Black carbon is not a gas but a particulate — a component of soot emitted by the incomplete combustion of fossil fuels and biomass. By some estimates, emissions from black carbon vie with methane as the second largest contributor to global warming after CO2 emissions. The largest source of black carbon in developed countries such as the United States is diesel fuel. Along with methane, black carbon is one of the two greenhouse contributors the Center has targeted for immediate reduction to curb global warming before the Arctic melts. Reducing black carbon could help keep the climate system from passing the tipping points for abrupt climate changes, including significant sea-level rise from the disintegration of the Greenland and/or Antarctic ice sheets.
Black carbon is generally thought to have both a direct warming effect (by absorbing incoming solar radiation in the atmosphere and converting it to heat radiation) and an indirect warming effect (by reducing the reflectivity of snow and ice). Yet it has not been addressed by international or national global warming regulations. Depending on the region, the atmospheric residence time for black carbon emissions from different regions varies between 4.6 to 7.3 days — in other words, it stays in the air for a period of days to weeks instead of more than a hundred years, as is the case with CO2.
Because of black carbon’s short atmospheric life span, decreasing its presence offers an opportunity to mitigate the effects of global warming quickly — within weeks. Control of black carbon, according to many scientists — particularly from fossil-fuel sources — could be the fastest method of slowing global warming in the near future.
In the Arctic, the most crucial climate-change battleground in the short term, black carbon appears to cause more warming than any other agent except CO2 because of its combined heating of the Arctic atmosphere and the surface. Recent studies suggest black carbon is one of the important contributors to the retreat of Arctic sea ice. On timescales of days to weeks, northern Eurasia is the strongest source of Arctic air pollution, especially in the lower troposphere; b lack carbon emissions from northern Eurasia, North America, and Asia have the greatest absolute impact on Arctic warming.
Globally, the United States is responsible for about 6.1 percent of total fossil fuel and biofuel soot emissions. The warming due to U.S. soot exceeds the warming due to either methane or nitrous oxide, making biofuel soot potentially the second leading source of U.S. global warming emissions overall. As snow and ice surfaces continue to warm, melt, darken, and lose contrast with black carbon, the net warming effect of black carbon on the Arctic will decrease. That means that reducing black carbon now will have more of an impact than delaying reductions.
Black carbon emissions come mainly from four sources: 1) diesel engines for transportation and industrial use; (2) residential solid fuels such as wood and coal; (3) open forest and savanna burning, both natural and initiated by people for land clearing; and (4) industrial processes, usually from small boilers.
To tackle black carbon reduction in the Arctic itself, we have to reduce intra-Arctic emissions of black carbon emissions such as generators and implement emissions controls on marine vessels operating within Arctic waters. This will be particularly important as industry seeks new opportunities in the thawing Arctic.
In the United States, the main sources of fossil-fuel soot are off-road vehicles, followed by on-road vehicles such as cars, buses and trucks, stack emissions, and lastly accidental spills or leaks. The good news: Black carbon emissions in the United States are already projected to decline by 42 percent from 2001 to 2020, primarily as a result of new diesel-vehicle regulations.
Globally, reducing black carbon will mean reducing prescribed burns in eastern Europe so that black carbon emission and depositing does not occur in spring as radiation is increasing and the area of snow/ice pack is large. In South Asia, the majority of soot emissions come from biofuel cooking, whereas in East Asia, coal combustion for residential and industrial uses plays a larger role. Providing alternative energy-efficient and smoke-free cookers and introducing transferring technology for reducing soot emissions from coal emissions in small industries could have major impacts on the contributions made to global warming by soot. Improved cookstove programs in China have been successful, and both pilot and full-scale projects have been conducted by the Partnership for Clean Indoor Air and the Shell Foundation. These programs can also be linked to substantial health benefits.
The use of certain cleaner fuels must also be promoted, especially in developing countries: Distillate fuels, such as kerosene and LPG, burn cleaner than solid fuels like charcoal.
The Center works to curb all forms of pollution that contribute to climate change, including black carbon, through education and legal tactics. In June 2011 we filed a notice of intent to sue the Environmental Protection Agency for its failure to reduce dangerous black carbon pollution. In February 2010, we’d petitioned the EPA to take action to reduce black carbon under the federal Clean Water Act. The petition is the first to explicitly seek protection of water in its solid form; it asks the EPA to set water-quality criteria for concentrations of black carbon on sea ice and glaciers under the Clean Water Act — the first step toward reducing black-carbon emissions from diesel engines and other sources due to their role in accelerating the loss of sea ice and glaciers.