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The amount of sea level rise that comes from the oceans warming and expanding has been underestimated, and could be about twice as much as previously calculated, German researchers have said.

The findings in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal, suggest that increasingly severe storm surges could be anticipated as a result.

Sea level can mount due to two factors - melting ice and the thermal expansion of water as it warms.

Until now, researchers have believed the oceans rose between 0.7 to 1mm per year due to thermal expansion.

But a fresh look at the latest satellite data from 2002 to 2014 shows the seas are expanding about 1.4mm a year, said the study.

"To date, we have underestimated how much the heat-related expansion of the water mass in the oceans contributes to a global rise in sea level," said co-author Jurgen Kusche, a professor at the University of Bonn.

The overall sea level rise rate is about 2.74mm per year, combining both thermal expansion and melting ice.

Sea level rise was also found to vary substantially from place to place, with the rate around the Philippines "five times the global rate."

Meanwhile, sea level on the US west coast is largely stable because there is hardly any ocean warming in that area, said the findings.

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Sea level rise from ocean warming underestimated, scientists say

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The amount of sea level rise that comes from the oceans warming and expanding has been underestimated, and could be about twice as much as previously calculated, German researchers have said. The findings in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal, suggest that increasingly severe storm surges could be anticipated as a result.

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New York City would be swallowed by the ocean. Tokyo and Shanghai, too, would vanish.

Sea levels stand to rise by a staggering 164 feet (50 meters) or more if the world goes for broke on fossil fuels, burning all its attainable resources. That's because the Antarctic Ice Sheet would melt entirely from the warming caused by those emissions, concludes a study published Friday in Science Advances. The researchers say their paper offers the first long-term look at how carbon dioxide emissions from oil, coal, and natural gas would affect the entire ice sheet.

"If we don't stop dumping our waste CO2 into the sky, land that is now home to more than billion people will one day be underwater," says study co-author Ken Caldeira, senior scientist at Stanford University's Carnegie Institution for Science. ( See an interactive map of the world if all the ice melted.)

There's a bright side, sort of: The full melt would take about 10,000 years. However, at least 100 feet of the swell, as modeled in the paper from scientists in Germany, California, and the United Kingdom, would occur in this millennium, at a rate of more than an inch per year-a harrowing prospect, given that many coastal areas are already seeing land loss and flooding from much more modest sea level increases.

To put it bluntly, if we burn it all, we melt it all.

Ricarda Winkelmann, scientist at Potsdam University

The study highlights the importance of the international aim to prevent global warming beyond 2°C (3.6°F) over pre-industrial levels, a goal that some scientists say is already unrealistic. Meeting that target, the new paper says, would keep Antarctic melting in check, limiting the attendant sea level rise to two meters.

Blowing past that threshold, on the other hand, would stoke the disintegration of the entire ice sheet. Its potential to raise sea levels "far exceeds all other possible contributions" from melting elsewhere, including the Greenland Ice Sheet, the researchers say.

The Intergovernmental Panel on Climate Change says that in order to avoid warming beyond the two-degree target, humans must not emit more than 1,000 metric gigatons (or 1,000 billion tons) total of carbon dioxide. However, current cumulative emissions are already more than halfway there, and burning all accessible fossil fuels would release 10 times that limit.

"To put it bluntly, if we burn it all, we melt it all," says co-author Ricarda Winkelmann of the Potsdam Institute for Climate Impact Research. The higher temperatures would create a runaway cycle of ice melting and instability, she says: "Once these processes are started, on human time scales, it's impossible to stop them."

Currently, the researchers say, Antarctica contributes less than 10 percent of the global sea-level rise. If emissions are kept under control, a predicted increase in snowfall would offset the ice loss. If not, the region becomes a driving force for dramatic sea level hike.

Recently, a separate study found that a melting West Antarctic Ice Sheet could boost sea level nearly 8 inches by the end of this century. Dan Martin, a staff scientist at Lawrence Berkeley National Laboratory who co-authored that study, says the idea that the whole ice sheet would melt away in a high-emissions scenario, "I don't think, is actually surprising to anyone who's actually thought about these things."

Still, he says, "it's useful to think about where we're ending up, and how we get there is also important." He notes that researchers are still trying to figure out the details of exactly how climate change will affect the dynamics of ice sheets.

Winkelmann also acknowledges the complexity of the coming picture. The melting that's occurring in the West Antarctic Ice Sheet might already be unstoppable, but it's hard to tell exactly how and when the entire mass will respond, she says, because the effects vary from region to region.

Regardless, she notes that we are facing a sea level rise that's unprecedented in human history.

"Our study shows how important it is to think beyond the next decades, or even the next century," Winkelmann says, "because we're changing the face of the planet over the next 10,000 years."

