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Cloudy future.
Excess CO2 in the atmosphere could worsen already hazy days, like this one in Los Angeles, California.

CO2 and Pollution Don't Mix Well

Eli is a contributing correspondent for Science magazine.

More bad news about the effects of global warming: The carbon dioxide (CO2) that's driving it could also be worsening air pollution around the world. A new climate-modeling study indicates that as the world warms, CO2 will cause more pollution-related deaths, although other forms of air pollution will continue to kill far more people. The effect may be small, but the work adds new detail to efforts to quantify how climate change might exacerbate pollution, researchers say.

Atmospheric scientist Mark Jacobson of Stanford University in Palo Alto, California, designed a computational model that included atmospheric chemistry and air-ocean interactions and simulated the distribution of gases, including the kinds of pollutants commonly found in cities. It also assumed that a warmer world would feature fewer winds to blow pollution up and out of cities, and that higher levels of water vapor around cities could help catalyze the production of ozone. Formed when pollution is struck by sunlight, ozone is the main component of smog and can worsen respiratory disease and asthma.

To determine how variations in CO2 would modulate these different variables, Jacobson ran his model in two ways: once with 2006 CO2 levels, and again with preindustrial levels from the year 1750. After analyzing pollution levels for carcinogens, ozone, and particulates, Jacobson found that each degree of warming caused by CO2 could be responsible for roughly 1000 deaths to the 50,000 to 100,000 annual deaths in the U.S. related to air pollution, he reports in an issue of Geophysical Research Letters published earlier this month. "You're not seeing a huge percentage of deaths, … but in terms of absolute numbers, it's still a lot of people," Jacobson says. Even more important than the total number, he argues, is the model's suggestion that climate change has the greatest pollution-related impact in areas that are already heavily polluted.

"This work adds an important component to our overall understanding of the links between greenhouse gas emissions and adverse health impacts today," says Kim Knowlton of Columbia University and the Natural Resources Defense Council in New York City. Bart Croes, a staff scientist at the state of California's official Air Resources Board in Sacramento, says the work's finding bolsters the state's argument that it should be allowed to set controls for air pollution that are stricter than national standards, including limits on CO2 emissions. The U.S. Environmental Protection Agency disagrees and today issued a final decision rejecting California's request for the rules. "We will review the study as appropriate, but the California waiver decision stands following the law," an EPA spokesperson said of Jacobson's paper.

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