You may have recently heard about “black carbon” and wondered if it was a climate epithet, a word reversal in a familiar product (carbon black), or simply redundant (carbon is black). But in fact “black” or elemental carbon is emerging as a particularly potent greenhouse agent that needs to be reckoned with on its own terms with special measures to prevent releases to the atmosphere. Recent studies suggest that black carbon emissions, which are not yet controlled by the Kyoto or Montreal Protocols, are the second largest contributor to global warming (after carbon dioxide) and that reducing them may be the fastest strategy for slowing climate change. Black carbon emissions are greatest from developing countries, a trend which is expected to increase, but the US and other developed nations can also do much more to address the problem. Reducing black carbon emissions offers a nearly instant return in lowering the greenhouse effect, because black carbon particles remain airborne for weeks while carbon dioxide remains in the atmosphere for more than a century (see footnote 24 of IGSD October 2008 Climate Briefing).

Recent studies and public testimony by scientists cited in the Intergovernmental Panel on Climate Change’s (IPCC) 2007 report suggest that emissions from black carbon are the second largest contributor to global warming after carbon dioxide emissions and that reducing these emissions may be the fastest strategy for slowing climate change. Soot and other forms of black carbon could have as much as 60 percent of the current global warming effect of carbon dioxide. Also recent studies suggest that black carbon plays a large role in the retreat of arctic sea ice and Himalayan glaciers. Black carbon warms the planet by absorbing heat in the atmosphere and by reducing albedo, a measure of the earth’s ability to reflect sunlight, when deposited on snow and ice. The effect is worse than imagined only months ago. Further, mitigation would have immediate health benefits in addition to the long term effect of reducing greenhouse gas emissions.

The largest sources of black carbon are Asia, Latin America, and Africa. Between 25 and 35 percent of black carbon in the global atmosphere comes from China and India when it is emitted from the burning of wood and cow dung in household cooking and through the use of coal to heat homes. Emissions from China doubled between 2000 and 2006. The problem is not just a question of climate forcing. The inhalation of smoke during indoor cooking has been linked to the deaths of an estimated 400,000 women and children in south and east Asia. Advocates for black carbon controls have pointed out that well-tested existing technologies used by developed countries, such as clean diesel and clean coal, could be transferred to developing countries to help reduce their black carbon emissions. A later blog will address an ancient but promising technology that offers promise worldwide for controlling black carbon releases -- biochar.

But developing nations are not the only significant sources. Countries in Europe and elsewhere that rely heavily on diesel fuel for transportation also contribute large amounts. Advocates for stronger soot controls in developed nations say that per capita emissions of black carbon from the United States and some European countries are still comparable to those from south Asia and east Asia.

The US emits about 21 percent of the world’s CO2, and only 6.1 percent of the world’s soot, but the US and the European Union could further reduce their black carbon emissions by accelerating implementation and sharpening the focus of existing air quality laws and regulations and by supporting the adoption of pending International Maritime Organization (IMO) regulations. Existing regulations also could be expanded to increase the use of clean diesel and clean coal technologies and to develop second-generation technologies. Senator Hilary Clinton of New York, together with Senators Carper and Kerry, has introduced a bill that would require the US Environmental Protection Agency to study black carbon emissions toward identifying US and global black carbon emissions inventories and levels, the science and extent of black carbon climate impacts including appropriate metrics, and best technology to control emissions. The House had already hosted a hearing a year ago on the issue.

In the US, black carbon is indirectly regulated as a component of fine particulate matter (PM 2.5) under the Clean Air Act. Black carbon constitutes 5-10 percent of PM 2.5 on an annual average basis, but can be much higher on particular days. For example, days affected by fire events show higher levels of black carbon than the annual average. Gasoline combustion, diesels, and fire are the major US sources. (Click here for more info on the composition of PM 2.5). Fine particulate matter affects millions of people who live in vehicle-rich cities and suburbs or near industrial smokestacks. Prolonged exposure to tiny particles up 1/30th the size of a human hair can cause serious respiratory problems such as asthma, cardiovascular disease, and heart attacks.

Black carbon may be indirectly reduced soon in the US because EPA has put more than 215 counties on notice -- including much of the Eastern Seaboard from New York to Washington, as well as Chicago, Los Angeles and San Francisco -- that they are unlikely to meet the national standard for fine particulates. The proposed designations cover metro areas in 25 states and could have major implications for economic growth and transportation planning across the country. Steps to control PM 2.5 include tightening pollution controls for cars, trucks, and non-road engines used for construction. Such programs can be expensive to design and implement, and some critics have argued that meeting such standards comes at the expense of economic growth. But now controlling black carbon in PM 2.5 adds a climate benefit to what otherwise has been viewed as primarily a cardiovascular and pulmonary health issue.

The debate over use of the Clean Air Act for climate management purposes now includes whether black carbon is a pollutant subject to direct CAA regulation. If CO2 is a CAA pollutant, why would black carbon be treated any differently? Senator Obama has said that he would urge use of the CAA tools if Congress does not promptly enact climate legislation. Senator McCain's representatives have said that the Senator would be “reluctant” to use them, while “Sen. Obama would not hesitate to use them as necessary to achieve climate goals,” according to his energy spokesman Elgie Holstein.