November 1, 2006 A NASA scientist, who died recently, helped understand how the color of aerosols determines its role in cloud formation. Light-colored aerosols -- such as those that predominantly occur in nature -- reflect light back to space, keeping cloud temperatures consistent to help form new clouds. Darker aerosols -- as most of those coming from man-made pollution -- absorb more sunlight, warming up clouds and evaporating water inside the cloud, causing the cloud to disappear.
BACKGROUND: NASA scientists have determined that the formation of clouds is affected by the lightness or darkness of air pollution particles, which in turn impacts the earth's climate. In a breakthrough study published in Science magazine, scientists explain why aerosols -- tiny particles suspended in air pollution and smoke -- sometimes stop clouds from forming, and in other cases increase cloud cover. Clouds deliver water around the globe, and also help regulate how much of the sun's warmth the planet holds. The capacity of air pollution to absorb energy from the sun is the key.
ABOUT THE STUDY: It is difficult to directly measure the impact of aerosols because aerosols only stay airborne for about one week -- unlike greenhouse gases like carbon dioxide, which can linger in the atmosphere for decades. So the NASA scientists conducted an extensive survey of sky conditions at 17 different locations, each representing different types of air pollution and weather patterns. They used automated instruments to record how much sunlight was coming from the sky; the measurements were made several times an hour at different times of the year.
THE RESULTS: Regardless of location or season, the same pattern emerged. There were lots of clouds when light-reflecting pollution filled the air, but many fewer clouds were recorded when the pollution was mostly light-absorbing. Here's why: When the overall mixture of aerosol particles in pollution absorbs more sunlight, it is more effective at preventing clouds from forming. When aerosols in pollution are lighter in color and absorb less energy, they actually help clouds form. The NASA researchers estimate that as a result, there could be as much as a 5 percent net increase in cloud cover worldwide. In badly polluted areas, such cloud cover changes could alter the availability of fresh water, and affect regional temperatures.
ABOUT GLOBAL WARMING: Global warming refers to an average increase in the earth's temperature -- which has risen about 1 degree Fahrenheit over the past 100 years -- which in turn causes changes in climate. A warmer earth may lead to changes in rainfall patterns, and a rise in sea level, for example, as polar glaciers melt. Some of this rise is due to the greenhouse effect: certain gases in the atmosphere trap energy from the sun so that heat can't escape back into space. Without the greenhouse effect, the earth would be too cold for humans to survive, but if it becomes too strong, the earth could become much warmer than usual, causing problems for humans, plants and animals.