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Targeting four pollutants could reduce sea level rise

Black carbon, a short-lived pollutant (shown in purple), shrouds the globe. Credit: NOAA, NSF.

The US National Research Council projects that sea levels will rise by up to a meter (20-39 inches) by 2100, specifically due to human-caused increases in greenhouse gases. However, a new study published in Nature Climate Change indicates that cutting four kinds of emissions - methane, tropospheric ozone, hydrofluorocarbons, and black carbon - could reduce the warming trend by 25 to 50 percent by 2050. Such progress is possible because these heat-trapping pollutants cycle through the atmosphere much faster than carbon dioxide.

Funded by the US National Science Foundation and the US Department of Energy, the study is a collaboration of the Scripps Institution of Oceanography, the National Center for Atmospheric Research (NCAR), and Climate Central. Using the NCAR-based Community Climate System Model and a second computer model, the research team simulates climate, carbon, and geochemistry. To document the potential impacts, they also incorporate emissions estimates under various socio-economic scenarios, as well as computer models of melting ice and sea level rise. Computing resources were provided by the Climate Simulation Laboratory at NCAR's Computational and Information Systems Laboratory (CISL), whose Bluefire supercomputer - a 4,064-processor IBM Power6 resource with a peak of 77 TeraFLOPS - provided more than 7.5 million computing hours.

"It is still not too late, by stabilizing carbon dioxide concentrations in the atmosphere and reducing emissions of shorter-lived pollutants, to lower the rate of warming and reduce sea level rise by 30 percent," says Veerabhadran Ramanathan, an atmospheric scientist at Scripps and a co-author of the Nature Climate Change study. "The large role of the shorter-lived pollutants is encouraging since technologies are available to drastically cut their emissions."

A separate analysis published in the Journal of Geophysical Research (JGR): Atmospheres shows that black carbon (soot) may be second only to carbon dioxide in the total amount of heat it traps in the atmosphere - playing a much bigger role in global warming than previously thought. Soot comes predominantly from combustion: fossil fuels for transportation (diesel); solid fuels such as coal; and biomass - a source of household energy in many parts of the world.

According to the JGR study, "for a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all emissions from these sources would reduce net climate forcing (i.e., produce cooling)." Yale's environmental journal, Environment 360, underscores this potential with estimates that two billion people use biomass to cook on open fires. Finding clean substitutes could go a long way toward reducing the impact of black carbon on sea level rise - not to mention preventing health problems and deaths caused by indoor air pollution from open biomass fires.

"To avoid potentially dangerous sea level rise, we could cut emissions of short-lived pollutants even if we cannot immediately cut carbon dioxide emissions," said Aixue Hu, a researcher at NCAR and lead author of the Nature Climate Change study. "This new research shows that society can significantly reduce the threat to coastal cities if it moves quickly on a handful of pollutants."

Read the full article in Nature Climate Change, here.

- Greg Moore, senior communications specialist, IT Communications at Indiana University

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