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

On Twitter: Follow Christina Nunez and get more environment and energy coverage at NatGeoEnergy.

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Just How Much Could the Sea Rise from Burning Fossil Fuels? A Lot.

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New York City would be swallowed by the ocean. Tokyo and Shanghai, too, would vanish. Sea levels stand to rise by a staggering 164 feet (50 meters) or more if the world goes for broke on fossil fuels, burning all its attainable resources.

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Doesn't have quote but good info

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Global average sea level has risen by about 17 cm between 1900 and 2005. This is a much faster rate than in the previous 3,000 years.

The sea level changes for several reasons, including rising temperatures as fossil fuel burning increases the amount of greenhouse gases in the atmosphere. In a warming climate, the seas are expected to rise at faster rates, increasing the risk of flooding along our coasts. But until now we didn't know what fraction of the rise was the result of human activities.

In research published in Nature Climate Change, we show for the first time that the burning of fossil fuels is responsible for the majority of sea level rise since the late 20th century.

As the amount of greenhouse gases we are putting into the atmosphere continues to increase, we need to understand how sea level responds. This knowledge can be used to help predict future sea level changes.

Image credit: CSIRO

Measuring sea level

Nowadays, we can measure the sea surface height using satellites, so we have an accurate idea of how the sea level is changing, both regionally and in the global mean.

Prior to this (before 1993), sea level was measured by tide gauges, which are spread unevenly across the world. As a result, we have a poorer knowledge of how sea level has changed in the past, particularly before 1960 when there were fewer gauges.

Nevertheless, the tide gauge measurements indicate that global mean sea level has increased by about 17 cm between 1900 and 2005.

What drives sea level rise?

The two largest contributors to rising seas are the expansion of the oceans as temperatures rise, loss of mass from glaciers and ice sheets, and other sources of water on land. Although we now know what the most important contributions to sea-level rise are, we did not know what is driving these changes.

Changes in sea level are driven by natural factors such as natural climate variability (for example El Niño), ongoing response to past climate change (regional warming after the Little Ice Age), volcanic eruptions, and changes in the sun's activity.

Volcanic eruptions and changes in the sun affect sea level across years to decades. Large volcanic eruptions can cause a temporary sea-level fall because the volcanic ash reduces the amount of solar radiation reaching the ocean, thus cooling the ocean.

Humans have also contributed to sea level rise by burning fossil fuels and increasing the concentration of greenhouse gases in the atmosphere.

Separating the causes

We used climate models to estimate ocean expansion and loss of mass from glaciers and ice sheets for each of the individual factors responsible for sea level change (human and natural). To this we added best estimates of all other known contributions to sea level change, such as groundwater extraction and additional ice sheet contributions.

We then compared these model results to the observed global mean 20th century sea-level change to figure out which factor was responsible for a particular amount of sea level change.

Over the 20th century as a whole, the impact of natural influences is small and explains very little of the observed sea-level trend.

The delayed response of the glaciers and ice sheets to the warmer temperatures after the Little Ice Age (1300-1870 AD) caused a sea-level rise in the early 20th century. This explains much of the observed sea-level change before 1950 (almost 70%), but very little after 1970 (less than 10%).

The human factor

The largest contributions to sea-level rise after 1970 are from ocean thermal expansion and the loss of mass from glaciers in response to the warming from increasing greenhouse gas concentrations. This rise is partly offset by the impact of aerosols, which on their own would cause a cooling of the ocean and less melting of glaciers.

The combined influence of these two factors (greenhouse gases and aerosols) is small in the beginning of the century, explaining only about 15% of the observed rise. However, after 1970, we find that the majority of the observed sea-level rise is a direct response to human influence (nearly 70%), with a slightly increasing percentage up to the present day.

When all factors are considered, the models explain about three quarters of the observed rise since 1900 and almost all of the rise over recent decades (almost 90% since 1970).

The reason for this difference can be found either in the models or in the observations. The models could underestimate the observed rise before 1970 due to, for instance, an underestimated ice sheet contribution. However, the quality and number of sea level observations before the satellite altimeter record is also less.

Tipping the scales

Our paper shows that the driving factors of sea-level change have shifted over the course of the 20th century.

Past natural variations in climate were the dominant factor at the start of the century, as a result of glaciers and ice sheets taking decades to centuries to adapt to climate change.

