GAINESVILLE, Fla., May 17, 2001 (UPI) -- Researchers have found evidence that a drought, believed to have caused the collapse of the Maya civilization in A.D. 900, may be linked to a 206-year cycle possibly related to solar activity.

Just a change in solar activity, however, would not be enough to cause the drought. Some type of climate amplification mechanism also would be necessary.

University of Florida researchers said sediments taken from Lake Chichancanab on the Yucatan Peninsula show evidence of drought during the time the Maya civilization is believed to have collapsed.

Lake Chichancanab is rich in gypsum and during dry periods it builds up on the lake bottom. The archaeological record shows abandonment of cities and a slowdown in building around the time the sediments show a drought. This portion of the researchers' work confirms earlier findings.

The gypsum deposits occur in a 208-year cycle. The researchers noted this corresponds to a 206-year cycle of isotope deposits in the sediment thought to be related to increased solar activity.

David Hodell, a University of Florida geologist, told United Press International: "Solar intensity, particularly the solar wind that emanates from the sun, actually deflects cosmic radiation away from the earth. So during times when the sun is more intense and the solar wind is more intense you deflect more cosmic rays away from the earth and the production rate of cosmogenic nuclides (nuclides caused by cosmic rays) is diminished."

Hodell said the fact that we see the same cycle of 208 years in the drought record from Yucatan "suggests that the causal mechanism, at least at this 208-year period of drought recurrence, is variations in the intensity of the sun."

When radiation from the sun increases, it prevents some cosmic rays from entering the earth's atmosphere. As a result, fewer isotopes of carbon-14 and beryllium-10 are formed in the stratosphere and subsequently fewer are found in lake sediments in a 206-year cycle observed going back at least 1,000 years.

Hodell, along with colleagues Mark Brenner and Jason Curtis, are candid that the increase in the solar radiation by itself would not be enough to cause climate change.

"The energy received by the earth at the peak of the solar cycle increases less than one-tenth of 1 percent, so it's likely that some mechanism in the climate is amplifying the impact in the Yucatan," Hodell said.

Although the theory exists that the cycle in beryllium-10 and carbon-14 deposits is related to a 206-year cycle in the sun, proof is lacking.

"There really is no understanding of whether there is such a variation in the sun or why it would exist in the sun," said David Rind, a climate modeler at NASA Goddard Institute of Space Studies. "The only way you'd get that from the sun is to be able to model the sun and see whether the sun has cycles like that, that are inherent in its radiance."

Rind said an increase of one-tenth of 1 percent in solar radiation would be associated with a mean global warming of about one-half of a degree Fahrenheit (about one-quarter of a degree Centigrade) if the radiation continued over a long period of time.

Rind explained how the mechanism of a fairly small change in solar radiation, one-tenth of 1 percent, might be amplified in the atmospheric climate system.

He said when more UV radiation comes in at low latitudes, such as the Yucatan Peninsula, it increases the amount of ozone created high in the stratosphere. The stratosphere warms and the temperature gradient increases, causing an increase in the west winds in the stratosphere.

With this altered west wind in the stratosphere, the huge waves of wind in the atmosphere start to propagate differently, including in the troposphere, the part of the atmosphere in contact with the earth's surface. And as they propagate differently, there is a change in the path that storms take.

"If you were to change storm tracks due to the change in solar (radiance) it's not inconceivable that one of the storm track differences would be to provide less moisture for a specific region, say, the Yucatan," Rind said.

Another theory involves the idea that decreased isotopes um -10 may be related to a decreased opportunity for the atmospheric conditions that lead to rainfall.

While saying he could not comment on the solar effect aspect of Hodell's work, Harvey Weiss, a Yale University professor and noted researcher into the Maya collapse, said: "Mayan archeology will never return to a pre-Hodell state. This is landmark work. The frequency and severity of drought is coincident with the frequency and severity of collapse. It makes drought the only convincing and tested casual agent."

The research is reported in the May 18 issue of Science.

Copyright 2001 by United Press International.