Scientists are increasingly finding that black carbon aerosols in the atmosphere and their deposition on snow and ice-covered areas are having more of a warming effect than earlier thought.
Recent research has found that black carbon is the second largest anthropogenic contributor to warming, adding a climate forcing about 55 percent of that of carbon dioxide, and nearly twice that of methane.
In Arctic and Antarctic areas, black carbon deposition on snow and ice causes the surfaces to absorb more of the sun’s heat, and may be responsible for as much warming in the Arctic as all other anthropogenic forcings combined. Because of their short residence life in the atmosphere, focusing on reducing black carbon emissions can result in immediate climate benefits and also help reduce health risks associated with incomplete combustion in many developing countries.
Black carbon is the soot produced from the incomplete combustion of biomass and fossil fuels. Small particles can remain airborne for up to a few weeks and travel considerable distances from their source of emission. While small amounts of black carbon are emitted by virtually all fuel consumption, the amount emitted decreases as combustion efficiency increases. Low-technology sources such as home cook fires in developing countries are responsible for a disproportionate amount of emissions per fuel consumed, while modern power plants produce relatively little carbon for the same quantity of fuel.
Black carbon emissions generally are concentrated in South and East Asia (in India and China, particularly), though the highest per-capita emissions are in the U.S. and Europe (see Figure One). In Asia, these large black carbon emissions contribute to the formation of atmospheric brown clouds and global dimming.
Figure One: Total and Per-Capita Black Carbon Emissions by Source. Taken from a presentation by Ramanathan at the International Workshop on Black Carbon, London, January 2009.
Climate scientists have known that black carbon is a positive forcing agent since at least the early 1990s. Estimates of its impacts have varied widely, with the most recent Intergovernmental Panel on Climate Change synthesis report estimating their global impact at 0.3 to 0.5 watts per meter squared.
More recent work by Hansen and Ramanathan estimates climate forcing from black carbon to be considerably higher – two to four times higher than IPCC estimates, making it the second largest anthropogenic forcing after CO2. Figure Two shows how estimates of black carbon forcings from Ramanathan and Feng compare to IPCC estimates and anthropogenic greenhouse gas emissions. The negative forcing resulting from sulphate aerosols (as discussed in a previous Yale Forum article) still considerably outweighs the positive forcing caused by black carbon, and the increases in estimated forcing by Hansen and Ramanathan relative to the numbers used in the IPCC 2007 report do not necessarily require any reassessment of the positive forcing associated with anthropogenic greenhouse gases, given the wide uncertainty range in negative aerosol forcings.
Figure Two: Estimates of Black Carbon Forcing. Taken from a presentation by Ramanathan at the International Workshop on Black Carbon, London, January 2009.
Although emissions of black carbon are concentrated in the tropics, they have a strong forcing influence in snow-covered areas because of albedo effects: black carbon that falls onto snow and ice covered surfaces absorbs rather than reflects sunlight, warming the surface and causing melting.
A number of scientific studies have found that black carbon may be responsible for as much ice and snow melt as greenhouse gas emissions in the Arctic and for springtime snow in Eurasia. The distribution of black carbon emissions is somewhat important because of its relatively short atmospheric lifetime. Emissions near Arctic areas or other areas with year-round snow cover will have a stronger albedo impact than emissions in the tropics, as more black carbon will fall onto the ice.
Given the concentration of these emissions in Asia, especially those associated with home cook fires having dire local air pollution effects, there is considerable room to focus on reducing black carbon emissions as part of international development projects. Projects like distributing or subsidizing inexpensive but efficient cook stoves for rural areas could help reduce premature mortality resulting from air pollution and also reduce black carbon emissions.
Controlling these emissions from industrial sources is somewhat more complicated, as many of the technologies to clean up black carbon will also reduce sulphate aerosol emissions, and therefore counteract some of the reduction in climate forcings.
Recent and ongoing research on the large role of black carbon emissions in contributing to atmospheric warming is another reminder that the issue is more complicated than CO2 emissions alone, and that cleaning up other forcings like black carbon may prove less expensive, more politically viable, and more effective in the short-term while other strategies are developed and implemented to address longer-term concerns.