In contrast, by the end of the 20th century, human influence has become the dominant driving factor for sea-level rise. This will probably continue until greenhouse gas emissions are reduced and ocean temperatures, glaciers and ice sheets are in equilibrium with climate again.
Aimée Slangen, Postdoctoral research fellow, Institute for Marine and Atmospheric Research, Utrecht University and John Church, CSIRO Fellow, CSIRO
This article was originally published on The Conversation. Read the original article. Main image: A glacier at South Georgia Island. Credit: Flickr/DavidStanley

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Burning Fossil Fuels is Responsible for Most Sea-Level Rise Since 1970

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By Aimée Slangen, Utrecht University and John Church, CSIRO Global average sea level has risen by about 17 cm between 1900 and 2005. This is a much faster rate than in the previous 3,000 years. The sea level changes for several reasons, including rising temperatures as fossil fuel burning increases the amount of greenhouse gases in the atmosphere.

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How sea level is effected by humans.

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This Saturday, March 28, 2015, the planet will be celebrating the ninth annual Earth Hour, where people around the world will turn off their lights at 8:30 p.m. local time for one hour. The organizers see this event as a planet-wide movement, reminding us-for just 60 minutes each year-that there are small things that we can do to minimize carbon emissions that are causing climate change.

There is one "continent" that will participate hardly at all in Earth Hour, yet it is arguably the one continent most directly affected by climate change, and which will, in turn, affect the climate on the rest of the planet. It has the highest mountains, the deepest valleys, and the vastest plains. It is inhabited by an unfathomable numbers of species, plant and animal. It has a total area of 155.557 million square kilometers, including an estimated 157,000 kilometers of coastline. It is the largest continent-larger than all the landmass in Earth put together. Oceania-the "Liquid Continent."

For decades it was fashionable to speak about the "Pacific Rim," which soon became equated simply with the word "Pacific." To speak of the Pacific is to speak of North America's West Coast, East and Southeast Asia, and-for the more bold-the Western countries of Latin America. That's the Pacific Rim. In between-what some of us called the Pacific Basin-is another land entirely. A land rendered invisible by the "Pacific Rim": Oceania.

One hears mention, in the talk about climate change, that certain small island nations in the Pacific- Tuvalu, in particular, and also Kiribati (pronounced KEE-ree-bahs)-are starting to disappear under the rising sea. As long as one thinks in terms of land continents, the loss of some tiny islands-just like the loss of some coastal Arctic native villages-can seem far away and insignificant. But Islanders are already cognizant of these effects:

Loss of coastal land and infrastructure due to erosion, inundation and storm surges;

Increase in frequency and severity of cyclones with risks to human life, health, homes and communities;

Loss of coral reefs with implications for the sea eco-systems on which the livelihood of many Islanders depends;

Changes in rainfall patterns with increased droughts in some areas and more rainfall with flooding in other areas;

Threats to drinkable water due to changes in rainfall, sea-level rise and inundation;

Loss of agricultural land due to salt-water intrusion into the groundwater;

Human health impacts with an increase in incidence of dengue fever and diarrhea.

But the impact of climate change on the ocean has huge implications not just for islanders, but for the planet.

Our own Enivornmental Protection Agency tells us that as greenhouse gases trap more energy from the sun, the oceans are absorbing more heat. Though it is less noticeable to us on land, this gradual rise in ocean temperatures will not only bring rising sea levels, but changes to the movement of heat around the planet by ocean currents. This will lead to alterations in climate patterns around the world.

Because the ocean, not the land, is the primary driver of our climatic system. Changes to the ocean affect changes to winds. One result already has been flooding events on the East Coast of the United States linked to sea level rise and changing wind patterns.

Coral bleaching (which kills corals), ocean acidification (which makes it harder for shell-building species to survive), migration of fish towards the poles (disrupting global fisheries), pollution and over-fishing are driving the ocean towards what some scientists see as a tipping point-not just for climate change, but for the ecology of the ocean itself.

What can we do, besides turn off our lights for an hour each year? Following on the meeting of Small Island Developing States (SIDS) in Apia, Samoa, last year, the voyage of the Hōkūleʻa-to raise awareness of the health of the oceans and therefore, of the Earth-has adopted the One Ocean, One Island Earth pledge. And we are all encouraged to do likewise. You can sign up for the pledge here. The pledge is simple:

I recognize that Earth is a blue planet. Our ocean is the cornerstone of life, and our planet's life-support system.

No matter where on Island Earth I live, the ocean produces the air I breathe and helps to regulate the climate.

I recognize that our ocean and Island Earth is changing because of the habits and choices of human beings.

I recognize that with supporters like me, and the community I reach out to around me, the future of our oceans and our Island Earth can improve.

The difference will start with me and spread to others. I pledge to support our oceans and Island Earth, and inspire people of all ages to do the same.

As the Polynesian voyaging canoe Hōkūleʻa circles the globe on its worldwide voyage, the crew seeks to find and share stories of hope that can bring us all together to care for the one ocean and the one island earth that we share. It takes a people from Oceania-who see the ocean not as empty space but as a dynamic realm much greater than land-to teach us the importance of caring for the oceans.

