Soot and global warming

Black Carbon emissions regionWe have long known that there are plenty of reasons to reduce emissions of soot, also called black carbon (BC). Black carbon has serious and well documented health effects; the same crud that turns buildings and your laundry black also coats your lungs. Industrialized countries recognized the dangers of billowing clouds of soot over a century ago; emissions in North America have been decreasing since their peak around 1910.

We now have an additional and urgent reason to minimize soot emissions: black carbon gives a short-term, but powerful boost to heating the planet. Black carbon (BC), contributes to climate warming in two ways. First, black soot particles in the air absorb sunlight and directly heat the surrounding air. Second, soot falling on snow or ice changes those reflecting surfaces into absorbing ones, that is, soot decreases the albedo. Therefore, soot deposits increase the melting rate of snow and ice, including glaciers and the arctic ice.

Black carbon is a “short-term” climate forcer, acting for a few days in the atmosphere and a few months on snow and ice. Over these short times it is an important contributor to warming. Reducing soot will have immediate benefits; by gradually eliminating BC emissions over the next 40 years we could slow warming, perhaps by 0.1-0.2°C globally. Decreasing black carbon deposits in the arctic may also slow amplification of feedbacks from melting arctic snow and ice.

Black carbon does not accumulate in the atmosphere like carbon dioxide (CO2). BC stops warming the air when it is washed out in a few days or weeks; it stops warming the snow when it gets covered with more snow or carried away by melting. So, reductions in BC have immediate, but not long-term effects on global warming. Each CO2 molecule continues to block heat loss from the Earth for the 100+ years that it stays in the atmosphere. That is why CO2 is known as the “biggest control knob” for our climate. Climate change cannot be prevented without reducing carbon dioxide emissions. Reductions in long and short-term forcers, CO2 and BC and methane and ozone will be necessary to keep global temperatures from rising more than 2°C above preindustrial levels in the next 50 years.

“It is important to emphasize that BC reduction can only help delay and not prevent unprecedented climate changes due to CO2 emissions.” (Ramanathan and Carmichael. Global and regional climate changes due to black carbon. Nature Geoscience (2008) vol. 1 (4) pp. 221-227)

“Short-lived climate forcers – methane, black carbon and ozone – are fundamentally different from longer-lived greenhouse gases, remaining in the atmosphere for only a relatively short time. Deep and immediate carbon dioxide reductions are required to protect long-term climate, as this cannot be achieved by addressing short-lived climate forcers.” (Integrated Assessment of Black Carbon and Tropospheric Ozone; United Nations Environment Programme,, 2011)

Because of its short lifetime in the atmosphere, the effects of BC are most important regionally, especially in East Asia and South America. Other hotspots occur in Mexico, Brazil, Peru, and parts of Africa. In Asia, BC contributes to regional heating and disrupts rainfall patterns. BC is of great concern in the Himalayas where it accelerates melting of glaciers, which supply water to millions.

The largest sources of BC are incomplete burning of biomass and unfiltered diesel exhaust. Major reductions could be achieved by replacing traditional cook and heat stoves in developing countries with clean-burning biomass stoves or alternative fuel systems. Installation of filters on diesel vehicles reduces BC, as can be seen by comparing bus exhaust in different countries. Industrial coke ovens and brick kilns should also be updated to employ newer, cleaner technologies. Finally, open field burning of agricultural waste should be eliminated. These old technologies are primarily used in developing countries and many are already acting to replace them.

In the industrialized northern hemisphere, residential heating stoves are the primary source of BC. Emissions from North America and Europe are major controllable sources of BC to the Arctic, contributing to northern warming and loss of ice. Recent work (Doherty, et a) suggests that BC in the Arctic has been relatively constant or slowly declining since the 1980s.

The major sources of BC (biomass burning for cooking and heating, and diesel engines) are not the biggest global sources of CO2 (coal and fossil fuel burning). Therefore, both problems can and must be addressed independently and simultaneously. Immediate reductions in BC can buy a little time as we convert to low carbon energy sources.

Health effects alone should motivate the rapid limitation of soot emissions. Worldwide reductions in soot emissions would prevent an estimated 2.4 million premature deaths. Furthermore, emissions of BC are accompanied by carbon monoxide, and volatile organic compounds (VOCs), which have additional adverse health effects. The climate change effects of BC add urgency to the issue because global warming will only exacerbate the known health consequences of breathing dirty air.



Integrated Assessment of Black Carbon and Tropospheric Ozone: Summary for Decision Makers. United Nations Environment Programme and World Meteorological Organization (2011) pp. 1-36.

Doherty et al. Light-absorbing impurities in Arctic snow. Atmos. Chem. Phys. (2010) vol. 10 (23) pp. 11647-11680.

Lacis et al. Atmospheric CO2: Principal Control Knob Governing Earth's Temperature. Science (2010) vol. 330 (6002) pp. 356-359.

Ramanathan and Carmichael. Global and regional climate changes due to black carbon. Nature Geoscience (2008) vol. 1 (4) pp. 221-227.

Graphic by Riccardo Pravettoni, UNEP/GRID-Arendal, 'Black Carbon Emissions', UNEP/GRID-Arendal Maps and Graphics Library, 2009, <> [Accessed 30 March 2011].

Posted by Sarah on Saturday, 2 April, 2011

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