Soot (sometimes called “black carbon”) has been widely identified as an important climate forcing, with recent research (e.g., here and here in PDF) suggesting that soot emissions have played a potentially “very large” role in warming of the Arctic. Here is how the New York Times recently characterized the role of soot in climate change:

While carbon dioxide may be the No. 1 contributor to rising global temperatures, scientists say, black carbon has emerged as an important No. 2, with recent studies estimating that it is responsible for 18 percent of the planet’s warming, compared with 40 percent for carbon dioxide. Decreasing black carbon emissions would be a relatively cheap way to significantly rein in global warming — especially in the short term, climate experts say. Replacing primitive cooking stoves with modern versions that emit far less soot could provide a much-needed stopgap, while nations struggle with the more difficult task of enacting programs and developing technologies to curb carbon dioxide emissions from fossil fuels.

In fact, reducing black carbon is one of a number of relatively quick and simple climate fixes using existing technologies — often called “low hanging fruit” — that scientists say should be plucked immediately to avert the worst projected consequences of global warming. “It is clear to any person who cares about climate change that this will have a huge impact on the global environment,” said Dr. Ramanathan, a professor of climate science at the Scripps Institute of Oceanography . . .

So it is with some surprise that the EPA has decided that the science of soot in the climate system is too uncertain to act. Courtesy of Environmental Capital here is what the EPA endangerment finding says about the role of soot in the climate system:

Like other aerosols, black carbon can also affect the reflectivity and lifetime of clouds. How black carbon and other aerosols, such as sulfates, alter cloud properties is a key source of uncertainty in quantifying the total human influence on the global climate. This total cloud indirect effect caused by all aerosols (e.g., sulfates, black carbon and organic carbon) is estimated to be causing a net cooling effect, with a large range of uncertainty. Given these reasons, there is considerably more uncertainty associated with black carbon’s warming effect compared to the estimated warming effect of the six long-lived greenhouse gases.

In other words, unlike carbon dioxide, soot does not qualify as a pollutant with respect to climate change due to what EPA calls “considerably more uncertainty” in the science. Really? I’m not sure how this would be assessed, but the role of soot in the climate system has been extensively studied.

Despite EPA’s judgment that the science is too uncertain to act, an interesting group of policy makers has decided otherwise:

Sens. Tom Carper (D-Del), Jim Inhofe (R-Okla.), Barbara Boxer (D-Calif.) and John Kerry (D-Mass.) introduced legislation directing the U.S. Environmental Protection Agency (EPA) to study the environmental impact of black carbon and the most cost-effective ways to reduce its emissions to improve public health and reduce global warming.

So if the science is robust and the political will is there, why would EPA steer away from black carbon as an “easy win” on climate change? Here is my guess:

EPA seems to have painted itself into a corner with its endangerment finding. If black carbon is a pollutant due to its role in global and regional climate change, then as a precedent it opens up the door to a lot of uncomfortable questions and potential actions. For instance, if black carbon is an important forcing that affects the climate system with negative impacts, then why not water vapor emissions from land use, which also has been shown to influence local and regional climates? What about other land use change that alters surface energy budgets, such as albedo changes, irrigation, urbanization, and land clearing? And so on. Black carbon is an inconvenient forcing, and thus for EPA, rather than open up a can of worms, they have decided to follow the tried and true approach of hiding behind uncertainty.