For those of us who grew up on the great land continents, and have always thought of the sea as something one ventures out onto once in a great while, and possibly with some hesitance, the sea is simply that great blue empty space. On many maps it is colored a uniform, solid blue. But for the people of Oceania, the sea is places, roads, highways. It is gods, and mysteries, and destinies. It is a medium that connects the small bits of land where people rest between journeys. And for all of humanity-indeed, all species of life on the planet-it is the Great Source.

Just as all rivers flow ultimately to the sea, so all of human activity is tied to the ocean, for better and for worse. For among the great metropolises of the world, the ocean is the dumping ground. It is where the polluted rivers flow, where the garbage ends up, where nuclear waste is stored. If you want to get rid of it, throw it in the sea. The sea is infinite after all, isn't it?

What we now know is that human garbage is ending up in the deepest and most remote parts of the ocean. Just as our lands have absorbed the toxic by-products of industry and agriculture, and the atmosphere has absorbed the carbon dioxide and other gaseous and particulate output of smokestacks and exhaust pipes, so too the ocean-that great being that has always seemed capable of absorbing everything without consequences-shows many of the same sad signs of abuse. The lesson is clear: we can no longer thoughtlessly throw things "away." There is no "away" anymore. Not even in the ocean.

In a previous article, I wrote how the Earth is like an island, and like a canoe (a large voyaging canoe, that is): what we have is all we have, and as we would do aboard ship, we must take care of the vessel that carries us, that we may survive and thrive. And strange as it seems, as we think about Island Earth, we must recognize that the ocean is part of that island too. Here I offer some thoughts in that regard.

In his book Thinking History Globally, Diego Olstein reminds us that "Oceanic histories transcend enclosed political and regional boundaries by privileging bodies of water rather than the terrestrial domains." An oceanic perspective focuses on sea-borne connections between human societies. And with globalization, those sea-borne connections have become much greater. The sea that surrounds us, which Tongan author Epeli Hau'ofa wrote about, in talking of how the peoples of Oceania see the ocean as connecting, rather than separating, now holds true for the entire planet. The once-mighty continents themselves are now islands in the sea, and we who dwell on them should learn to understand them as such.

As climate change and other human-induced environmental issues increasingly close in on us, we are standing on the edge of a new journey forward: one that demands the best of us, the greatest wisdom, the wisest plotting of the course forward. Where the ancestors of Oceania's peoples used painstaking observation of the oceanic elements, trial and error, commitment, determination and innovation to design vessels that could journey out into the unknown, so too do we now need deep vision and courage to visiualize a new beginning that will take us beyond the horizons set by the dominant worldview. As Tongan scholar Winston Halapua has said, "We need new ways of thinking and new ways to approach the huge challenges that confront us and will demand the best of all our energies and will be a deeply authentic way to journey forward."

One Ocean, One Island Earth. The earth is the canoe taking us on our journey into the future. We are all in the same boat. And 70 percent of that "boat" is the ocean.
The Smithsonian Institution will participate in this year's Earth Hour. Tonight the American Indian Museum, Air and Space Museum, Natural History Museum, National Zoo, Hirshhorn Museum, and Castle will go dark to show commitment to sustainability.

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What Climate Change Will Mean for the People of Oceania

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This Saturday, March 28, 2015, the planet will be celebrating the ninth annual Earth Hour, where people around the world will turn off their lights at 8:30 p.m. local time for one hour. The organizers see this event as a planet-wide movement, reminding us-for just 60 minutes each year-that there are small things that we can do to minimize carbon emissions that are causing climate change.

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No quote but good information

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Overview

The seas of the Earth are rising, a direct result of a changing climate. Ocean temperatures are increasing, leading to ocean expansion. And as ice sheets and glaciers melt, they add more water. An armada of increasingly sophisticated instruments, deployed across the oceans, on polar ice and in orbit, reveals significant changes among globally interlocking factors that are driving sea levels higher.

Yet the globally averaged trend toward rising sea levels masks deeper complexities. Regional effects cause sea levels to increase on some parts of the planet, decrease on others, and even to remain relatively flat in a few places, including, in recent decades, on the California coast. Thermal expansion of seawater can be the product of regional phenomena, such as El Niño, the periodic warming of the eastern tropical Pacific. But some of these regional cycles so far show no direct link to long-term global climate change-despite, at times, independently exerting a powerful short-term influence on global climate.

And while Greenland, Antarctica and most of the world's glaciers are melting, a distinction must be made between glacial discharge into the oceans, a more permanent type of ice loss, and changes in the precipitation and evaporation that is feeding those glaciers and ice sheets, which fluctuate on the scale of decades.

