The report -- based on geological evidence from one of South America's largest lakes -- demonstrates how nature can produce sudden, unexpected climate changes that affect the entire planet.

The study, which appeared in the Jan. 26 issue of Science, uses sediment samples taken from the bottom of Lake Titicaca -- the world's highest lake navigable to large vessels.

Straddling the border between Bolivia and Peru, Titicaca is 120 miles long, 50 miles wide and has average depth of 500 feet. The lake is located more than 2 miles above sea level on the Altiplano, or High Plateau, of the northern Andes Mountains.

"The Altiplano is like a giant cup, and Titicaca is the deepest point in the vast plateau, so most of the precipitation in the Altiplano drains into the lake," says Stanford geologist Robert B. Dunbar, one of the authors of the Science study.

Because very little water drains out of Titicaca, the lake serves as a reliable archive of rainfall patterns over many centuries -- not just on the Altiplano, but in a large portion of tropical South America, according to Dunbar and his co-authors.

"Titicaca is the only large and deep freshwater lake in South America, and in deeper portions of the lake, sediment has accumulated continuously for at least the past 25,000 years," they add.

The authors point out that earlier studies of Titicaca relied on coring samples from the lake bottom taken at depths of 150 feet or less. To obtain an older and more complete climate record, a team of geologists led by Science co-author Paul A. Baker of Duke University collected three new samples at 270 feet, 450 feet and 690 feet below the surface.

Baker, Dunbar and their colleagues were able to reconstruct a history of precipitation in the Altiplano by determining how water levels in Lake Titicaca changed during the last 25,000 years. The researchers used a variety of techniques to analyze the salinity, chemistry and microfossil content of the ancient lakebed.

The most direct method involved counting fossilized diatoms -- microscopic single-celled algae often found in lakes. Some diatom species live near the surface, while others inhabit the deep. By comparing the abundance of deep- versus shallow-water fossils in each core sample, researchers were able to determine whether the lake level was high or low in a particular season.

Dramatic changes
After analyzing all three core samples, the scientists concluded that the lake -- and therefore the entire Altiplano -- has undergone a series of dramatic changes since the Ice Age was at its peak between 26,000 and 15,000 years ago.

"Lake Titicaca was a deep, fresh and continuously overflowing lake during the last glacial stage," according to the Science study, "signifying that the Altiplano of Bolivia and Peru and much of the Amazon basin were wetter than today."

Then, about 15,000 years ago, the Altiplano underwent a significant change. A dry era was launched, which continued for the next 2,000 years, causing Lake Titicaca to drop significantly.

Between 13,000 and 11,500 years ago, Titicaca began overflowing once again. This wet period was followed by 1,500 years of relative dryness, followed by another 2,500 years of heavy precipitation as the lake again rose to overflow levels.

Then, about 8,500 years ago, the lake level fell sharply as the Altiplano again became dry. But heavy precipitation would return for another 1,000 years, only to be followed by an extremely dry period between 6,000 and 5,000 years ago, during which Titicaca fell some 250 feet below its present-day level -- its lowest level in 25,000 years.

Titicaca finally began rising again 4,500 years ago. Since then, the southern portion of the lake has overflowed its banks numerous times.

Millennial time-scale
What caused these thousand-year cycles of extreme wetness and aridity?

For an answer, the authors turned to geological climate studies of the Atlantic Ocean. It turns out that, since the last Ice Age, the North Atlantic has experienced periods of unusually cold surface temperature, often lasting 1,000 years or more and accompanied by centuries of intense precipitation.

According to the authors, these periods of plunging sea temperatures match the cycles of extreme wetness revealed in the Lake Titicaca core samples.

The fact that alternating periods of dryness and wetness occur on a millennial time-scale or longer may be influenced, in part, by the behavior of the Earth as it orbits the sun.

For example, the Earth's rotational axis gradually changes direction every 26,000 years -- a process called precession. As a result, parts of the Earth that are relatively close to the sun during summer today will be farther away during summer thousands of years from now.

So far, scientists do not have a complete explanation for the periodic climate changes in the Altiplano. For example, why did the water level of Lake Titicaca suddenly plunge to its lowest level 6,000 years ago?

"This drop occurred very suddenly in just two or three centuries," notes Dunbar, "suggesting that there can be rapid changes that occur in nature that we don't know much about. Natural variability can be enormous, so we'd better get a full understanding of how these systems work before we try to tease out the impact of humans on climate change."

In addition to Dunbar and Baker, other co-authors of the Science study are Harold D. Rowe of Stanford; Geoffrey O. Seltzer of Syracuse University; Sherilyn C. Fritz and Pedro M. Tapia of the University of Nebraska; Matthew J. Grove of Duke University; and James P. Broda of the Woods Hole Oceanographic Institution.

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