The even darker side of brown clouds

Atmospheric aerosols compete with carbon dioxide as an agent of warming.

In the charge against global warming, carbon dioxide has long held sway as public enemy number one. But now, less-recognized molecules are entering the fray as significant agents of global warming. Aerosols emitted from smokestacks, exhaust pipes and domestic cooking fires consist of substances such as sulphates and nitrates that scatter light and have a local cooling effect; they also contain black carbon — or soot — a byproduct of incomplete combustion, which absorbs light.

Scientists modelled the behaviour of the cooling particles years ago, but so few direct measurements have been made of the heat-absorbing effects of black carbon that, even now, models do not adequately represent their influence. The most recent assessment of the Intergovernmental Panel on Climate Change1 reported that the total contribution of aerosols to climate warming since the onset of the industrial era was about 20% of that caused by greenhouse gases. As much as half of the recent warming trend attributed to CO2 and other greenhouse gases is thought1 to have been cancelled out by cooling from aerosols. But new observations show that in some regions black carbon is as culpable as CO2 for the warming, and in some cases, has a greater effect.

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