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The seas of the Earth are rising, a direct result of a changing climate. Ocean temperatures are increasing, leading to ocean expansion. And as ice sheets and glaciers melt, they add more water. An armada of increasingly sophisticated instruments, deployed across the oceans, on polar ice and in orbit, reveals significant changes among globally interlocking factors that are driving sea levels higher.

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How sea level is effected by humans.

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WASHINGTON, DC - Human-generated emissions are largely responsible for warming the top layer of the ocean over the past four decades, causing water to expand and sea level to rise, according to a new study.

Climate models show that human activities, like burning fossil fuels, are responsible for 87 percent of the sea level rise since 1970 that's been caused by swelling volume of the upper ocean. Natural forces, like solar radiation and volcanic activity, are responsible for the remainder of the increase in the upper ocean's share of warming-induced, or thermosteric, sea level rise, according to a new study recently published in Geophysical Research Letters, a journal of the American Geophysical Union.

"The fact that greenhouse gases from human activity have an impact (on thermosteric sea level rise) is well known but nobody up to now has been worried about putting a number on that," said Marta Marcos, a researcher at the Mediterranean Institute for Advanced Studies in Mallorca, Spain and the lead author of the new research. The analysis is the first to quantify how much of an influence human activity has had on thermosteric sea level rise in the top layer of the ocean, she added.

Figuring out how much of an influence humans are having on specific climate variables, like thermosteric sea level rise, is important for assessing what will happen as the planet warms, Marcos said. Sea level rise can cause flooding and erosion in low-lying coastal areas.

Thermosteric sea level rise is the increase in ocean height caused by the increasing volume of the ocean that occurs when seawater warms up and expands. Since 1970, global sea level has been rising by 2 millimeters (.08 inches) per year, of which .8 millimeters (.03 inches) per year* is due to increased warming in all layers of the ocean. More than half of that rise, or .46 millimeters**, results from thermal expansion specifically in the upper-ocean layer, or the top 700 meters of seawater. The rest of the increase in global sea level is caused by melting ice, and changes in reservoir levels and the shape of the Earth.

The new study, showing that the top layer of the ocean is warming mainly because of human activity, "means that all the consequences of this warming - sea level rise, higher storm surges - are because of our activity," Marcos said. "There's almost nothing natural going on."

To calculate the human influence on thermosteric sea level rise, the researchers used the latest climate models that simulate the rise in thermosteric sea level in the top 700 meters (2,297 feet) of the ocean from 1950 to 2005. This historical simulation included both the influence of natural variability, and greenhouse gases and aerosols generated by human activity. They then looked at another simulation that examined only the contribution of natural variability to thermosteric sea level rise over this same time period. The difference between these two simulations allowed the researchers to calculate how much of the thermosteric sea level rise can be attributed to emissions of gases associated with human activity.

The researchers found that human activity was responsible for an increase in thermosteric sea level rise of .40 millimeters (.016 inches) per year from 1970 to 2005, compared with an increase of .46 millimeters (.019 inches) per year during the same period as a result of both natural variability and human activity. This means that 87 percent of the observed global thermosteric sea level rise of the upper water column of the ocean since 1970 is associated with human activity, according to the paper.

"The difference is so big, the number is so high, that quite clearly the impact of human activity is evident," Marcos said.

The study also found that the human contribution to thermosteric sea level rise varies by region. Human activity is responsible for all of the increase in thermosteric sea level rise in the Tropical Pacific and the Southern Ocean, while natural variability accounts for 35 percent of the warming in the North Atlantic, according to the study.

Marcos said although the concentration of greenhouse gas and aerosol emissions has increased ocean temperature globally, the entire ocean does not warm at the same rate. Changes in wind conditions and ocean circulation can affect how much heat each region of the ocean takes up and how it is distributed around the world. Each ocean region has its own characteristics and will respond differently to global warming, she said. Marcos said she plans to more closely examine these regional differences.

* This number comes from Intergovernmental Panel on Climate Change, Fifth Assessment (2013) and covers the period from 1970 to 2010.

** This number is based on numerical modeling of the period from 1970 to 2005 - the time period studied by Marcos and her colleagues. Marcos says that the numerical models on which her study is based stop in 2005.

Press release from the Mediterranean Institute for Advanced Studies (in Spanish): http://www.imedea.uib.es/noticias.php?nid=MTMwMA==
- Nanci Bompey, AGU Public Information Specialist - Writer

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Humans caused nearly 90 percent of sea level rise from warming of upper ocean, study says - GeoSpace

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WASHINGTON, DC - Human-generated emissions are largely responsible for warming the top layer of the ocean over the past four decades, causing water to expand and sea level to rise, according to a new study.

