For the basic science of black carbon, Wikipedia has a pretty good primer. Black carbon warms the atmosphere directly by absorbing infrared radiation in the atmosphere. However, it mostly effects global warming through changing the Earth's albedo, or ability to reflect incoming radiation rather than absorb it. It does so by settling on snow or ice and making those surface absorb more incoming radiation. In the long run, black carbon reduces the extent of snow and ice coverage by warming those surfaces, which further decreases albedo by exposing either ocean water or land. A study by Ramanathan and Carmichael in 2008 found that black carbon is currently causing 60% of the global warming that CO2 is, making it the second highest contributor to warming by greenhouse gases after CO2.
One study recently found that aerosols settling on snow are causing 90% of the melting of Himalayan glaciers as opposed to study 10% from global warming. Under the heading of aerosols, black carbon causes 30% of the melting. In addition to lowering the Earth's albedo, the melting of Himalayan glaciers threatens the water supply for hundreds of millions of people in South Asia.
Thanks to the fact that we don't burn a lot of wood relative to fossil fuels and air pollution regulations for coal plants and diesel engines, the US contributes just 6.1% of global black carbon emissions. That's not much more than our share of the global population, unlike our disproportionately huge share of CO2 emissions.
However, there are several reasons that increase the urgency of the US cutting its black carbon pollution. First, as Wikipedia notes, black carbon from North America disproportionately impacts the Arctic sea ice, which is so crucial for maintaining the Earth's albedo. Second, the US is one of the foci of international shipping by sea. Ocean-going vessels often are fueled by dirty diesel engines, and they often pass closer to the Arctic. Finally, the US would set a powerful example for other countries, particularly India and China.
Reducing black carbon is easier than the shrinking fossil fuel consumption from CO2 because reducing black carbon often involves technology fixes like particulate filters. If we take aggressive steps to reduce it now, we can help avoid climate tipping points, especially in light of the failure of the global community to commit to serious enough CO2 reductions.
Will President Obama's EPA listen? Our bedrock environmental laws like the CWA, Clean Air Act, and Endangered Species Act give the agency wide latitude to regulate pollution. It has taken initial steps to regulate CO2 under the Clean Air Act, but might be reluctant to regulate air pollution under the Clean Water Act, not for legal reasons but for fear of stepping on toes. As I've written earlier, Obama and the Democrats may realize that global warming exists and that we must cut CO2 pollution, but they definitely do not understand how gigantic the problem is and the scale of response it requires. Fortunately EPA head Lisa Jackson is more progressive than most in the administration, so we'll have to see. Whatever they decide, the outcome will be as much a political determination as a legal one.
PS. The IPCC seriously underestimated the effect of black carbon in its 2007 report, as the Wikipedia article notes. Yet another reason why emissions cuts need to be more severe than what the IPCC report dictates because the report is so conservative. This drives in the absurdity of the mainstream media's recent criticisms of the IPCC for minor mistakes. The IPCC hasn't created the idea of global warming out of its imagination, but rather has significantly under-estimated the extent and effect of this very real and pernicious phenomenon!