Do plants prefer the hazy skies brought on by pollution to the clean atmosphere envisioned by environmentalists, regulators and the public? That's the implication of a new study of exactly how much plants, ranging from broadleaf trees to grasses, have been benefiting from the pollution brought on by the particles—from soot to sulfur dioxide molecules—that burning fossil fuels leaves in the air.

That apparent benefit is because plants do their best photosynthesis—the chemical process that uses chlorophyll in their leaves to turn sunlight and carbon dioxide (CO2) into plant food and oxygen—under so-called diffuse radiation, or hazy skies, that scatters the sunlight, thereby distributing it more evenly. Ecosystem modeler Lina Mercado from the Center for Ecology and Hydrology headquartered in Wallingford, England, and her colleagues' study, published in Nature, found that plants stored 23.7 percent more CO2—the leading greenhouse gas causing climate change—between 1960 and 1999 thanks to more efficient photosynthesis brought on by air pollution scattering sunlight. Less CO2 storage in the plant "carbon sink" means more in the atmosphere, accompanied by more global warming.

"Plants often thrive in hazy conditions such as those that exist during periods of increased atmospheric pollution," Mercado says. But as the skies are cleaned up, "the contribution of diffuse radiation to the land carbon sink could disappear by the end of the 21st century."

The modeling results mimic the impact of the 1991 eruption of Mount Pinatubo in the Philippines that increased atmospheric haze and resulted in lower global atmospheric concentrations of CO2 in 1992 and 1993. All told, polluted skies resulted in the uptake of an extra 440 million metric tons of carbon per year between 1960 and 1980, declining to just 300 million metric tons of carbon per year between 1980 and 1999. Further declines are expected as skies clear going forward as U.S. and European Union regulations reduce sulfur dioxide and particulate emissions by attaching scrubbers to smokestacks, among other efforts. Although the scrubbers and other pollution controls are being added to combat aerosols, among other air pollution problems, few such measures are in place for CO2. "We conclude that steeper cuts in fossil-fuel emissions will be required to stabilize the climate if anthropogenic aerosols decline," the researchers wrote.

To be clear: there is no doubt that aerosols are bad for human health. The tiny particles contribute outsize health effects ranging from asthma to heart disease. That's why environmental regulators have focused on reducing and removing the aerosols from the atmosphere.

And aerosols have other environmental impacts, which this modeling study did not examine. "Aerosols and clouds affect not only diffuse and direct-beam forms of radiation but also other factors such as temperature and precipitation that are also important for the dynamics of the terrestrial carbon cycle," says environmental scientist Lianhong Gu of Oak Ridge National Laboratory, who first showed the effect of the Pinatubo eruption on photosynthesis. This "indicates that the two most uncertain factors in projecting climate change—clouds and aerosols—are more uncertain than we thought."

And that points up the complexity of trying to design policies to mitigate both air pollution and climate change. Hazy interactions can be important, both for carbon-sink reasons and the link between irrigated agriculture and declining rainfall, says climatologist Dev Niyogi of Purdue University. Further studies will be needed and the lifetimes of the various pollutants taken into account—aerosols last just a few weeks at most in the atmosphere, whereas CO2 can linger for a century—and atmospheric changes must be examined as a whole, he notes.

But it now seems clear that "global dimming" "has acted to increase the uptake of carbon by the land [plants], which has helped to slow the rate of increase in atmospheric CO2," Mercado notes. And her co-author climate modeler Peter Cox of the University of Exeter in England adds: "As we continue to clean up the air in the lower atmosphere, which we must do for the sake of human health, the challenge of avoiding dangerous climate change through reductions in CO2 emissions will be even harder."