SAN FRANCISCO?Contrary to the conventional wisdom, new computer modeling from the University of Michigan suggests that global warming might not be a product of human activity. Ironically, argues Joyce Penner, professor of atmospheric, oceanic and space sciences, carbon and sulfur emissions can have the reverse effect, serving to cool down the planet.

Penner, an expert in computer simulations of climate change, recently found that, whereas greenhouse gases have led to a warming of 2.5 Watts per meter squared, aerosols like soot particles and sulfuric acid reflect nearly twice as much energy under certain conditions.

“This effect clouds our understanding of climate change over the last 100 years, but still cannot protect us from the larger increases in greenhouse gases expected in the future,” Penner said. “If further research serves to uphold these initial findings, the warming we’ve seen over the last 100 years may simply be due to natural variability,” she said. Penner is presenting the work Friday (Dec. 12) at the fall meeting of the American Geophysical Union in San Francisco.

When floating freely in air, carbon aerosols from fossil fuel emissions add between .16 and .20 Wm-2 to the atmosphere, and thus heat it. But, according to the latest simulations by Penner and her colleagues at the Lawrence Livermore National Laboratory, in Livermore, Calif., and the Centre des Faibles Radioactivites, in Gif sur Yvette, France, carbon aerosols trapped in clouds may cool the earth’s atmosphere by as much as -4.4 Wm-2?or a net decrease of roughly 0.7 to 2.1 degrees Celsius, provided nothing changed in the future.

Penner said that the models are uncertain because they rely on poorly known estimates for natural sources of aerosols, so the actual number for negative forcing could be as low as -2.4 Wm-2. Even so, she said, the results are startling. “I had not expected to get such a large negative forcing from carbon aerosols in clouds. If these results hold up, we are going to have to do a lot more work to understand how climate might change in the future.”

U-M News and Information ServicesUniversity of Michigan