Aerosols affect climate more than satellite estimates predict

Science Daily: Aerosol particles, including soot and sulfur dioxide from burning fossil fuels, essentially mask the effects of greenhouse gases and are at the heart of the biggest uncertainty in climate change prediction. New research from the University of Michigan shows that satellite-based projections of aerosols' effect on Earth's climate significantly underestimate their impacts.

...Aerosols are at the core of "cloud drops" -- water particles suspended in air that coalesce to form precipitation. Increasing the number of aerosol particles causes an increase in the number of cloud drops, which results in brighter clouds that reflect more light and have a greater cooling effect on the planet.

As to the extent of their cooling effect, scientists offer different scenarios that would raise the global average surface temperature during the next century between under 2 to over 3 degrees Celsius. That may not sound like a broad range, but it straddles the 2-degree tipping point beyond which scientists say the planet can expect more catastrophic climate change effects. The satellite data that these findings poke holes in has been used to argue that all these models overestimate how hot the planet will get.

"The satellite estimates are way too small," said Joyce Penner, the Ralph J. Cicerone Distinguished University Professor of Atmospheric Science. "There are things about the global model that should fit the satellite data but don't, so I won't argue that the models necessarily are correct. But we've explained why satellite estimates and the models are so different."

Penner and her colleagues found faults in the techniques that satellite estimates use to find the difference between cloud drop concentrations today and before the Industrial Revolution. "We found that using satellite data to try to infer how much radiation is reflected today compared to the amount reflected in the pollution-free pre-industrial atmosphere is very inaccurate," Penner said. "If one uses the relationship between aerosol optical depth -- essentially a measure of the thickness of the aerosols -- and droplet number from satellites, then one can get the wrong answer by a factor of three to six....

Soot and cinders... Three locomotives adjacent to the coaling stage at Eastleigh locomotive depot in the UK. Shot by OLU, Wikimedia Commons via Geograph UK, under the Creative Commons Attribution-Share Alike 2.0 Generic license