A team of scientists from Syracuse University, Duke University
and the University of Nebraska proved what many suspected all along--changes in
climate in the Northern Hemisphere have a direct effect on climate conditions
south of the equator.
In a study published in the Jan. 26 issue of Science, the
researchers documented the first record of climate change in tropical South
America over the past 25,000 years. Contrary to previous conclusions, the
researchers found that when the Northern Hemisphere was buried under layers of
glacial ice, unusually wet conditions dominated South American tropical areas.
It was previously believed that during the last glacial maximum (LGM), the
climate in tropical South America was characterized by arid conditions.
"We have a unique record of climate change in tropical
South America that shows when global climate conditions cooled and the glaciers
advanced, wetter climates prevailed in the Andes, and when things warmed up and
glaciers receded, conditions in Amazonia got drier," says Geoffrey Seltzer,
associate professor of earth sciences at Syracuse University.
Seltzer, Paul A. Baker of Duke University and Sherilyn C.
Fritz of the University of Nebraska are the lead researchers on a series of
projects to study long-term climate changes in the South American tropics. The
group has done extensive studies of sediment cores they obtained from Lake
Titicaca, which straddles the border of Peru and Bolivia high in the Andes
Mountains, and from the Altiplano of Bolivia. The Bolivian Altiplano includes
one of the world's largest salt flats. Their work has been providing
scientists with a better understanding of how long-term climate variations
occur.
"If we understand climate variations that occur from
natural forces, we will be in a better position to predict what could happen as
a result of the things humans do that affect the environment," Seltzer
says.
As the only large and deep freshwater lake in South America,
Lake Titicaca holds an important climatic record in the layers of sediment
hidden beneath the water. Sediment has been building in the lake for more than
25,000 years, says Seltzer, who, with Baker and Fritz, has returned to Lake
Titicaca to prepare to drill the sediments beneath the lake in an effort to
uncover a longer record of tropical wet and dry phases.
"Unlike the Northern Hemisphere, where weather patterns
originate in the west, the dominant source of moisture in the low latitudes
comes from the east," Seltzer says. "Therefore, major changes in
climate that occur over the Amazon will be similar to what we see in the Andes
and which is recorded in the lake sediments."
At least two forces drive global climate changes--periodic
fluctuations in the Earth's orbit around the sun, which affects the amount of
solar radiation various regions of the Earth receive, and changes in sea surface
temperatures, Seltzer says.
According to the study in Science, an analysis of the sediment
cores revealed periods of time during the past 25,000 years when the water level
in Lake Titicaca dropped some 80 to 100 meters, indicating dry climate
conditions, and times when the lake overflowed continuously to the south,
indicating wet climate conditions. The wet conditions corresponded to existing
records of unusually cold sea surface temperatures in the northern Atlantic
Ocean, which occurred about 21,000 years ago. The cold sea surface temperatures
occurred during the last glacial maximum and were strong enough to alter climate
conditions in the South American tropics. When the glaciers receded and the sea
surface temperatures warmed, drier conditions predominated in the Andes, Seltzer
says.
During the past 2,000 years, Lake Titicaca has developed an
overflow outlet, Seltzer says. "We believe these latest changes are due to
orbital variations where the Earth's position changes in relationship to the
sun. Things are getting a bit wetter and a little colder, which is an indication
of a move toward another period of glaciation in the Andes."
