Black Carbon’s Big Impacts

For years, efforts to study and reduce the impacts of climate change have focused on one very specific gas—carbon dioxide (CO2). But when it comes to global warming, other gases and atmospheric particles may play more than just a supporting role. Black carbon, a form of particulate air pollution produced mainly from burning biomass, cooking with solid fuels and diesel exhaust, has a warming potential in the atmosphere three to four times greater than scientists originally thought. Unlike CO2, black carbon absorbs solar radiation, and current estimates show that black carbon could have up to 60% of the current global warming effect of carbon dioxide.Black carbon is part of a category of emissions called “short-lived climate forcings” (SLCFs) that includes methane, hydrofluorocarbons and tropospheric ozone. These emissions are more powerful than CO2 in terms of their warming potential, but they don’t last anywhere near as long. Black carbon only remains in the atmosphere for a few weeks at most. “Black carbon is removed quickly by rain, whereas CO2 has a lifetime of about 40 years,” explains Mark Jacobson, Ph.D., a professor at Stanford University. But black carbon’s short, concentrated lifespan offers major global warming reduction potential. Jacobson’s research indicates that controlling soot could reduce warming above parts of the Arctic Circle by almost three degrees Fahrenheit within five years, equivalent to reversing virtually all warming that has occurred in the Arctic in the last 100 years.

Black carbon warms the climate in two ways. When it is suspended in the air it absorbs sunlight and generates heat which warms the air and affects cloud formation and precipitation patterns. It absorbs sunlight at all wavelengths and transfers that warmth to the atmosphere with almost a million times the heat-trapping power of CO2. Also, as black carbon falls out of the atmosphere with precipitation, it deposits on snow and ice where it accelerates melting.

More than three-quarters of the world’s black carbon is believed to come from developing countries, discharged from wood-burning stoves, open pit burning and old diesel engines. Between 25% and 35% of black carbon in the atmosphere comes from China and India. North America and Europe contribute black carbon emissions mainly via diesel engines. And scientists have observed that North American emissions contribute to black carbon falling on Greenland’s ice, while Europe’s emissions reach the rest of the ArcticA study in Nature Geoscience suggested that black carbon may be responsible for more than 30% of warming in the Arctic, contributing to the acceleration in sea ice melting, but some uncertainty remains about the direct connection. “The evidence of warming due to any chemical is through computer modeling,” Jacobson says. “Data show trends, for example, changes in temperature, but cannot be used to determine causes of the trends.”Aside from playing a significant role in global warming, black carbon also has detrimental impacts on human health. The inhalation of smoke during indoor cooking is linked to an estimated 400,000 deaths of women and children in south and east Asia. Respiratory diseases account for around 1.9 million deaths annually around the world, and a leading cause of respiratory diseases is exposure to soot. Cookstove smoke contributes to chronic illnesses and acute health impacts such as childhood pneumonia, emphysema, cataracts, lung cancer, bronchitis and cardiovascular disease.The United Nations Environment Programme (UNEP), in conjunction with the World Meteorological Association, published a report on black carbon, methane and ground-level ozone in February 2011. The report examined the methods to reduce these emissions and the impact of those reductions. Steve Seidel, vice president for policy analysis at the Pew Center on Global Climate Change, says, “the [UNEP] report found that if effective policies were taken to reduce these SLCFs that temperature changes could be reduced in the near-term, but it also showed that longer-term complementary reductions in CO2 are critical to limiting long-term changes in climate.”

Potential measures to reduce black carbon emissions include using diesel particle filters and cleaner-burning cookstoves. In the U.S., there is an effort to reduce particulate emissions from new cars and trucks, and Seidel notes that “the EPA [Environmental Protection Agency] recently provided $50 million in grants to states for the Clean Diesel Program to reduce black carbon emissions from older vehicles.” The Global Alliance for Clean Cookstoves is working to bring clean and efficient stoves to the estimated three billion homes that rely on biomass, wood, dung or coal. Other global emission policies are being investigated under the Montreal Protocol, the Arctic Council and the Convention on Long-range Transboundary Air Pollution. Ultimately, limiting black carbon and other short-lived emissions will not only alleviate warming, but also improve air quality and human health across the globe.