Tuesday, September 29, 2009

Up in Smoke: Black Carbon's Role in Climate Change

[Update: I am moving to a once-a-week blog post.  Check here each Tuesday for a brand new entry of Brave Blue Words!]

The most significant anthropogenic greenhouse gas (GHG) is carbon dioxide, CO2. Comprehensive reductions in the atmospheric concentration of CO2 (expressed as “parts per million or ppm) are the only way to bring down global temperatures and reduce the impacts of climate change. However, because CO2 and most other GHGs have such a long atmospheric lifetime – hundreds of years – reductions made today will not be felt for a very long time.Black carbon, one of the small components of soot, may be responsible for as much warming as CO2 at the “Third Pole,” the Himalayan-Tibetan region and more than one third of the warming caused by CO2 in the Arctic. Because BC is so short-lived and only stays in the atmosphere only a week or so, BC reductions can have immediate benefits.

Black carbon (BC) is a solid, not a gas. When microscopic bits of a solid get suspended in the air, it is known as an aerosol. BC has such a short atmospheric lifetime because these tiny particles can get washed out of the air by rainfall. Greenhouse gases (GHGs) warm the atmosphere by absorbing infrared radiation, the heat that is given off by the Earth and would otherwise be transmitted out to space. BC absorbs sunlight directly, in the atmosphere and on surfaces.  When BC is deposited on otherwise highly-reflective surfaces such as snow and ice, the surfaces gets darker, reducing the amount of sunlight reflected back, the albedo. The heat absorbed by the BC accelerates melting of the underlying ice. This snow-albedo feedback is one of the most significant causes of of sea ice melting in the Arctic and the retreat of glaciers in the Himalayas.

BC’s short lifetime also means that it does not travel far from its emission source. Therefore its impacts – and the benefits of reductions – are felt close to its source. BC sources vary from region to region: the BC that gets deposited in the Arctic comes primarily from dirty diesel sources in areas north of 40 degrees N (North America and Europe), while much the BC darkening the slopes of the Himalayas is produced by cookstoves and primitive brick kilns in China and India. All of these sources produce air pollution that is responsible for hundreds of thousands of premature deaths a year. Therefore, in addition to producing dramatic climate benefits, reductions in BC can have immediate health benefits in the most populated regions of the world.

Primitive brick kiln

Dirty diesel smoke


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