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How sea level is effected by humans. No Quotes

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Global sea level has been rising over the past century, and the rate has increased in recent decades. In 2014, global sea level was 2.6 inches above the 1993 average-the highest annual average in the satellite record (1993-present). Sea level continues to rise at a rate of about one-eighth of an inch per year.

Higher sea levels mean that deadly and destructive storm surges push farther inland than they once did, which also means more frequent nuisance flooding. Disruptive and expensive, nuisance flooding is estimated to be from 300 percent to 900 percent more frequent within U.S. coastal communities than it was just 50 years ago.

The two major causes of global sea level rise are thermal expansion caused by warming of the ocean (since water expands as it warms) and increased melting of land-based ice, such as glaciers and ice sheets. The ocean is absorbing more than 90 percent of the increased atmospheric heat associated with emissions from human activity.

With continued ocean and atmospheric warming, sea levels will likely rise for many centuries at rates higher than that of the current century. In the United States, almost 40 percent of the population lives in relatively high-population-density coastal areas, where sea level plays a role in flooding, shoreline erosion, and hazards from storms. Globally, eight of the world's 10 largest cities are near a coast, according to the U.N. Atlas of the Oceans.

Sea level rise at specific locations may be more or less than the global average due to local factors such as land subsidence from natural processes and withdrawal of groundwater and fossil fuels, changes in regional ocean currents, and whether the land is still rebounding from the compressive weight of Ice Age glaciers. In urban settings, rising seas threaten infrastructure necessary for local jobs and regional industries. Roads, bridges, subways, water supplies, oil and gas wells, power plants, sewage treatment plants, landfills-virtually all human infrastructure-is at risk from sea level rise.

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national oceanic and atmospheric administration

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Global sea level has been rising over the past century, and the rate has increased in recent decades. In 2014, global sea level was 2.6 inches above the 1993 average-the highest annual average in the satellite record (1993-present). Sea level continues to rise at a rate of about one-eighth of an inch per year.

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How is sea level effected by humans. No Quote

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Photo By shayanlinux/ CC BY-NC-ND 2.0

Erin A. Thead

October 2016

One of the most dire impacts of anthropogenic climate change is a rise in the global sea level caused by the melting of glaciers and land-based ice caps, as well as a smaller increase from expansion due to the higher temperature of the water itself. Unlike some other predicted effects of climate change, this impact has already been observed for some time. Indeed, not only is there evidence that sea levels are rising; there is also evidence both that the rate of sea level rise has been increasing in recent years and that it will continue to increase. Figure 1 shows the average sea level increase from its 1993-2013 midpoint.

Figure 1. 2014 sea level compared to the mean sea level from 1993-2013. Credit: National Oceanic and Atmospheric Administration, Climate.gov

As Figure 1 makes clear, the impact of sea level rise is not felt equally around the globe. Some locations experience greater rise than others because of local terrain, local hydrological factors, and oceanic currents, among other regional factors. Unfortunately, many large cities are located on coastlines that are particularly vulnerable to sea level rises. Improvements in high-resolution modeling have made it possible for the expected sea level rises in specific locations to be mapped, both in worst-case and expected-case scenarios. This very detailed awareness of the hazard has, in many locations, led to positive planning and actions to mitigate the impact. This article will describe the latest scientific thinking about the magnitude of global sea level rise, detail some solutions that coastal cities around the globe are implementing to mitigate risk, and offer general suggestions for good resilience planning for locations that will be impacted by this threat.

Recently, a team of scientists published a study that found that the rate of sea level rise in the 20 th century was greater than it had been in 2,800 years. Sea levels have risen almost 3 inches globally in the most recent 20 years and rise on an average of 1/8 inch each year. Melting land ice is responsible for a larger-and ever-increasing-amount of the global sea level rise in recent decades, as opposed to thermal expansion of seawater. A compounding problem with ice melt is that it can accelerate through positive feedback. Snow-covered ice has a high reflectivity or albedo, which means that radiation is reflected back from it and not as much is absorbed as heat. Water, however, has a very low albedo and does absorb high amounts of solar radiation and heat held in the earth's lower atmosphere. Once ice has started melting, the process accelerates without an external influence to cause refreezing. Short-term climatic patterns that temporarily raise global temperature averages, such as a strong El Niño event, can therefore exacerbate the long-term trend unless a strong cooling event occurs quickly thereafter.

