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There’s some fascinating new research about “CO2 domes,” invisible clouds of carbon pollution that hover above urban areas. Bradford Plumer at The New Republic does a great job setting the context:

Does it matter where carbon dioxide is emitted? From a climate perspective, at least, the standard answer has always been, “Not really.” Carbon dioxide mixes pretty evenly and uniformly throughout the atmosphere, so that the heat-trapping gases coming out of a factory in China have the same effect on global temperatures, pound for pound, as the greenhouse gases emitted by, say, cars in Delaware. (This is in contrast to a number of other air pollutants, whose effects are often localized—sulfur dioxide only causes acid rain in discrete areas.)

The new finding:

But a new study just published in Environmental Science and Technology by Stanford’s Mark Jacobson adds a slight twist to this standard view. Older research has found that local “domes” of high CO2 levels can often form over cities. What Jacobson found was that these domes can have a serious local impact: Among other things, they worsen the effects of localized air pollutants like ozone and particulates, which cause respiratory diseases and the like. As a result, Jacobson estimates that local CO2 emissions cause anywhere from 300 to 1,000 premature deaths in the United States each year. And presumably the problem’s much worse in developing countries.

Mark Jacobson, professor of civil and environmental engineering and director of the Atmosphere/Energy Program at Stanford, has been vocal about the need for a complete clean-energy transformation. This week, with the political world consumed by health care, his work offers a reminder that carbon pollution is a serious health problem. It makes traditional air pollution—such as particulates and ozone—more harmful, so it poses particular threats to the places with the worst air pollution—cities.

Here’s a map of CO2 released from fossil fuels (with red and yellow marking the biggest pollution points), compiled from 2002 data by the Vulcan Project at Purdue University. It’s a map of emissions, which isn’t quite the same as airborne concentrations, but it gives a sense of where pollution happens:

Map courtesy of Purdue University Department of Earth and Atmospheric Sciences

Jacobson’s urban-dome research presents two implications worth teasing out:

Trouble for cap-and-trade? The new evidence adds a wrinkle to cap-and-trade plans by suggesting that it matters where pollution happens. Cap-and-trade rests on the assumption that a ton of carbon has the same impact regardless of where it’s emitted, so it doesn’t matter if a factory in Nashville and a power plant in Phoenix trade emission permits. It only matters where emissions can be reduced most cheaply.

But, says Jacobson, “This study contradicts that assumption.” Stanford’s press release on the research plays up the contradiction; “Urban CO2 domes increase deaths, poke hole in cap-and-trade proposal,” blares the headline.

If the research proves correct, it doesn’t argue against cap-and-trade so much as highlight the need for a multi-pronged approach to CO2 regulation. The Clean Air Act can set plant-by-plant performance standards while a declining cap covers the broader economy. (That’s the approach taken by the Kerry/Boxer Clean Energy Jobs & American Power Act.) So the study shouldn’t be used to entirely discount the idea of cap-and-trade plans–but that doesn’t mean it won’t be.

Urban vs. rural. Jacobson’s research also pits the interests of rural and urban communities against each other. Cities could stand to suffer more under climate change, but the senators representing large urban areas already have proportionately less power to push through legislation that would curb CO2 pollution. California, with its 37 million residents and numerous polluted urban areas, has two senators who want to enact climate legislation; Wyoming, with 540,000 residents and vast expanses of rural land, has two senators who oppose climate legislation.

Urban and rural areas have already been at odds over climate policy—and that was before we had any evidence that cities might really get the short end of the stick. The “domes” research provides more fodder for the fight. It underscores the essential unfairness of the effects of carbon pollution, and raises the question of just how much Wyoming should have to say about the health of Californians.