Methane and black carbon cuts won’t buy us time

The warming effect of soot has been exaggerated. Photo: © Buriy - Fotolia.com

Two new scientific studies say that the effectiveness of cutting a range of non-CO₂ gases such as methane and black carbon to limit global warming to 2°C has been strongly and consistently overestimated.

The Guardian reports that the global warming effect of black carbon, or soot, has been greatly exaggerated due to mistaken assumptions about the atmospheric altitude at which its particles are concentrated, according to a study by Norwegian resarchers. “Uncertainty surrounds the exact influence of black carbon on global warming, partly because of the difficulties involved in estimating how much of it is there is. Soot particles are also enigmatic substances, with warming and cooling properties that depend on the atmospheric conditions they encounter as they drift upwards. In the upper troposphere at tropical and middle latitudes, they have the potential to absorb and emit heat and solar radiation. But if they do not rise that far, they may stabilise lower-lying clouds that block the sunlight, so reducing temperatures. Global warming efforts should focus on CO₂, not soot particles known as black carbon”, says the Norwegian team.

The New Scientist reports on a new study published by the International Institute for Applied Systems Analysis, which states that to prevent dangerous climate change, there is no alternative to ending human emissions of carbon dioxide. The study says that “suggestions that cutting a range of non-CO₂ gases like methane might do at least part of the job are based on faulty accounting.” Some scientists and governments have in the past argued that there is a third way to curb warming. Action to reduce other types of greenhouse gas emissions might be cheaper and quicker, buying time for later action on CO₂. The US government and the UN Environment Programme have both promoted cutting emissions of methane and soot as an effective emergency response to global warming. The study says “that the potential for preventing 2°C of warming by controlling these pollutants has been strongly and consistently overestimated.”

The authors of the study explain according to the New Scientist that “the problem is twofold. Because gases like methane have much shorter atmospheric lifetimes than CO₂ they don’t accumulate in the atmosphere in the same way, lessening their contribution to warming. Secondly, these other gases often have the same sources as CO₂ emissions, such as burning fossil fuels. This means that the impact of cutting these gases on curbing climate change has already been included in calculations of cutting CO₂ emissions, so those gases have in effect been ‘double-counted’. Many studies haven’t considered this.”

Reinhold Pape

Sources:
New Scientist, 5 November 2014 by Fred Pearce,  Journal reference: PNAS, DOI: 10.1073/pnas.1415631111
Disentangling the effects of CO2 and short-lived climate forcer mitigation. By Rogelj J, Schaeffer M, Meinshausen M, Shindell D, Hare W, Klimont Z, Velders G, Amann M, Schellnhuber HJ. Proceedings of the National Academy of Sciences (PNAS). Link:

Arthur Neslen, Guardian, 26 September 2014
How shorter black carbon lifetime alters its climate effect. By Hodnebrog O, Myhre G, Samset BH. Nature Communications 5.