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Underwater: how much will sea level rise?

May 17, 2020

Ice
Photo credit: Elaine Heron

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How much will sea level rise in the next hundred years? A team of researchers posed this question and others to 500 of the world's leading experts on sea level rise. The results from this survey, published May 8 in the journal Nature, give us an overview of their understanding.

The surveyed scientists were asked to consider the likely sea level rise under two greenhouse gas scenarios proposed by the Intergovernmental Panel on Climate Change (IPCC). In the best case scenario (called the RCP 2.6), greenhouse gas emissions will decrease substantially by the end of the 21st century, with energy sources shifting away from oil and toward bio-energy, natural gas, nuclear, solar, wind, and geothermal power production. In the panel's worst case scenario (RCP 8.5), greenhouse gas emissions continue to increase along their current trajectory.

These scientists predicted that by the year 2100, the sea levels will likely rise by 0.3 – 0.7 meters (1.0 – 2.1 ft) in the best case scenario and 0.6 – 1.3 meters (2.1 – 4.3 ft) in the worst case scenario. By the year 2300, they predicted sea levels would rise 0.5 – 2.2 m (1.8 – 7.1 ft) in the best case, and 1.7 – 5.6 m (5.5 – 18.4 ft) in the worst case scenario.

The next hundred years: where are we going?

Which scenario will come to fruition is not yet known. In the best case scenario, the increase in global average surface temperatures remains within 2°C of pre-industrial levels, preventing some of the most serious impacts of climate change. The global community has coalesced around this target, codifying it in the Copenhagen Accord in 2009 and again in the the Paris Agreement of 2016.

However, as of 2017 none of the major industrial countries had met their pledge under the voluntary accord, and in 2019 the United States formally indicated its intention to withdraw from the Accord later this year. According to NASA, average temperatures have already increased just over 1°C.

The worst-case scenario, RCP 8.5, may be equally unlikely. For instance, maintaining our current level of emissions over the long term would require a significant increase in coal production, which some scientists have noted is inconsistent with our current understanding of coal reserves and coal markets (unless coal subsidies are introduced in large scale).

Where's the ice?

According to the IPCC, there are four major contributors to sea level rise: the thermal expansion of oceans as temperatures rise, melting glaciers, and the shrinking of the two major ice sheets of Greenland and Antarctica.

Thermal expansion, caused by the fact that warmer water is less dense and so takes up more space, is estimated to be responsible for about half of the sea level rise in the last 25 years.

Ice
The Columbia Glacier. The loss of ice from this Alaskan glacier alone was responsible for 1.3% of global sea level rise from 1993-2010. Photo credit: C Watts.

An enormous amount of water is contained in the Greenland and Antarctic ice sheets. If both of these ice sheets were to completely melt, scientists calculate that sea levels would rise an astonishing 66 meters (217 ft). Measuring changes to land ice masses was historically difficult, but modern satellite imagery have revolutionized scientists' ability to track ice melt.

In the 1990s, the Greenland Ice Sheet began to consistently melt more ice each year than it regains in winter months. According to one NASA study, melting of the Antarctic Ice Sheet's glaciers may be at least partially offset by snow compacting into ice, from snow accumulation over the past 10,000 years. More recently, a study in the journal Science found that from 2003-2019, melting from these two ice sheets totalled 318 gigatons of ice mass per year, contributing to 14 mm sea level rise from 2003-2019.

Change in ice sheet mass (since 2002)

Source: Land ice sheet data (Greenland and Antarctica) from the National Aeronautics and Space Administration's (NASA) GRACE and GRACE Follow-On satellites. Wiese, D. N., D.-N. Yuan, C. Boening, F. W. Landerer, and M. M. Watkins (2019) JPL GRACE and GRACE-FO Mascon Ocean, Ice, and Hydrology Equivalent Water Height RL06M CRI Filtered Version 2.0, Ver. 2.0, PO.DAAC, CA, USA. Dataset accessed 2020-05-01 at http://dx.doi.org/10.5067/TEMSC-3MJ62. Accessed May 2, 2020.

