Clouds Greatly Affect Earth’s Temperature, Report Says

Clouds play a far greater role in maintaining the Earth’s atmospheric temperatures than previously believed, according to scientists who have revealed a “surprising” finding that could mean global warming from the predicted “greenhouse effect” could be either significantly more or less severe than expected.

The greenhouse effect, caused by increasing concentrations of carbon dioxide in the upper atmosphere, is expected to cause temperatures to rise significantly in the years ahead with potentially catastrophic results for wide areas of the planet.

On the surface, the scientists would seem to have discovered the obvious: that a cloudy day is cooler than a sunny day. But by using sophisticated instruments on three orbiting satellites, the scientists determined that the rate of cooling caused by clouds is far greater than had been thought, and the cooling affects the atmosphere on a global scale.

The scientists, who studied their data for four years before publishing the results in today’s issue of the journal Science, said the cooling from clouds is three to five times greater than the expected warming from the greenhouse effect.

That would suggest that clouds could offset, if not cancel, any global warming trend, but the scientists said that may not be true.

That is because some clouds have just the opposite effect--trapping the Earth’s heat and raising temperatures--so any major changes in global temperatures could create clouds that would enhance, not retard, the greenhouse effect.

The discovery adds a major new source of complexity to the greenhouse equation because the scientists do not know if clouds will make the situation better or worse, but they know now that clouds will be a major factor.

The finding also illustrates once again that “the globe is extremely delicately balanced,” said Veerabhadran Ramanathan of the University of Chicago, the lead author of the report.

The study would not have been possible without the use of instruments on satellites that can collect data on the entire planet, documenting even minor changes in widely separated areas, Ramanathan said in a telephone interview.

The key tool in the project is a satellite called the Earth Radiation Budget Experiment, which was launched from the space shuttle in 1984. The satellite, built by TRW of Redondo Beach, studies the Earth’s heat “budget,” which is the balance between the radiation the planet receives from the sun and the heat that escapes from the Earth into space. In addition, two polar-orbiting weather satellites are equipped with the same instruments, giving scientists three sources of data covering the entire globe.

Ramanathan called the finding both “exciting and surprising,” but he added that it raises more questions than it answers.

Scientists have long debated the role that clouds play in global temperatures, but in the past, research was limited to isolated geographical locations and it was impossible to tell whether the impact from clouds was anything more than a very localized phenomena.

As anyone who has ever been to the beach on a cloudy day knows, clouds block the warming rays of the sun, lowering temperatures in that area. But clouds can also warm local areas. In the tropics, for example, clouds trap the Earth’s heat near the ground, preventing the heat from being lost into space. That is why a cloudy evening is often warmer than a clear evening.

“The question has been, which of these effects, averaged over the Earth, is stronger,” Ramanathan said. In other words, do clouds make the planet warmer or cooler?

Ramanathan and five other scientists involved in the project believe their research has now answered that question.

The planet overall “would be significantly warmer” without the cooling from clouds, Ramanathan said. But that may hold true only for the present, because any long-term trends could upset the current delicate balance, he added.

Warming of the ocean, for example, would cause clouds to rise to altitudes as high as 100,000 feet. At that height, clouds would literally “suck the water from the ocean and pump it into the upper atmosphere,” he said.

That would create “an enormous greenhouse effect 50 times larger than all the human impact for the last 100 years,” Ramanathan said. On the other hand, clouds might stem the greenhouse effect by keeping overall temperatures low enough to protect the “critical balance.”

“It could go either way,” he said.

The other members of the team are R. D. Cess of the State University of New York, E. F. Harrison, P. Minnis and B. R. Barkstrom of NASA’s Langley Research Center, E. Ahmad of the University of Chicago and D. Hartmann of the University of Washington.