't always blow, the sun doesn't shine at night. Do your plans rely a lot on energy storage, which remains a tough challenge?

If you get the [power] transmission grid right you don't need a whole lot of storage. By combining wind and solar and geothermal and hydroelectric, you can match the power demand. And if you oversize the grid, when you're producing extra electricity you use it to produce hydrogen [for fuel-cell vehicles and ships as well as some district heating and industrial processes]. You can also spread the peak demand by giving financial incentives [for consumers to use power at off-peak times]. Some storage certainly would help; we have storage in the form of hydrogen and in concentrated solar power plants. There are many ways to tackle the intermittency issues.

The other concern that is usually raised about renewable energy is that it is more expensive than fossil fuels. What would electricity prices be like in New York?

The residential electricity cost in the U.S. on average is 13.1 cents per kilowatt-hour. In New York it's 18.1 cents. If you look at the states that have the highest percentage of electricity generation from wind, the average electricity price increase from 2003 to 2011 was 2 cents a kilowatt-hour, whereas all the other states averaged 3.6 cents. So prices in the states that didn't put in a lot of wind went up more.

Given the radical nature of your proposals, is it fair to say you are an advocate for renewable energy?

I'm doing the science part of it. I'm not really advocating. The job of a scientist is to make sure that information is provided clearly and appropriately, so people can make a better decision. I don't advocate. But there is a larger group of people that I'm doing science work for that is more policy oriented, the Solutions Project. It involves scientists, industrial people, business people, finance people, artists and entertainers, and some media. It tries to take clean energy plans and get them implemented.

If you're not an advocate, what is your motivation? You've done this exercise three times now.

My career has always been based on trying to understand large-scale pollution and climate problems-with the goal of trying to solve them. This is the "trying to solve them" part. If society is going to do it, at least we now know that it's technically and economically feasible. Whether it actually happens depends on political will.

You've plotted a course for the world, and for New York. What's next?

We've almost finished a California plan, which is similar. Ultimately, we'd like to do 50 plans for 50 states. I've started Washington State. The California plan should be done in two months or so. Then we have to decide what we're going to do with it.

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How to Power the World without Fossil Fuels

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Three times now, Mark Jacobson has gone out on the same limb. In 2009 he and co-author Mark Delucchi published a cover story in Scientific American that showed how the entire world could get all of its energy-fuel as well as electricity-from wind, water and solar sources by 2030.

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This website will help explain what fossil fuels are!

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Fossil fuels are hydrocarbons, primarily coal, fuel oil or natural gas, formed from the remains of dead plants and animals.

In common dialogue, the term fossil fuel also includes hydrocarbon-containing natural resources that are not derived from animal or plant sources.

These are sometimes known instead as mineral fuels.

The utilization of fossil fuels has enabled large-scale industrial development and largely supplanted water-driven mills, as well as the combustion of wood or peat for heat.

Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.

The burning of fossil fuels by humans is the largest source of emissions of carbon dioxide, which is one of the greenhouse gases that allows radiative forcing and contributes to global warming.

A small portion of hydrocarbon-based fuels are biofuels derived from atmospheric carbon dioxide, and thus do not increase the net amount of carbon dioxide in the atmosphere.

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energy and the environment; renewable energy; oil spills; fossil fuels; energy and resources; petroleum

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Fossil fuels are hydrocarbons, primarily coal, fuel oil or natural gas, formed from the remains of dead plants and animals. In common dialogue, the term fossil fuel also includes hydrocarbon-containing natural resources that are not derived from animal or plant sources. These are sometimes known instead as mineral fuels.

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Everything you need to know about the carbon cycle!

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Everything you need to know about the carbon cycle!

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Click here!

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by John Arthur Harrison, Ph.D.

Table of contents

Terms you should know

atmosphere : the collective mass of gases that surrounds Earth or another planet.

organic : originating from a living organism; a compound that contains hydrocarbons

species : a distinct type of plant or animal; a related group of living things that can produce offspring

Carbon is the fourth most abundant element in the universe, and is absolutely essential to life on Earth. In fact, carbon constitutes the very definition of life, as its presence or absence helps define whether a molecule is considered to be organic or inorganic. Every organism on Earth needs carbon either for structure, energy, or, as is the case of humans, for both. Discounting water, you are about half carbon. Additionally, carbon is found in forms as diverse as the gas carbon dioxide (CO ₂), and in solids like limestone (CaCO ₃), wood, plastic, diamonds, and graphite.