The current best estimates predict that sea level will rise up to 6.6 feet, or 2 meters, by the year 2100. Until recent years, this figure was viewed as pessimistic, with a rise of 3 feet considered more likely. Recent studies raise the concern that the 6.6-foot estimate is actually the more probable one with "business as usual" carbon emissions. Earlier work accounted for glacial and Arctic melt, but had greater uncertainty about the West Antarctic Ice Sheet. The new research, developed in the last three years, modeled that the West Antarctic sheet would be undermined by warmed seawater, accelerating its decline. The study also found that adhering to the agreements in the Paris climate summit of 2015, and thereby keeping the mean global temperature increase under 2°C, would lessen the melting of the West Antarctic Ice Sheet. Nonetheless, even in this optimistic case, some sea level rise will continue to occur due to current greenhouse gas levels in the atmosphere and the attendant warming.

Several American coastal cities have begun plans to minimize the effects of sea level rise. In New York City, the areas of the city that are in the "100-year flood zone" and "500-year flood zone"-in other words, that have a 1 percent chance and 0.2 percent chance of flooding each year-are expected to expand. Figure 2 shows a map of New York City with expected flood zones that are based on the projection of 2.5-foot global sea level rise by the year 2050.

Figure 2. Current and projected future flood risks for New York City assuming 2.5 foot sea level rise by 2050. Credit: Federal Emergency Management Agency/National Oceanic and Atmospheric Administration

In response to the flood analysis and the destruction wrought by Sandy, the city developed a comprehensive resilience plan. In this plan, the city specifically analyzed the effects of Sandy as a near-worst-case impact and the projected future flood zones. It determined several actions to take to minimize the risks. To protect coasts against tidal flooding, the city plans to reinforce beaches, build bulkheads, and protect sand dunes that act as natural barriers. The city may also enact rock breakwaters offshore to attenuate waves associated with storms, and erect storm walls and levees in areas that are particularly vulnerable to storm surge. The city's plan contains a rigorous geological analysis of the landscape and makes recommendations specific to boroughs and neighborhoods based on what types of mitigation strategies the rock and soil in each locale can support.

Boston is another American city that has developed a comprehensive climate resiliency plan. Since it is on a coastline, Boston's greatest risks from climate change are flooding and storm surge. Boston's plan emphasizes community awareness and education as critical tools for climate preparedness. As Brian Swett, a lead developer of Boston's climate plan and formerly the chief of Boston's Department of Environment, stated with regard to storm surge,

If I told somebody, even in July, that we're expecting 20 inches of snow in two days, everybody has that mental checklist up here in Boston: Make sure I know where the shovel is; I know the streets not to park on because there'll probably be a parking ban; I got my bread and my milk; I've got salt for the walkway. We don't have that mental checklist up here for hurricanes and we certainly don't drill for it [...].

The Boston climate plan also puts emphasis on outreach to low-income households, small business owners, and other vulnerable residents, by working closely with other city departments that focus on these residents' concerns. Low-income residents in particular utilize public services such as buses more than other residents, and as the tragedy of Hurricane Katrina in New Orleans showed, these residents are most vulnerable in the event of a needed evacuation.

Cities that have smaller populations than the East Coast metropolises and rely heavily on tourism for economic development are also making preparations. Tybee Island, a barrier island near Savannah, Georgia, is a popular tourist destination during the summer months. The sea level on this island's coastline has risen 10-11 inches since 1935, and as in most areas of the world, it is expected to continue rising. The city, also named Tybee Island, developed a resilience plan to cope with rising sea levels. The city has a single road that allows access on and off the island, so shoring up this road-U.S. Highway 80-and improving it to minimize traffic bottlenecks was a key part of the city's resilience plan. As in other cities, reinforcing natural barriers to inland flooding was also important, so protecting Tybee Island's beaches from erosion was another crucial part of the plan.

Cities elsewhere around the world have begun to grapple with the risks of sea level rise as well. In Australia, coastal cities face the threats of tidal flooding, non-tropical storm flooding, and tropical cyclone storm surge just as cities in the U.S. do. Australian states and municipalities also have significant authority over their own policies, comparably to the U.S. The national government of Australia has issued a strategic plan for climate resilience and adaption, which recommends procedures to states and municipalities.

The Australian resilience plan acknowledged that coastal cities were built with the assumption that weather and tidal conditions would fall within a known historical range that includes a stable sea level, and therefore that expected rises from climate change pose a threat. The government of Australia is in the process of developing an online tool, known as CoastAdapt, that will help local officials understand specific risks their areas face and provide specialized information about resilience options.

A specific example of this type of local resilience planning is the analysis of seawalls in Sydney. The city has several older seawalls, and authorities were unsure of their reliability in the face of climate change and extreme events. The government of Australia oversaw a project to assess the seawalls for their current condition that included analysis of the materials used in the seawalls, maintenance, stability, and strength. The project officials then proposed improvement suggestions for each seawall examined.

Many cities in Europe are also vulnerable to sea level rise. European cities are not at risk of hurricane storm surge due to their northerly latitudes and location on coastlines that do not experience tropical cyclones, but they are vulnerable to tidal flooding and non-tropical storm flooding. Some are also built below sea level and rely on levees for protection.

