In December, the world's attention will fall on climate-change negotiations at the 20th United Nations Framework Convention on Climate Change (UNFCCC) Conference of the Parties in Lima, Peru. The emphasis will be on reducing emissions of long-term atmospheric drivers such as carbon dioxide, the effects of which will be felt for centuries. At the same time, the mitigation of short-lived climate-forcing pollutants (SLCPs) such as methane, black carbon and ozone — which are active for days or decades — must be addressed .

Current air-quality legislation falls short. Existing measures would prevent just 2 million premature deaths by 2040. We estimate that around 40 million more such deaths would be avoided if concentrations of methane, black carbon and other air pollutants were halved worldwide by 2030 (see 'Clean air').SLCPs cause poor air quality and are responsible for respiratory and cardiovascular diseases. Particulate matter in the atmosphere is the leading environmental cause of ill health, and air pollution is causing about 7 million premature deaths annually¹. Interactions between warming, air pollution and the urban heat-island effect (which causes cities to be markedly warmer than their surrounding rural areas) will raise health burdens for cities worldwide by mid-century². Air pollution also damages ecosystems and agriculture.

This is not an 'either-or' decision: coordinated action on both climate change and air pollution is necessary. And it is tractable: for example, electric-car sharing or shifting from fossil fuels to renewable power generation would reduce consumption and overall emissions and lead to behavioural shifts that are beneficial in both the near and long term³.

But defining joint CO2 and SLCP reduction goals is difficult. Researchers need to spell out the benefits and trade-offs of separate and joint air-pollution and climate-change mitigation in terms of public health, ecosystem protection, climate change and costs. A suite of mitigation policies must be designed and applied on all scales — from cities to the global arena.

Double jeopardy

Studies^4, 5 estimate that rigorous reductions of global methane and black-carbon-related emissions by 2030 could prevent around 2.4 million premature deaths per year that result from air pollution, and save 50 million tonnes of crops through avoided ozone damage (methane is a precursor for ozone production). Global mean temperature rise would be slowed by about 0.5 °C by mid-century. The rate of sea-level rise would be reduced by 20% in the first half of this century by such measures alone, and by 50% in the second half if CO2 and SLCP mitigation are combined⁶.