Paper: Strategies to limit temperature rise

A paper published in the Proceedings of the National Academy of Sciences identifies three steps that major greenhouse gas emitting countries should take so that global temperature rise does not exceed a rise of 2⁰C.

Recommended steps include stabilizing CO₂ concentrations in the atmosphere, and fashioning warming-neutral pollution laws that will balance the removal of aerosols that have an atmospheric cooling effect with the removal of warming agents such as soot and ozone. Finally, the authors advocate achieving reductions in methane, hydrofluorocarbons and other greenhouse gases that remain in the atmosphere for short periods of time. The authors write that aggressive simultaneous pursuit of these strategies could reduce the probability of reaching the temperature threshold to less than 10 percent before the year 2050.

Ramanathan V and Xu Y. (2010). The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues. PNAS, May 4, doi: 10.1073/pnas.1002293107

Abstract
At last, all the major emitters of greenhouse gases (GHGs) have agreed under the Copenhagen Accord that global average temperature increase should be kept below 2 °C. This study develops the criteria for limiting the warming below 2 °C, identifies the constraints imposed on policy makers, and explores available mitigation avenues.

One important criterion is that the radiant energy added by human activities should not exceed 2.5 (range: 1.7–4) watts per square meter (Wm−2) of the Earth's surface. The blanket of man-made GHGs has already added 3 (range: 2.6–3.5) Wm&minus.

A second criterion is that even if GHG emissions peak in 2015, the radiant energy barrier will be exceeded by 100%, requiring simultaneous pursuit of three avenues:

1. reduce the rate of thickening of the blanket by stabilizing CO₂ concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task),
2. ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and
3. thin the blanket by reducing emissions of short-lived GHGs.

Methane and hydrofluorocarbons emerge as the prime targets. These actions, even if we are restricted to available technologies for avenues 2 and 3, can reduce the probability of exceeding the 2 °C barrier before 2050 to less than 10%, and before 2100 to less than 50%. With such actions, the four decades we have until 2050 should be exploited to develop and scale-up revolutionary technologies to restrict the warming to less than 1.5 °C.

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