ESRL Quarterly Newsletter - Spring 2009

International Study: Climate Change Irreversible

ESRL’s Susan Solomon leads study showing effects will last 1,000 years+

Significant sea level rise, temperature increases, and shifts in rainfall are irreversible consequences of people’s emission of carbon dioxide, CO2, according to a new study led by ESRL’s Susan Solomon. Climate changes wrought by the greenhouse gas will persist for more than 1,000 years, the new study shows, even if people on this planet instantly stopped emitting carbon dioxide—by curbing the burning of fossil fuels in cars and power planets, among other actions.

“Current choices regarding carbon dioxide emissions will have legacies that will irreversibly change the planet,” Solomon said.

Solomon and her co-authors used climate models to ask what would happen if CO2 were allowed to build up to several different peak levels (all beyond today’s 385 parts per million), and if emissions were then cut off completely. The cutoff scenario is not a realistic one—CO2 emissions are increasing rapidly. Rather, it was selected to demonstrate the longevity of climate impacts.

Not only does some CO2 persist for millennia in the atmosphere, the planet’s surface would remain warmer and sea level would remain higher long after CO2 levels in the atmosphere began dropping, the authors found. The lingering effect is primarily due to the oceans, which currently buffer climate change. Today, oceans absorb much of the excess CO2 emitted by human activities, removing it from the atmosphere.

But the ability of the oceans to store carbon dioxide will reach a limit and slow down, notes Solomon. As it does, the ocean’s uptake of excess heat also slows, so the atmospheric temperature stays nearly constant for a millennium.

In one of the scenarios considered in the new study, CO2 was allowed to peak at 450-600 parts per million before being cut off. That triggered persistent decreases in dry-season rainfall in several subtropical areas, comparable to the 1930s North American Dust Bowl. Affected areas included southern Europe, northern Africa, southwestern North America, southern Africa, and western Australia—and the effects lasted far beyond the year 3000. Regional impacts of that kind of drying could include diminished water supplies, increased fire frequency, ecosystem changes, and expanded deserts, Solomon and her colleagues wrote.

Figure 3 from Solomon et al., PNAS. Expected decadally-averaged changes in the global distribution of precipitation per degree of warming (% change in precipitation per degree of warming, relative to 1900-1950 baseline) in the dry season at each grid point, based upon a suite of 22 Atmosphere-Ocean General Circulation Models, for a mid-range future scenario (A1B). White areas are where fewer than 16 of 22 models agree on the sign of change. Data are monthly averaged over several broad regions for the inset plots. Red lines show the best estimate (median) of the changes in these regions, while the red shading indicates the +/-1-sigma likely range (i.e., 2 out of 3 chance) across the models.

ESRL Director Sandy MacDonald incorporated the new results into a February talk at the National Renewable Energy Laboratory, part of the Sustainable Energy and Atmospheric Sciences seminar series. “This is a really important paper,” MacDonald said. “We’re on a real freight train.”

Pieter Tans, from ESRL’s Global Monitoring Division, said Solomon et. al did an excellent job focusing attention on the very serious problem of carbon dioxide’s longevity.

Solomon’s co-authors were Gian-Kasper Plattner of the Institute of Biogeochemistry and Pollutant Dynamics in Switzerland, Reto Knutti of the Institute for Atmospheric and Climate Science in Switzerland, and Pierre Friedlingstein of the Institut Pierre Simon Laplace in France.

Their research was widely covered by international media, with reports appearing in the New York Times, The Province (Canada), Gazeta do Sul (Brazil), and Le Monde (France). Andy Revkin, a New York Times reporter, asked John Sterman, an expert in risk perception at the Massachusetts Institute of Technology, for perspective.

“It’s important that people not react to Solomon’s work with despair,” Sterman replied. “The Solomon paper should finally bury the idea that we can wait and see. It further strengthens the case for immediate, strong mitigation. The good news is that it’s getting cheaper every day to cut carbon emissions.”

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