The movement of carbon, in its many forms, between the atmosphere, oceans, biosphere, and geosphere is described by the carbon cycle, illustrated in Figure 1. This cycle consists of several storage carbon reservoirs and the processes by which the carbon moves between reservoirs. Carbon reservoirs include the atmosphere, the oceans, vegetation, rocks, and soil; these are shown in black text along with their approximate carbon capacities in Figure 1. The purple numbers and arrows in Figure 1 show the fluxes between these reservoirs, or the amount of carbon that moves in and out of the reservoirs per year. If more carbon enters a pool than leaves it, that pool is considered a net carbon sink. If more carbon leaves a pool than enters it, that pool is considered net carbon source.

Figure 1: A cartoon of the global carbon cycle. Pools (in black) are gigatons (1Gt = 1x10⁹ Tons) of carbon, and fluxes (in purple) are Gt carbon per year. Illustration courtesy NASA Earth Science Enterprise. image © NASA

The global carbon cycle, one of the major biogeochemical cycles, can be divided into geological and biological components. The geological carbon cycle operates on a timescale of millions of years, whereas the biological carbon cycle operates on a timescale of days to thousands of years.

The geological carbon cycle

The geological component of the carbon cycle is where it interacts with the rock cycle in the processes of weathering and dissolution, precipitation of minerals, burial and subduction, and volcanic eruptions (see The Rock Cycle module for information). In the atmosphere, carbonic acid forms by a reaction with atmospheric carbon dioxide (CO ₂) and water. As this weakly acidic water reaches the surface as rain, it reacts with minerals at Earth's surface, slowly dissolving them into their component ions through the process of chemical weathering. These component ions are carried in surface waters like streams and rivers eventually to the ocean, where they precipitate out as minerals like calcite (CaCO ₃). Through continued deposition and burial, this calcite sediment forms the rock called limestone.

This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction. As seafloor carbon is pushed deeper into the Earth by tectonic forces, it heats up, eventually melts, and can rise back up to the surface, where it is released as CO ₂ and returned to the atmosphere. This return to the atmosphere can occur violently through volcanic eruptions, or more gradually in seeps, vents, and CO ₂-rich hotsprings. Tectonic uplift can also expose previously buried limestone. One example of this occurs in the Himalayas where some of the world's highest peaks are formed of material that was once at the bottom of the ocean. Weathering, subduction, and volcanism control atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years.

Comprehension Checkpoint

Carbonic acid reaches the Earth's surface by way of

The biological carbon cycle

Biology plays an important role in the movement of carbon between land, ocean, and atmosphere through the processes of photosynthesis and respiration. Virtually all multicellular life on Earth depends on the production of sugars from sunlight and carbon dioxide (photosynthesis) and the metabolic breakdown (respiration) of those sugars to produce the energy needed for movement, growth, and reproduction. Plants take in carbon dioxide (CO ₂) from the atmosphere during photosynthesis, and release CO ₂ back into the atmosphere during respiration through the following chemical reactions:

Respiration:

C ₆H ₁₂O ₆ (organic matter) + 6O ₂ → 6CO ₂ + 6 H ₂ O + energy

Photosynthesis:

energy (sunlight) + 6CO ₂ + H ₂O → C ₆H ₁₂O ₆ + 6O ₂

Through photosynthesis, green plants use solar energy to turn atmospheric carbon dioxide into carbohydrates (sugars). Plants and animals use these carbohydrates (and other products derived from them) through a process called respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and changes carbohydrate "fuel" back into carbon dioxide, which is in turn released back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.

On land, the major exchange of carbon with the atmosphere results from photosynthesis and respiration. During daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere. At the same time plants, animals, and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere. Photosynthesis stops at night when the sun cannot provide the driving energy for the reaction, though respiration continues. This kind of imbalance between these two processes is reflected in seasonal changes in the atmospheric CO ₂ concentrations. During winter in the northern hemisphere, photosynthesis ceases when many plants lose their leaves, but respiration continues. This condition leads to an increase in atmospheric CO ₂ concentrations during the northern hemisphere winter. With the onset of spring, however, photosynthesis resumes and atmospheric CO ₂ concentrations are reduced. This cycle is reflected in the monthly means (the light blue line) of atmospheric carbon dioxide concentrations shown in Figure 2.

