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Climate change measures must focus on CO2 first, CIFAR fellow argues

by Lindsay Jolivet
Aug 28 / 14
Image above: CIFAR Senior Fellow Raymond Pierrehumbert argues in a review paper that we must take action on CO2, because its effects on the environment are much more damaging than those of other, shorter-lived greenhouse gases such as methane.

Delaying measures to reduce CO2 emissions in favour of mitigating shorter-lived greenhouse gases such as methane could cause virtually irreversible damage to the Earth’s climate, according to a review paper by CIFAR Senior Fellow Raymond Pierrehumbert (University of Chicago).

Pollutants such as methane, hydrofluorocarbons, black carbon and ozone break down in the atmosphere within decades, whereas carbon dioxide’s effect on the atmosphere lasts thousands of years. Based on these lifetimes, Pierrehumbert argues that governments and companies should dedicate their resources to reducing CO2 emissions and set action aside on the other pollutants for now.

“The harm done by short-life gases like methane and HFCs is reversible,” Pierrehumbert says. “You can defer action on those and still get the benefit.”

Humans add about 0.3 gigatonnes of methane to the atmosphere each year, which warms the atmosphere by about 0.014 degrees Celsius. However, nearly all of that methane dissipates after about a decade, Pierrehumbert writes in the paper published in the Annual Review of Earth and Planetary Sciences.

By contrast, the nearly 10 gigatonnes of carbon humans pump into the atmosphere every year in the form of C02 increases the Earth’s temperature by about 0.02 degrees Celsius, and that rising temperature is mostly irreversible. It’s an argument that attracted the attention of The New York Times and other media outlets in recent weeks.

Pierrehumbert compares the problem to saving for retirement: starting early leaves more time for the money to grow and therefore more savings when retirement day comes. “Methane mitigation is like trying to stockpile bananas to eat during retirement. Given the short lifetime of bananas, it makes little sense to begin saving them until your retirement date is quite near,” he writes in the paper.

However, current measures of climate change contributors don’t always take atmospheric lifetimes into account, Pierrehumbert says. For example, one of the events that led him to become interested in this issue was a complaint from a New Zealand official that high levels of methane emissions from sheep were damaging the country’s climate change assessments by the Intergovernmental Panel on Climate Change.

“I was also thinking a lot about methane because of various discussions as part of the Earth System Evolution program on the role of methane in past climates,” Pierrehumbert said.

Researchers have known the differences in atmospheric lifetime between CO2 and other greenhouse gases for decades, Pierrehumbert says. CO2 is removed from the atmosphere by a very slow process that absorbs it into the deep ocean.

“The implications of this very slow uptake have only made it into the policy dialogue in fairly recent years,” Pierrehumbert says. However, few have considered the implications of CO2’s long lifespan on the way we manage shorter-term gases.

Pierrehumbert hopes his research will lead to efforts to bridge the gap between scientific research and climate policy, improving our approaches to issues from emissions trading schemes to farming practices.