Atmospheric Aerosols Brighten Clouds
By Cat Lazaroff
UPTON, New York, March 5, 2002 (ENS) - Aerosols - tiny particles of chemicals and other pollutants - make clouds reflect more sunlight, which could help cool Earth's climate, a new report suggests. This brightening effect must be considered by climate researchers working to assess the magnitude of global climate change, argue scientists studying the phenomenon.
Atmospheric scientists have long suspected that microscopic aerosol particles from industrial processes increase the brightness of clouds, resulting in greater reflection of sunlight and cooling of Earth's climate. However, this supposition was based on model calculations rather than observations, and these model calculations are very uncertain.
Scientists at Brookhaven National Laboratory and Purdue University have now combined satellite measurements of cloud brightness, water content and other variables with model calculations of atmospheric aerosols to demonstrate the brightening effect.
"We're not saying that aerosols can counteract the greenhouse effect, but rather that we need to know how much of a cooling effect they have so we have a clearer picture of the greenhouse effect," said lead scientist Stephen Schwartz, an atmospheric chemist at Brookhaven. "To whatever extent aerosols are offsetting greenhouse warming, then the offset is the unseen part of the greenhouse 'iceberg'."
One difficulty in measuring the effect of aerosols is knowing their concentration, Schwartz explained. Aerosols such as sulfur compounds result from emissions by fossil fuel burning power plants and other industrial processes.
Other aerosols may include nitrates, organic particles, fly ash and mineral dust, formed by fossil fuel combustion and burning of forests and other biomass. Aerosols are typically found in the lowest three to four kilometers above Earth's surface, and fall out of the atmosphere in about a week.
"Because of this short residence time, aerosols are highly variable as a function of location and time, which makes it tough to measure their concentrations on a global scale," Schwartz said.
Schwartz's team has been working for more than a decade to develop and refine a "chemical transport model" to calculate aerosol distribution. The model uses archived weather data and weather prediction models to track the distribution of aerosols from industrial sources to various parts of the atmosphere.
"This model is the key to knowing where and when to look for the aerosol effect," Schwartz said.
By analyzing data from the model, the Brookhaven-Purdue team identified two one week episodes during April 1987 when the modeled concentration of sulfate aerosol over the North Atlantic Ocean - far from any local sources of aerosol emissions - increased significantly and then decreased over the course of each week.
These large variations in aerosol concentration and the fact that there were no high atmosphere clouds during these events made them ideal episodes for studying the effect of aerosols on cloud brightening.
The next challenge was to get the data on cloud brightness for that area over the same time period. For this, the scientists retrieved satellite measurements of radiance - how much light the clouds reflect - and optical depth - a value related to how much light is transmitted through the cloud. The team used these measurements to calculate the size of the cloud droplets and the liquid water path - the amount of liquid water in the cloud.
The findings show that, for a given liquid water path, cloud reflectivity was higher on the days with higher aerosol content than on the days with lower aerosol levels.
"If the effect is as widespread as we think it is, it would produce quite a substantial cooling effect on climate," Schwartz said.
"This new study provides a method of quantifying the phenomenon globally over the past 15 years using archived satellite data," added Schwartz. "Once this is done, we will have a much better idea of the true magnitude of the greenhouse effect."
Some researchers have suggested that deliberately increasing the reflectivity of clouds could help offset the greenhouse effect.
"This is an attractive thought, but it cannot work in the long run - because aerosols are so short lived in the atmosphere, whereas greenhouse gases accumulate over time," Schwartz said. "An ever increasing amount of aerosols would be required. We'd never solve the long term problem."
Also, Schwartz said, the aerosol effect may have a different geographical distribution from the greenhouse effect, and "the consequence of this mismatch is unknown."
The study appears in the February 19 issue of the "Proceedings of the National Academy of Sciences."
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