An example of the latter is the city of Rotterdam in the Netherlands, which is 90 percent below sea level and surrounded by several rivers. Dikes protect the city from inundation. In 1953, Rotterdam suffered an extremely deadly flood that breached the dikes, killing 1,800 people, a tragedy that foreshadowed Hurricane Katrina in New Orleans fifty-two years later. In response, the Dutch government directed funding to a massive project to build dikes around areas of the city. Unfortunately, over the years the structures have damaged the environment and aquatic ecosystems. In a more sustainable, ecologically sound, and physically robust plan, the government is now shoring up the natural coastline of the city by rebuilding sand dunes and expanding shores with sediment.

As we have seen, governments at all levels are assessing the dangers for specific locations and analyzing the current infrastructure the world over. It is possible to model expected flood risk at an extremely high resolution and perform engineering analyses on existing infrastructure-natural and manmade-with a great degree of precision. The results are a set of plans tailored for the specific needs and capabilities of each location. Climate change does not affect all parts of the earth in the same way, and even sea level rise will not be globally uniform, so highly individualized resilience planning is a must. Their diversity notwithstanding, however, the plans do have some things in common. The ideas that appear repeatedly, in resilience plans around the globe, do so because they are broadly applicable, and in many cases, planners have arrived at them through past experience. Cities that wish to develop their own coastal resilience plans should look to these repeat ideas as guidance.

First, coastal cities at particular risk of flooding should protect any natural barrier islands that are present. These islands are the first line of defense against storm surge, whether from tropical cyclones or other storms at sea. Cities that can afford it, and can do so in an environmentally friendly way, might follow the example of New York City and erect artificial breakwaters offshore if they do not have any natural barrier islands. These structures could also serve as artificial reefs for marine life if oceanic conditions permit.

Many coastal cities have artificial seawalls and levee structures. These structures tend not to be as robust against extreme events as natural barriers, but cities that have them should follow the example of Sydney and examine them in close detail to determine their robustness. Of course, to be fully effective, civil authorities should conduct this type of analysis with an eye to the specific level and type of risk that a given city is expected to face from sea level rise. Resilience planners in very low-lying locations should also keep in mind the lessons of New Orleans and Rotterdam, emphasizing shoreline and wetland restoration as the first and best defense instead of relying wholly on a system of levees to fight the natural course of rivers. For locations that are below sea level, natural restoration approaches are more robust against flooding and have proved far better for surrounding ecosystems. Cities should not neglect existing seawalls and levee structures, but they should be part of a broader strategy.

Finally, resilience analysts should always consider the human factor, particularly in the context of extreme flood events that would pose a high threat to life and require partial or full evacuation of the city during the emergency. This type of risk is especially acute for cities that are low-lying, prone to storms, or located at the mouths of significant river systems. Officials should take guidance from Boston's plan, which emphasizes promoting community education about the flood threat and devoting special attention to vulnerable populations that rely on public services. The plan for Tybee Island, a much smaller municipality, takes into account its reliance on tourism and the dependence of its evacuation route on a single highway. Planners should always consider the specific local needs of a city, whatever its size.

Climate change is already causing sea levels worldwide to rise, and we can only expect this trend to continue. Our best, most current science predicts that ice cap melting and thermal expansion of seawater will produce a combined average rise of up to 6.6 feet by the beginning of the next century. This level of rise would inundate some beaches and overflow many barrier islands that serve as natural protection against storm surge from tropical and non-tropical cyclones. It would also raise the risk of tidal flooding, and in areas that are expected to see an increase in rainfall, flash flooding and river flooding would compound the flood risk associated with coastal waters. The risks of sea level rise to coastal cities must be taken seriously, and the kinds of concrete, specific, individually tailored flood resilience plans illustrated here are a very positive step. While emissions reductions can lessen the magnitude of this impact, some rise is going to occur. It is imperative for areas at risk to adapt to this new hazard, and fortunately, they are beginning to do just that. These pioneering coastal cities have created plans that offer excellent guidance. Hopefully, resilience planners in other locations around the world will follow their lead.
Erin A. Thead is a Graduate Research Fellow at the Climate Institute and a doctoral candidate in Earth and Atmospheric Sciences at Mississippi State University.
Downloadable pdf with references available here.

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Sea Level Rise: Risk and Resilience in Coastal Cities

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Photo By shayanlinux/ CC BY-NC-ND 2.0 Erin A. Thead October 2016 One of the most dire impacts of anthropogenic climate change is a rise in the global sea level caused by the melting of glaciers and land-based ice caps, as well as a smaller increase from expansion due to the higher temperature of the water itself.