For nearly a decade, University of Washington atmospheric chemist Robert Charlson has advanced the notion that, in some regions, tiny particles from industrial pollution are actually countering the atmospheric warming effects of greenhouse gases. The theory is based on nearly 30 years of observations, using technology patented by the UW, that relate the amount of visible haze to sunlight reflected back into space and to the mass and composition of haze particles.
Now NASA is planning a mission, called PICASSO-CENA, that will provide crucial data to help scientists better understand the role of those submicron-sized particles in human-caused climate change. The mission will use light detection and ranging, or lidar, the optical cousin of radar, to globally measure how much sunlight is reflected into space by clouds and pollution particles. Those particles, called aerosols, make up the familiar hazes in places like the Eastern United States or the Los Angeles area.
Aerosols are of particular interest to Charlson, an emeritus professor of atmospheric sciences and chemistry, because even on a clear day they are present in the skies in and downwind from industrial regions of the Northern Hemisphere. They are about the same size as the wavelength of the light they reflect, which is what makes them so effective - if they were larger or smaller their effect would be far less. The particles partially reflect the sun's heat and partially counter greenhouse warming, particularly in the Northern Hemisphere.
"You could reach the frightening conclusion that we have learned how to pollute just right to prevent climatic disaster. But we haven't learned how to pollute just right because they don't neatly offset each other," Charlson said. "The greenhouse effect works 24 hours a day everywhere and the aerosol effect only works during the day and only in certain places."
In addition, he said, greenhouse gases last for years while aerosols dissipate in days or weeks. With a serious reduction of industrial emissions, the atmosphere's aerosol content could decline quickly, raising the prospect of more rapid climate warming.
Charlson was among the first to propose that aerosols reflect enough sunlight away from Earth to offset the effects of greenhouse warming regionally. He was the lead author for a 1992 article in the journal Science that led to the formation of a National Academy of Science panel to study aerosol effects. That panel's recommendation was the impetus for the U.S.-French mission PICASSO-CENA (Pathfinder Instruments for Cloud and Aerosol Spaceborne Observations-Climatologie Etendue des Nuages et des Aerosols), scheduled for launch in 2003.
For three years the satellite will travel in a north-south polar orbit 435 miles high, making repetitive passes and constantly gathering information while the Earth spins below. The sun-synchronous orbit means the spacecraft will cross the equator alternately around noon and midnight, collecting data both in daylight and darkness. Lidar will sense the density of aerosols and clouds by how much light is reflected. The mission will provide the most comprehensive information on aerosol and cloud reflection, far surpassing small and widely separated readings taken on the ground or from airplanes. The work will supplant theoretical models with hard data.
The mission is a collaboration of NASA's Langley Research Center; the French space agency, Centre National d'Etudes Spatiales; the Institut Pierre Simon Laplace near Paris; Hampton (Va.) University; Ball Aerospace and Technology Corp. of Boulder, Colo.; and NASA's Goddard Space Flight Center.
Charlson, part of the 17-member science team, wants a successful satellite mission that will bring a better understanding of climate change issues among scientists and by the public. For instance, he notes the idea of climate change is often expressed simplistically as "global warming." But, while the Earth's mean surface temperature has risen about 0.6 degree Celsius this century, some places are actually cooler, perhaps because of the aerosol influence. He is concerned that oversimplification will lend credence to those who argue there is no reason for concern because the temperature has risen only a fraction of a degree. That view, he said, doesn't take into consideration a host of other possible climatic changes - such as the amount and form of precipitation, severe storms and unusual weather extremes.
"The changes in the chemical composition of the atmosphere influence all of these things," Charlson said. "The driving concern is the fact that the human population has changed the chemical composition of the entire atmosphere of the planet."
The best-documented change is a 30 percent increase in carbon dioxide during the last century that, among other things, has triggered a fundamental change in photosynthesis for all plants, he said. It remains to be determined when and where that change might be good or bad.
Other atmospheric changes aren't as well documented. PICASSO-CENA will provide greater data about particles from industrial pollution. But there are many other sources of aerosols, including volcanoes, burning forests and agricultural waste, and diesel engines, and their effects remain to be measured, Charlson said.
Additional information about the PICASSO-CENA mission can be found on the Internet at http://www-arb.larc.nasa.gov/picasso-cena/picasso.html
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