Sea-Level Rise Will Continue Even with Aggressive Emission Mitigation
Researchers working on Jaguar discover commitment to climate change effects
The thought of rising sea levels often conjures images of melting glaciers and ice sheets collapsing into the sea. However, estimates are that these dramatic events currently account for less than half of the rising ocean levels.
There is another cause, one that is just as important but far less dramatic: thermal expansion. As water warms, it expands, and when it warms at the scale of the world’s oceans, the outcome is rising ocean levels. Researchers led by Gerald Meehl of the National Center for Atmospheric Research (NCAR) used Oak Ridge National Laboratory’s (ORNL’s) Jaguar supercomputer to explore just how much sea level is likely to rise and under what circumstances.
The scientists found that sea level will continue to rise substantially, even if the global community takes aggressive action to slow climate change. The group released its findings in a Nature Climate Change article in July 2012.
“We can mitigate temperature increase, but it’s much harder to mitigate sea-level rise.” Says Meehl, “The reason it’s harder to mitigate sea-level rise is that the climate system has a very long memory. There’s this aspect called climate change commitment.”
The researchers ran simulations for four greenhouse gas mitigation scenarios, but the article published in Nature Climate Change focused on three, ranging from a very aggressive approach under which more carbon dioxide would be removed from the atmosphere than added by the year 2070 to a hands-off approach under which essentially nothing is done to curb carbon emissions. They concluded that while aggressive measures could slow the rise in sea level, it could not halt it altogether.
Under the most aggressive approach, the global average temperature would actually cool, but thermal expansion alone would cause sea level to rise about 5½ inches by 2100. The added effect of melting glaciers and land ice is uncertain; Meehl and his team used estimates from previous studies to predict that the total rise would be about 10 inches by 2100. By 2300 the total rise would be over 1½ feet.
Under the middle-of-the-road strategy, the simulations predicted a total rise of over 3 feet by 2300. The least aggressive mitigation scenario led to a rise of over 8 feet by 2300. Such an outcome would be devastating.
Sea level does not rise uniformly around the globe. Because of ocean circulation and wind pressure patterns, certain coastlines will see greater rises in their sea levels and will suffer more harmful effects. However, an average rise of over 8 feet could be expected to be catastrophic.
According to the Intergovernmental Panel on Climate Change, a rise in sea level of less than one foot would leave more than 700,000 Nigerians homeless as their coastal towns are destroyed. Approximately 10 percent of the world’s population currently lives in low-lying areas that will be much more susceptible to flooding. Big cities like London, New Orleans, and New York City will have to increase their storm surge defenses even under the most optimistic scenario.
If the sea level were to rise by 8 feet, cities would find it costly or even impossible to protect themselves from flooding, reports the Strategic Environmental Research and Development Program, which is affiliated with the Department of Defense, the Environmental Protection Agency, and the Department of Energy. Sea-level rise of this magnitude would also create major environmental problems, like damaging natural defense systems and contaminating fresh water sources of water with saltwater, which would in turn negatively impact humans.
Using the emission mitigation scenarios, the researchers ran climate simulations using the Community Climate System Model (CCSM4), which integrates simulations of atmosphere, ocean, land and ice. To draw their conclusions, the scientists needed models to go out to 2300 running five simulations for each of the four mitigation scenarios. All told, they simulated a total of 4,500 years, which would have been impossible without a powerful supercomputer like Jaguar.
“One of the limitations for including the level of detail we need in climate modeling is how fast the computers are.” Says Meehl, “The faster the supercomputer, the more realistic climate models we can run.”
The sea’s thermal expansion will continue even after temperatures have leveled off or declined because the surface of the ocean will heat first. Then, that warmer water will be pushed downward and deeper layers of the ocean will warm.
“It takes a long time to warm up the whole depth of the ocean, but it will eventually,” says Meehl. “That heat that you put into the surface will work its way down deeper and deeper into the ocean. The time scale for thermal expansion to stabilize is the time scale it takes to warm up basically the entire ocean.”
According to the Nature Climate Change paper, sea level will continue to rise regardless of the aggressiveness of mitigation, but it will rise much more if less stringent measures are taken. And it will rise faster, leaving less time for coastal areas to adjust. The researchers hope that their findings will help in urging governments to adopt more aggressive greenhouse gas mitigation techniques.
“If we are able to mitigate and stabilize the climate change, it would buy us more time to actually adapt to this ongoing rise of sea level,” says Meehl.
Now that the researchers have begun to quantify how much sea-level rise from thermal expansion can be expected, future studies will focus mainly on calculating how much rise ice melt will contribute. Meehl expects that sea level will continue to rise from this source as well, even after temperatures have leveled off. The question is how much more ice will melt under each mitigation scenario and for how long the melt will continue.
Says Meehl, “[Big ice sheets like Greenland and Antarctica] won’t just sit there like big ice cubes and slowly melt. There will actually be dynamic processes that would make them disintegrate faster than you’d expect. We have to observe how they behave and the physics of how they function and then try to capture those processes in models.”—by Leah Moore
Meehl, G.A., et al. 2012. “Relative outcomes of climate change mitigation related to global temperature versus sea-level rise,” Nature Climate Change, 2, pp 576-580.