Figure 2: The "Keeling Curve," a long-term record of atmospheric CO₂ concentration measured at the Mauna Loa Observatory (Keeling et al.). Although the annual oscillations represent natural, seasonal variations, the long-term increase means that concentrations are higher than they have been in 400,000 years (see text and Figure 3). Graphic courtesy of NASA's Earth Observatory. image © NASA

In the oceans, phytoplankton (microscopic marine plants that form the base of the marine food chain) use carbon to make shells of calcium carbonate (CaCO ₃). The shells settle to the bottom of the ocean when phytoplankton die and are buried in the sediments. The shells of phytoplankton and other creatures can become compressed over time as they are buried and are often eventually transformed into limestone. Additionally, under certain geological conditions, organic matter can be buried and over time form deposits of the carbon-containing fuels coal and oil. It is the non-calcium containing organic matter that is transformed into fossil fuel. Both limestone formation and fossil fuel formation are biologically controlled processes and represent long-term sinks for atmospheric CO ₂.

Comprehension Checkpoint

The major biological exchange of carbon with the atmosphere is from

Human alteration of the carbon cycle

Recently, scientists have studied both short- and long-term measurements of atmospheric CO ₂ levels. Charles Keeling, an oceanographer at the Scripps Institute of Oceanography, is responsible for creating the longest continuous record of atmospheric CO ₂ concentrations, taken at the Mauna Loa observatory in Hawaii. His data (now widely known as the "Keeling curve," shown in Figure 2) revealed that human activities are significantly altering the natural carbon cycle. Since the onset of the industrial revolution about 150 years ago, human activities such as the burning of fossil fuels and deforestation have accelerated, and both have contributed to a long-term rise in atmospheric CO ₂. Burning oil and coal releases carbon into the atmosphere far more rapidly than it is being removed, and this imbalance causes atmospheric carbon dioxide concentrations to increase. In addition, by clearing forests, we reduce the ability of photosynthesis to remove CO ₂ from the atmosphere, also resulting in a net increase. Because of these human activities, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer.

Because CO ₂ increases the atmosphere's ability to hold heat, it has been called a "greenhouse gas." Scientists believe that the increase in CO ₂ is already causing important changes in the global climate. Many attribute the observed 0.6 degree C increase in global average temperature over the past century mainly to increases in atmospheric CO ₂. Without substantive changes in global patterns of fossil fuel consumption and deforestation, warming trends are likely to continue. The best scientific estimate is that global mean temperature will increase between 1.4 and 5.8 degrees C over the next century as a result of increases in atmospheric CO ₂ and other greenhouse gases. This kind of increase in global temperature would cause a significant rise in average sea-level (0.09-0.88 meters), exposing low-lying coastal cities or cities located by tidal rivers, such as New Orleans, Portland, Washington, and Philadelphia, to increasingly frequent and severe floods. Glacial retreat and species range shifts are also likely to result from global warming, and it remains to be seen whether relatively immobile species such as trees can shift their ranges fast enough to keep pace with warming.

Even without the changes in climate, however, increased concentrations of CO ₂ could have an important impact on patterns of plant growth worldwide. Because some species of plants respond more favorably to increases in CO ₂ than others, scientists believe we may see pronounced shifts in plant species as a result of increasing atmospheric CO ₂ concentrations, even without any change in temperature. For example, under elevated CO ₂ conditions, shrubs are thought to respond more favorably than certain grass species due to their slightly different photosynthetic pathway. Because of this competitive inequality, some scientists have hypothesized that grasslands will be invaded by CO ₂-responsive grass species or shrubby species as CO ₂ increases.

Figure 3: CO₂ over the past 140,000 years as seen in an ice core and in the modern Mauna Loa record. The red line represents predicted concentrations. Figure courtesy of : Rebecca Dorsey, University of Oregon.

