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Clouds May Indeed Have Silver Lining

May 11, 1989

BALTIMORE (AP) _ Cloudy skies may indeed have a silver lining when it comes to the ″greenhouse effect,″ say researchers who have calculated that just a few more clouds could offset rising temperatures.

But scientists gathered for a meeting of the American Geophysical Union declined to predict that cloud cover will actually increase to offset the rising temperatures that have been widely discussed in recent years.

The greenhouse theory has attracted widespread attention with scientists warning that increases in carbon dioxide and other gases could trap heat and warm the earth, disrupting climate and crops and causing major changes for mankind.

Some researchers believe the planet’s temperature has risen slightly during the last century, although debate continues among scientists.

Rising temperatures could result in more water evaporating and, thus, more clouds.

″If the climate warms and the amount of liquid water in the cloud increases that can lead to more reflecting clouds. More reflecting clouds means that the total amount of energy available to the Earth’s energy system is reduced,″ explained Tony Slingo of the National Center for Atmospheric Research.

″That acts as a thermostat, bringing the climate back into equalibrium,″ he said.

The sensitivity of climate to clouds is so enormous that only very modest changes in particle size are needed to wipe out the (carbon dioxide) warming. Now whether those process really are operating and they really are wiping out the warming remains a question, he added.

″Let’s be very careful about that. We don’t know that, as the climate warms up with the greenhouse effect that these clouds are going to increase sand compensate for it...don’t jump to any conclusions,″ commented Bruce A. Albrecht of Penn State University.

But Albrecht agreed that the relatively low-level stratocumulus clouds - the thick sheets of cloud that are fairly common - can have a major effect on cooling the atmosphere.

″The number that we toss around ... if you increase the amount of stratocumulus by 4 percent″ that would be enough to compensate for a doubling of the carbon dioxide in the atmosphere, he said.

″The way these clouds work, they are highly reflective in the short wave but they have very little effect on the long-wave radiation,″ he explained. Thus, radiation arriving from the sun that would ordinarily warm the surface of the planet gets reflected back out into space, while long wave radiation cooling the earth isn’t blocked from leaving.

On the other hand, cirrus clouds, those high wispy ones that are hard to see, actually add warmth to the Earth by trapping heat from the sun in much the same way that carbon dioxide and other so-called greenhouse gases do, explained David Starr of the National Aeronautics and Space Administration.

These clouds are the most common type, covering 25 percent to 40 percent of the Earth’s surface, Starr said.

″The importance of cirrus clouds to climate is similar to that of the greenhouse gases,″ added Howard Hanson of the University of Colorado at Boulder.

Recent cloud studies have convinced researchers that the effects of clouds are much more complex than they had previously thought, Hanson said.

Cirrus clouds are not well understood and are difficult to study, he said, with scientists currently engaged in trying to learn more about these ice clouds high in the atmosphere.

Tiny aerosol particles in clouds caused smaller drops to form, and drizzle removes water from clouds, both factors that complicate the process of calculating their effect on climate, said Albrecht.

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