Aerosols cloud the climate picture

Modeling the climate just got a little more complex. A new simulation that considers chemical interactions between various gases and atmospheric aerosols is giving scientists and policy makers better estimates of the climate-altering effects of those gases, scientists report.

Some atmospheric gases — known as greenhouse gases — trap heat and boost the planet’s surface temperature. This process keeps Earth habitable, but nowadays, many scientists say, the planet may be getting too much of a good thing. Though most climate simulations include the direct, heat-trapping effects of these atmospheric constituents, which can readily be measured in a lab, few account for how their presence either increases or decreases atmospheric concentrations of planet-cooling aerosols, says Drew Shindell, a climate scientist at NASA’s Goddard Institute for Space Studies in New York City. “These effects are generally understood but not well quantified,” he notes.

Aerosols, particles so small that they remain suspended in the air, come from natural sources such as volcanoes and sea spray, but also form in chemical reactions involving the gases spewing from tailpipes and smokestacks. Light-colored aerosols, such as sulfate droplets, scatter sunlight and reflect some of it back into space, cooling Earth’s surface just as natural clouds do.

Recently, Shindell and his colleagues modified a NASA climate model to consider chemical reactions among major atmospheric constituents and the resulting effect on aerosol concentration. Their report, in the Oct. 29 Science, reveals that some greenhouse gases have a substantially stronger warming effect than previously recognized because they take part in reactions that destroy aerosols, while others actually tend to boost concentrations of cooling aerosols.

The new study “shows that you can’t make efficient climate policy without considering the effects of air pollution,” says Almut Arneth, an ecosystem modeler at Lund University in Sweden.

New portions of the revised model consider the influence of methane, carbon monoxide and nitrogen oxides on the atmospheric concentration of hydroxyl radicals, highly reactive molecules sometimes referred to as the atmosphere’s detergent. Hydroxyl concentrations can be depleted as these radicals react with gases in the atmosphere, and this slows the reactions that produce light-colored, light-scattering sulfate aerosols, Shindell says. “And a lower number of aerosols means a lower cooling effect,” he notes.

Analyses using the revised model suggest that the aerosol-stifling power of methane and carbon monoxide considerably boosts the planet-warming effect of these gases. Previous studies have shown that a kilogram of methane, over the course of a century, warms Earth about 25 times more effectively than a kilogram of carbon dioxide does. But add in methane’s hydroxyl-consuming effect, and its planet-warming potential jumps to 28 times that of CO2, an increase of 12 percent, Shindell says. (Scientists use carbon dioxide as a baseline largely because it is a common, long-lived greenhouse gas in the atmosphere and its warming effects are well known.)

Similarly, carbon monoxide’s greenhouse warming potential rises from 2.2 times to 3.3 times that of CO2 when its hydroxyl-consuming effect is considered. If the inhibiting influence of these two gases on the formation of planet-cooling clouds is also incorporated into the model, their greenhouse effect increases even further.

But the news isn’t all bad: The team’s model suggests that various nitrogen oxides produced by fossil-fuel burning tend to increase aerosol concentrations through a complex series of reactions, thereby tripling their cooling power. NOx emissions are increasing in developing countries, but still play a relatively minor role in the overall global greenhouse effect.

Shindell and his colleagues haven’t yet used these new values to make long-term climate predictions under various emissions scenarios. For now, however, the new model could give scientists insight into recent climatic trends. For instance, Shindell notes, “We’ll be able to learn how much greenhouse gas warming that aerosols have been masking.” Recent studies have found that as much as 20 percent of the warming in Europe since the 1970s stems from a decline in aerosols, such as fog and haze, during that period (SN: 2/14/09, p. 9).

Results of the new model will also enable policy makers to better determine the possible climatic effects of significantly reducing specific types of emissions, especially those that substantially affect pollutants and other aerosols.

Because many greenhouse gases also trim aerosol concentrations, Arneth says, people will have to cut emissions even further to keep Earth’s average temperature from increasing 2 degrees Celsius above pre–Industrial Revolution levels. “We not only have to think about greenhouse gases, but about pollution too,” she says.

A commentary in the same issue of Science suggests that megacities, areas with more than 10 million residents (SN: 9/8/07, p. 152) are tempting targets for emissions controls. While those areas are large economic engines, they also can provide a source of funding to address issues of climate change and air quality, says David D. Parrish, an atmospheric scientist at NOAA’s Earth System Research Laboratory in Boulder, Colo. Megacities also have a sufficient population density to make energy-efficient buildings and cars, as well as mass transit systems, effective tools in the fight to reduce CO2 emissions, he and colleague Tong Zhu of Peking University in Beijing note.

The newly revised NASA model only begins to address the complexities of atmospheric chemistry, Shindell says. It doesn’t, for example, consider how pollutants such as ozone and acid rain suppress the uptake of carbon dioxide by trees and other plants. “What we’re doing is state-of-the-art, but we need to advance substantially,” he adds. However, he admits, “some of the effects we’re missing locally may not be important globally.”

Arneth agrees, “There’s quite a lot of work is left to be done.”