Black carbon adds 2 to 3 orders of magnitude more energy to the climate system than an equivalent mass of CO2

Black carbon is the light-absorbing, carbonaceous component of soot. It is an aerosol and combustion by-product of inefficient burning of fossil fuels, biofuels, and biomass. It is released as fine particulate matter into the troposphere, and from there can move into the stratosphere.

The direct climate forcing effect of black carbon occurs when the particles absorb sunlight and release that energy as heat into the atmosphere. The current estimate for black carbon forcing at the “top of the atmosphere” is as much as 60% of the current radiative forcing due to carbon dioxide’s greenhouse gas effect. It is now thought to be the second most powerful contributor to global warming after carbon dioxide, and it adds two to three orders of magnitude more energy to the climate system than an equivalent mass of CO2.

Black carbon is also a potent climate warming agent when deposited on snow and ice, which may be responsible for a quarter of observed global warming. This effect represents a significant fraction of Arctic climate forcing. Black carbon aerosols stay aloft in the atmosphere for an average of only 4.6 days, and rarely more than one week. Once deposited, black carbon reduces the albedo, or reflectivity, of these surfaces, and increases the rate of melting, even when air temperatures are below freezing. In many areas, when these surfaces melt, the darker water or land exposed below absorbs more incoming sunlight, causing additional warming. About half the warming effect of black carbon on snow comes from its dark color; the other half occurs if darker earth or water below snow or ice becomes exposed. This is an example of a positive feedback, where a system responds to a perturbation in the same direction as the perturbation. Recent studies show that this effect of black carbon is much greater than had been previously assumed.

Black carbon is having a significant melting effect on glaciers and sea ice in the Arctic, including the Greenland ice sheet. Over 80 percent of the forcing caused by black carbon on snow comes from black carbon from fossil fuels. As black carbon increases melting on the surfaces of glaciers, the resulting meltwater percolates down through cracks in the ice and may increase lubrication at the bottom of the glacier causing the glacier to flow more quickly. This is another example of positive feedback loop for black carbon’s climate impacts, as the downward movement of ice to lower (and warmer) altitudes increases its melt rate.

Source: RE: Inclusion of Alaska in EPA’s Application to IMO for a North American Emission Control Area – Earth Justice (PDF)

Date: 27 March 2009