According to NASA, as of 2019 the Greenland ice sheet has lost 4,779 gigatons of mass since 2002. Antarctic ice shelf melting has been slower, with 2,316 gigatons lost in the same timeframe. For reference, one gigaton is equivalent to 400,000 Olympic swimming pools: so about 2.8 billion pools have melted in the polar ice sheets in 17 years. Another way of comparing it: seven gigatons of melt is more than four times the amount of water that went over the Niagara Falls in the same time period.

Since sea ice (such as floating icebergs) is already in the water, it does not contribute to sea level rise when it melts. However, this kind of ice is a crucial component of polar animal habitats. As sea ice disappears, species such as polar bears, seals, walruses, fish, ice algae, and crustaceans are likely to experience devestating loss of habitat.

Polar sea ice extent, 2004-2019

Source: Sea ice data from the northern (left) and southern (right) hemispheres are from the U.S. National Ice Center's Daily Ice Analysis Products, from the National Oceanic and Atmospheric Administration (NOAA). Data are from September in the northern hemisphere and March for the southern hemisphere.

What's in a meter?

One-half meter, one meter, three meters: what is the impact of sea level rise on the planet?

Sea level rise, in combination with increased storms and extreme weather, will risk the erosion and destruction of coastal regions inhabited by birds, turtles, and sea life. Coastal regions are a critical part of ocean ecosystems; in Florida, for example, an estimated 90% of commercial fish species are dependent on mangrove forests, the thick wooded areas in salt marshes and tidal estuaries.

The loss of coastal habitat makes cities and waterfront communities increasingly vulnerable: according to one study, the number of people exposed to flooding, erosion, and physical harm could be reduced by half simply by protecting habitats like coral reefs, seagrass beds, kelp forests, and marshes. Yet the destruction of coastal ecosystems has weakened these natural barriers against sea level changes.

The IPCC warns that the impact of sea level rise is not limited to land becoming permanently flooded. Rather, greater frequency of storm surges, changing tides, and extreme storms like hurricanes and typhoons will make coastal regions increasingly unstable and vulnerable to flooding and erosion.

The human impact

The number of people that live in low elevation coastal areas is immense. By the year 2100, the number of people that live at elevations less than 10 meters above sea level is projected to be 1.06 billion. By the end of the 21st century, coastal communities will face more frequent storms, more severe flooding, and permanent land erosion.

Population in low elevation coastal zones (<10m) in 2100

Select a metric:

Source: Center for International Earth Science Information Network - CIESIN - Columbia University. 2013. Low Elevation Coastal Zone (LECZ) Urban-Rural Population and Land Area Estimates, Version 2. Palisades, NY: NASA Socioeconomic Data and Applications Center (SEDAC). https://doi.org/10.7927/H4MW2F2J.

China has the largest population impacted by sea level rise, with an estimated 230 million people projected to be living in low elevation coastal regions by the year 2100. Major cities such as Shanghai (see below) and Tianjin, in the northeast of the country, are in low-lying areas.

Proportionally, small island countries are evidently the most at risk. At least three-quarters of the total population of several small island nations will be less than 10 meters above sea level, including Maldives, Cayman Islands, Turks and Caicos Islands, Bahamas, and Marshall Islands.

Other countries with large at-risk populations include the Netherlands, where 72% of the country's population is projected to live in low elevation coastal regions by 2100. Flooding today is held back by a network of dikes, canals, and floodgates, with Amsterdam lying about 2 meters below sea level. Large proportions of Greenland (58%), Viet Nam (55%), and Egypt (46%) will be similarly at risk by 2100.

Low elevation cities: less than 10m (yellow) or 1m (blue) above sea level

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Source: Raster files are NASA's Worldwide 3" digital elevation model, taken from Viewfinder Panoramas. Basemaps are from © Esri — (Esri, i-cubed, USDA, USGS, AEX, GeoEye, Getmapping, Aerogrid, IGN, IGP, UPR-EGP, and the GIS User Community). Shaded regions represent elevations less than 1 meter (blue) and 10 meters (yellow).




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