Comprehension Checkpoint

Carbon dioxide is considered a "greenhouse gas" because

How scientists study past atmosphere and climate

In an attempt to understand whether recently observed changes in the global carbon cycle are a new phenomenon, or have instead occurred throughout geologic history, scientists have devoted considerable effort to developing methods for understanding Earth's past atmosphere and climate. These techniques include analysis of gas bubbles trapped in ice, tree rings, and ocean and lake floor sediments for clues about past climates and atmospheres. Together, these techniques suggest that over the past 20 million years, the Earth's climate has oscillated between relatively warm and relatively cold conditions called interglacial and glacial periods. During interglacial periods, atmospheric CO ₂ concentrations were relatively high, and during glacial periods, CO ₂ concentrations were relatively low. We are currently in an interglacial warm period, and human activities are pushing CO ₂ concentrations higher than they have been for hundreds of thousands of years (Figure 3).

Understanding and mitigating negative impacts of atmospheric CO ₂ enrichment constitute two of the most central challenges that environmental scientists and policy makers currently face. In order to address this issue, the scientific community has formed the Intergovernmental Panel on Climate Change (IPCC), an international, interdisciplinary consortium comprised of thousands of climate experts collaborating to produce consensus reports on climate change science. Many nations have agreed to conditions specified by the Kyoto Protocol, a multilateral treaty aimed at averting the negative impacts associated with human-induced climate change. The United States, which is currently responsible for approximately one quarter of global CO ₂ emissions, has so far declined to participate in the Kyoto Protocol.

Summary

Carbon, the fourth most abundant element in the universe, moves between the atmosphere, oceans, biosphere, and geosphere in what is called the carbon cycle. This module provides an overview of the global carbon cycle, one of the major biogeochemical cycles. The module explains geological and biological components of the cycle. Major sources and sinks of carbon are discussed, as well as the impact of human activities on global carbon levels.

Key Concepts

The presence of carbon determines whether something is organic or inorganic; all living things require carbon to live.

Carbon cycles through the ecosystem in various ways, from photosynthesis and respiration to weathering and other geologic processes.

Many factors, such as seasons and human activities, influence the concentration of carbon in the global atmosphere.

NGSS

HS-C1.5, HS-C5.2, HS-C7.2, HS-ESS2.A1, HS-ESS2.A3, HS-ESS2.D3, HS-LS2.B3

John Arthur Harrison, Ph.D. "The Carbon Cycle" Visionlearning Vol. EAS-2 (3), 2003.

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The Carbon Cycle | Earth Science | Visionlearning

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Carbon, the fourth most abundant element in the universe, moves between the atmosphere, oceans, biosphere, and geosphere in what is called the carbon cycle . This module provides an overview of the global carbon cycle, one of the major biogeochemical cycles. The module explains geological and biological components of the cycle.

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This website has some information about the carbon cycle and offers a clear diagram!

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All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move!

In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.

Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.

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All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move! In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.

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Watch this video for an explanation of the nitrogen & phosphorous cycles!

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Nitrogen & Phosphorus Cycles: Always Recycle! Part 2 - Crash Course Ecology #9

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Watch this video for an explanation of the carbon and water (hydrologic) cycles!

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The Hydrologic and Carbon Cycles: Always Recycle! - Crash Course Ecology #8

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Hank introduces us to biogeochemical cycles by describing his two favorites: carbon and water. The hydrologic cycle describes how water moves on, above, and below the surface of the Earth, driven by energy supplied by the sun and wind. The carbon cycle does the same... for carbon!

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More footage of researchers exploring Movile Cave!

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Movile Cave, Romania - Part III - The Lake Room, Lower Galleries and Air Bell

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Filmed from my helmet camera in April 2011 during my second expedition into Movile Cave, Romania. Movile Cave is the only known totally sealed environment on Earth that contains life.

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Footage of researchers descending into Movile Cave

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Movile Cave, Romania - Part I - Descending The Shaft

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NEW* Complete footage video: https://youtu.be/vTj5n9W15yM THERE ARE TWO MORE VIDEOS IN THIS SERIES - PARTS II AND III! Filmed from my helmet camera in April 2011 during my second expedition into Movile Cave, Romania. Movile Cave is the only known totally sealed environment on Earth that contains life.