If you are concerned about man-made climate change, keep an eye on aerosol pollution. The concentrations of tiny particles, called aerosols, that float in the global atmosphere are on the rise. They come from dusty deserts and industrial emissions. They can change the way clouds form and can redistribute rainfall. They heat or cool parts of the atmosphere and Earth's surface depending on their composition. Their influence is one of the biggest unknowns in climate science. Until scientists know more about what aerosols are up to, they can't fully predict future climate change. Global warming due to carbon dioxide and other greenhouse gases is only part of the picture.
And so, the aerosol hunt is on.
On Monday, NASA released the first images from Calipso, one of two satellites launched in April to make detailed observations of clouds and aerosols. More images are needed to produce meaningful conclusions.
But two weeks ago, NASA took what it called "a big step forward" in its understanding of aerosols. The late Yoram Kaufman at NASA's Goddard Space Flight Center in Greenbelt, Md., and colleagues uncovered what aerosol particles do to clouds. It depends on the aerosols' color. Goddard's Lorraine Remer explains: "When the overall mixture of aerosol particles in pollution absorbs more sunlight, it is more effective in preventing clouds from forming. When pollutant aerosols are lighter in color and absorb less energy, they have the opposite effect and actually help clouds to form."
The researchers explained in the online edition of the journal Science how they used observations from robot observers at 200 sites around the world to pin down this effect. They also made extensive surveys of sky conditions from 17 locations such as Beijing and Rome. They estimate that the net result, world-wide, has been a 5 percent increase in global cloud cover.
Satellite data shows that aerosol pollution comes from many sources and travels quickly. China's pollution reaches North America in five days. North American pollution reaches Europe in three days.
Last May, Timothy Garrett and Chuanfeng Zhao at the University of Utah in Salt Lake City reported a study showing how aerosol pollution helps heat up the Arctic. Dr. Garrett explained that "particulate pollution from factories and cars can be transported long distances to the Arctic, where it changes clouds so they become more effective blankets, trapping more heat and further aggravating climate warming." The effect is especially strong in winter when Arctic air is very stable and pollution lingers. It makes the surface 2 to 3 degrees F. warmer when aerosol-laden clouds cover the area than when the air is clean.
Also in May, Chul Eddy Chung and Veerabhadran Ramanathan at Scripps Institution of Oceanography in La Jolla, Calif., showed how aerosols can modify greenhouse warming. Summarizing their paper in the Journal of Climate, Dr. Ramanathan said that aerosol pollution, "also called 'brown haze,' appears to be masking the greenhouse warming in the northern Indian Ocean, while the greenhouse warming continues unabated in the southern Indian Ocean." He added that we are starting to see that air pollution affects sunlight and is potentially causing a major disruption to rain patterns, with some regions getting more and some less.
But much more data and research is needed to fully understand what all this means to our climate.