Reducing C02 emissions must be priority in mitigating climate change, study says

Politically expedient solutions will have little effect on climate change without real steps to rein in carbon dioxide emissions, according to a comprehensive new study by Raymond Pierrehumbert, the Louis Block Professor in Geophysical Sciences.

Among the pollutants that humans put into the atmosphere in significant quantities, carbon dioxide’s impact is the longest-lived, with effects on climate that extend thousands of years after emissions cease. Finding the political consensus to act on reducing carbon dioxide emissions, however, has been nearly impossible. As a result, there is a movement to make up for that inaction by reducing emissions of other, shorter-lived gasses that contribute to climate change, such as methane, hydrofluorocarbons, nitrous oxide, and particulates such as soot and black carbon.

Pierrehumbert’s study shows that effort to be, as he puts it, a delusion. “Until we do something about CO2, nothing we do about methane or these other things is going to matter much for climate,” he said.

Pierrehumbert is the holder of the King Carl XVI Gustaf Chair in Environmental Sciences at Stockholm University for 2014-2015. His study, published in Annual Review of Earth and Planetary Sciences, brings together findings from the scientific literature with new research and analysis. Its conclusions are clear.

“Ray convincingly shows the benefit and importance of doing everything we can to lower CO2 emissions, and as soon as possible,” said Katherine H. Freeman, professor of geosciences at Pennsylvania State University. “We should lower short-lived pollutants like methane too. But, as he makes clear, we should not let them distract us from the urgent need to stop burning fossil fuels.”

The basic physics of climate pollutants has been well known for a long time. The warming effect of methane and other short-lived climate pollutants disappears quite quickly after the pollutants are removed from the atmosphere. When they’re removed, there is a one-time, lump-sum benefit. Carbon dioxide, on the other hand, lingers in the atmosphere.

Perhaps as a result of wishful thinking, the policy implications of those facts became confused, said Pierrehumbert. Part of the problem is that the statistical tool used to compare the climate effect of gasses is badly flawed. The measure, called Global Warming Potential, predicts the effect on climate by comparing the emission rate of carbon dioxide with the emission rate of methane. But a one-ton-per-year reduction in the amount of methane emitted translates into a single lowering of the global thermostat, while a one-ton-per-year reduction in carbon dioxide yields a climate benefit that increases over time. That’s because each extra ton of carbon dioxide that would have been emitted would have irreversibly ratcheted up the global thermostat by an additional increment.

Despite its well-known defects, GWP has been used since 1990 and was incorporated into the Kyoto Protocols in the climate-trading schemes implemented by Europe. Pierrehumbert proposes a different metric, which looks at the climate effect of reducing carbon dioxide emission by a fixed number of tons and then finds the rate by which the methane emissions must be reduced to get the same effect.

Pierrehumbert’s study doesn’t propose a single, “right” policy on climate change, said Richard Alley, the Evan Pugh Professor of Geosciences at Penn State. “But it is a very useful analysis that will be viewed carefully by people who are interested in making good policies, and the main conclusions will help inform those policies.”

Pierrehumbert himself hopes that his work will help lead policymakers to abandon Kyoto-style multi-gas trading schemes, which treat the gasses equivalently, and put the emphasis on carbon dioxide for the next 50 years or so. “I see puncturing the excessive enthusiasm about short-lived climate pollution control as a step in the right direction,” he said, “because it takes away one of the grounds for procrastination on CO2. If you’re serious about protecting climate, it’s the CO2 you’ve got to deal with first.”

Citation: “Short-Lived Climate Pollution,” by R.T. Pierrehumbert, Annual Review of Earth and Planetary Sciences, Vol. 42, 2014, DOI: 10.1146/annurev-earth-060313-054843.