Remediating long-term effects of fossil fuel combustion and other human-driven processes filling the atmosphere with invisible particles will be even costlier than previously thought, a Cornell earth scientist is claiming in a new study.

Natalie Mahowald, associate professor of earth and atmospheric sciences, reports in the journal Science, Nov. 11, that man-made aerosols affect climate through long-term effects on biogeochemical processes, largely due to changes in the atmospheric concentration of carbon dioxide. The result is that policies for reaching lower carbon targets will require extra effort, due to the hidden effects of aerosols.

"The aerosol indirect effect on biogeochemistry that people have been ignoring is actually the same order of magnitude as both the [better-known] aerosol direct and indirect effects, so it can be quite important," she said.

An aerosol is a solid or liquid suspended in the atmosphere -- for example, air pollution haze on a clear day. A large contributor to man-made aerosols is the burning of fossil fuels. Typically, aerosols have a cooling effect on the planet, due to the way they reflect incoming solar radiation.

Aerosols also have a well-known effect on clouds -- their size, shape and precipitation, which in turns affects the planet's surface radiation and how it is absorbed and readmitted.

Mahowald draws attention to a lesser known further impact of aerosols on a longer time scale: on land and ocean biogeochemical cycles. Aerosols from the burning of fossil fuels and other human activities can add nutrients or pollutants to ecosystems, which appears to have enhanced carbon dioxide uptake in the land and oceans.

In addition, aerosols have resulted in a cooler climate, which also can enhance carbon dioxide uptake by natural systems. Overall these changes in biogeochemical cycling result in a net uptake of carbon dioxide from aerosols, equivalent to a -0.5 +/- 0.4 W/m2 radiative forcing, similar in magnitude to the direct radiative effect and indirect aerosol-cloud impact.

While aerosols do have a known cooling effect, Mahowald says, it is important to recognize their long-term effects in terms of cleanup efforts. According to her study, aerosol remediation efforts to improve public health over the next 100 years will result in less carbon dioxide being taken up by the land and ocean, and will have a net warming effect.

The result will be the need for a more aggressive emissions-control effort to make up the difference. In fact, she argues that in the last 150 years, the increase in atmospheric carbon dioxide would have been as much as 150 parts per million, instead of the observed 100 parts per million, were it not for the direct and indirect effects of aerosols on biogeochemical cycles.

The study was supported by the National Science Foundation, NASA and the Atkinson Center for a Sustainable Future at Cornell.