Black Carbon: A Key Cause Of Warming Not Well Recognized

May 1, 2009

By CHERYL HOGUE

Quick – after carbon dioxide, what's the second largest contributor to human-induced global warming?

It's not methane from giant manure pits and rice paddies.

It's not even a gas. It's a solid.

It's black carbon, a type of particulate pollution. Scientists say black carbon is a major cause of warming in the Arctic and the Himalayas.

Black carbon didn't figure prominently in the Intergovernmental Panel on Climate Change's landmark 2007 scientific assessment of global warming. But research since then, especially a 2008 paper published in Nature Geoscience, has boosted black carbon's significance among the emissions that cause warming.

Policymakers are increasingly interested in black carbon these days because reducing emissions of it could curb global warming in the short term. This could buy time for the development and deployment of new technologies to reduce carbon dioxide releases over the long haul. However, black carbon is not among the emissions targeted for control under the United Nations treaty to combat climate change.

Black carbon comes from incomplete combustion. Sources include forest fires and the burning of diesel, coal, crop residues, wood, dung, or other biomass that people use for heating and cooking. It's a constituent of fine particulate matter, a type of air pollution linked to respiratory and cardiovascular problems.

JOURNALISTS, TAKE NOTE: BLACK CARBON ISN'T CARBON BLACK.

A commercial product used as a pigment in inks and car tires, carbon black is also starting material for many products of nanotechnology. There are key chemical and physical differences between the intentionally produced commercial material and the widespread pollutant, according to the International Carbon Black Association, an industry-sponsored group that conducts health, safety, and environmental research. The big difference, the association says, is that carbon black is generally composed of at least 97% of carbon in its elemental form — meaning the atoms of carbon aren't bonded with other elements.

Black carbon, in contrast, has less elemental carbon than the commercial product. It generally contains a fairly hefty amount of other chemicals, including a family of hazardous air pollutants called polyaromatic hydrocarbons.

Black carbon contributes to global warming in several ways. It absorbs the sun's energy and heats up the air around it, just like a black dashboard and seats can make a car's interior stifling on a cool, sunny day.

When wind currents sweep black carbon bits high into the atmosphere above most clouds — at an altitude of about 6,500 feet — these particulates have a two-fold warming effect. At a place scientists call the top of the atmosphere, black carbon absorbs the sun's energy directly and causes warming. Plus, it reduces the global cooling effect of clouds by intercepting and absorbing sunlight that clouds are reflecting into space.

Black carbon particles last about a week in the atmosphere, and eventually fall out of the air as part of rain or snow. If they land on icy or snowy areas like glaciers, they contribute to global warming just as they do above clouds — by cutting down ice and snow's ability to help cool the planet by reflecting the sun's rays into space.

Plus, snow or ice dirtied with black carbon melts faster. In some places, this means dark soil that readily absorbs heat is exposed more quickly in the spring, exacerbating planetary warming.

In the Arctic, black carbon also makes winter clouds thicker. This makes clouds more able to trap heat in the atmosphere and less likely to allow heat to dissipate into space during the dark polar winter.

Last September, the U.S. government issued a scientific report recommending sharp cuts in global levels of black carbon and ground-level ozone (or smog, as most of us call it) to forestall climate change in the short run. Cuts in emissions of black carbon and ozone over the next decade would have an almost immediate cooling effect on the climate, the report argues.

This is because these two pollutants only exert their warming for the few days to weeks they last in the atmosphere, the report explains. In contrast, most other greenhouse gases, such as carbon dioxide, stay in the atmosphere for decades to centuries. But there's a complication.

Many activities that form black carbon, such as the burning of coal, release a mixture of air pollutants. Some of these contaminants, such as sulfates, reflect light and help cool the planet. This means any efforts to cut black carbon emission need to be carefully targeted on sources that don't supply significant amounts of cooling pollution.

The U.S. plan says reducing emissions from biomass burned for cooking and heating homes in Asia is the most cost-effective policy for reducing black carbon pollution and cut global warming in the short term. But logistically and politically, this idea may prove difficult to implement.

More scientific research is under way on black carbon. Watch for new studies as well as novel policy proposals, nationally and globally, to reduce emissions of this pollutant.

Cheryl Hogue covers climate change and other pollution issues for Chemical & Engineering News in Washington, D.C. 

** From SEJ's quarterly newsletter, SEJournal Spring, 2009 issue.

CHERYL HOGUE