ICE CORES SHOW RECORD OF CLIMATE DATING BACK 20,000 YEARS

COLUMBUS, Ohio -- A new analysis of ice cores retrieved from the heart of a Peruvian glacier has unlocked a record of past climate dating back at least 20,000 years.

That record is painting a new picture of the climate in the tropics during the last ice age. The Amazon River basin environment was probably drier at the time, with less extensive rain forests and more savanna vegetation such as exists in Oklahoma today.

The report by researchers from Ohio State University and Louisiana State University was published in the July 7 issue of the journal Science.

These findings are the latest from nearly two decades of study of ice cores from glaciers in the South American Andes, Greenland, Antarctica, China and the former Soviet Union and they provide what may arguably be the best view of how world climate varied over the last 20,000 years.

Surprisingly, the ice cores provide the best record of

changing nitrate levels in the tropical latitudes, an indicator which may be extremely useful in gauging shifts in the growing patterns of major forest regions near the ice caps. Nitrate records have been retrieved from past ice cores but whatever evidence they provided about ancient climate was never very clear.

Studies of these tropical regions are important for understanding climate changes in the rest of the world, explained Lonnie Thompson, professor of geological sciences and research scientist with the Byrd Polar Research Center at Ohio State.

"From 30 degrees north to 30 degrees south, you have 50 percent of the land's surface and 75 percent of the world's population. It's an enormously important region of the planet," Thompson said. "It's also an area where we really don't understand the climate and that has been because we lacked the data. The new findings will help resolve that."

The research team analyzed two cores taken during a 1993 expedition to the col, or saddle, of a glacier atop Huascarán, the highest tropical mountain located in the Cordillera Blanca in west central Peru. The two cores were drilled through the glacial ice to the underlying bedrock and were 530 feet (160.4 meters) and 548 feet (166.1 meters) long respectively.

The smaller of the two cores was cut into nearly 2,700 sections, melted and packaged into small bottles for later study. The longer second core was shipped frozen back to Ohio State where it was cut into nearly 4,700 samples for analysis.

Researchers examined several basic indicators as clues to the climate that existed when the ice formed from fallen snow. First, they looked for changes in the ratio of two isotopes trapped in the ice -- oxygen-18 and oxygen 16. Shifts in this ratio have long been viewed as evidence of temperature change in the area where the ice formed.

They also looked at pollen trapped within the ice. An analysis of the types of pollen can indicate the kinds of plants that thrived in nearby areas and the amount of pollen can suggest how expansive the vegetation was, both indicators of certain climate conditions.

They also looked for dust trapped in the ice cores. Both the type and amount of dust are good indicators of regional climate conditions at the time the ice was formed.

Lastly, the team looked at the amount of nitrate compounds trapped in the ice. "Nobody has ever had a good nitrate record from an ice core that they have been able to interpret in terms of climate variability," said Ellen Mosley-Thompson, an associate professor of geography at Ohio State and a member of the research team.

In the bottom portion of the ice cores, in the ice age ice, they found dramatic changes in both the nitrate levels and the dust concentrations. Nitrate levels fell by a factor of two to three and dust concentrations increased 200-fold. Taken together, Thompson says they indicate that there was substantial drying and a significant reduction of the rain forest in the nearby Amazon River basin which could only have been caused by changing climate.

A shift in the oxygen isotope ratios retrieved from the ice suggest that temperatures at higher elevations in the tropics may have been as much as 8 to 12 degrees Celsius cooler near the end of the Late Wisconsinan Glacial Stage, or the last ice age.

The researchers were able to reconstruct a record of changing climate from the present dating back into the ice age. Based on their analyses, climate conditions warmed abruptly at the end of the ice age and then suddenly began another cooling period. This cooling is called the Younger Dryas phase and the cores show the middle of that period occurring around 12,250 years ago. This is one of the best records yet of the Younger Dryas in the tropics.

The cores also show that the period between 8,400 years ago and 5,200 years ago was the warmest since the ice age. After that, a long, persistent cooling period began and which culminated in the Little Ice Age from 500 years to 200 years ago. There is evidence of an abrupt warming over the last 200 years in the core.

Based on pollen retrieved from the Huascarán cores, the vegetation patterns on the eastern slope of the Andes Mountains have remained relatively stable last 3,000 years.

The evidence provided by the cores is troublesome to modern climate predicting tools, Thompson says. "The findings of these cooler temperatures conflict with reconstructions by CLIMAP. Knowing how sensitive the tropics are to global climate changes is essential for models that try to simulate how the Earth's climate system works. These same models are used to predict future climate changes.

"These results challenge the current view of tropical climate history and the role of the tropics during the Late Glacial Stage," he said.

Along with Thompson and Mosley Thompson, the research team included J.F. Bolzan, senior research associate at the Byrd Center; M.E. Davis, P-N. Lin, and J. Cole-Dai, all research associates with the Byrd Center; K.A. Henderson, a master's degree student in geological sciences; and K-b.Liu, an associate professor at Louisiana State University. The project was supported by the National Oceanic and Atmospheric Administration in cooperation with ElectroPeru and SENAHMI (the Meteorological Bureau of Peru).

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Contact: Lonnie Thompson, (614) 292-6652

Written by Earle Holland

EDITOR'S NOTE: Photos may be available to accompany this release. Please contact Earle Holland, University Communications, Ohio State University, (614-292-8384) or via e-mail (Holland.8@osu.edu).