Does dirty air cool the climate?

If the impact of aerosol cooling is small and the climate is less sensitive to CO2 increases than current estimates hold, warming might fall toward the low end of projections by the IPCC's science working group. If cooling has been large and the climate is very sensitive, warming could exceed their projections.

Uncertainties about aerosol cooling and climate sensitivity to greenhouse gasses remain large. But as new studies have emerged, the importance of these two factors has appeared to grow, says Dr. Andreae, who heads the biogeochemistry department at the Max Planck Institute for Chemistry in Mainz, Germany. Aerosol cooling may have been greater in the past than we believed, and the climate may be more sensitive to greenhouse-gas accumulation.

While the European team's broad description of cooling and sensitivity is the latest warning, the devil is in the details, other climate scientists say.

For example, the team's calculations appear to be based in part on the assumption that all of the warming since the 1850s is due to human activities. The IPCC, in contrast, has been willing to attribute "most" of the warming over the past 40 years. The difference seems small, only a few tenths of a degree. But it leads to a lower climate sensitivity than Andreae's team calculates, notes Theodore Anderson, a climate researcher at the University of Washington in Seattle.

Moreover, ice core data from the last glacial maximum some 20,000 years ago place limits on the climate's sensitivity, notes Gavin Schmidt, a climate researcher at NASA's Goddard Institute for Space Studies in New York. This leads to a climate system less responsive to changing CO2 levels than the European team's model implies - and in line with estimates that underlie current climate-change projections.

In addition, all pollutants are not created equal, both researchers add. Sulfur-based aerosols from burning fossil fuels tend to reflect sunlight back into space, acting as an atmospheric coolant. But other aerosols, such as black-carbon soot, warm the atmosphere. The relative contribution of each is highly uncertain. But their offsetting effects could lead to weaker aerosol cooling than Andreae's team suggests. Moreover, aerosols have indirect but important effects on clouds that the model fails to capture.

If nothing else, the effort highlights the importance of a set of satellite missions aimed at solving the cloud-aerosol problems. Two satellites are set for launch later this year that will round out a suite of five satellites dubbed "the A Train." The satellites orbit Earth in a line so their unique instruments can cover the same swaths of the atmosphere. The satellites fill wide gaps in measurements of aerosols, clouds, their movements, and their interactions with each other and with climate, helping to zero in on the climate's true sensitivity to a doubling of CO2 concentrations.

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