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Monsoon Has Long History In World's Driest Desert

Julio Betancourt in the Atacama Desert of northern Chile
Tucson - September 11, 2000
by Lori Stiles
Scientists have used techniques developed to reconstruct climate history in the American Southwest in reconstructing the past 22,000 years of climate in the world's driest desert with striking results.

Fossil rodent middens and paleospring deposits from the heart of the Atacama Desert in northern Chile document millennia-long wet and dry climate regimes that are linked to changes in tropical monsoons far removed from the region.

Julio L. Betancourt of the U.S. Geological Survey, Claudio Latorre of the University of Chile, Jason A. Rech and Jay Quade of the University of Arizona, and Kate.A. Rylander of the USGS report on it in the current issue of Science (Sept. 1).

Their article, "A 22,000-year record of monsoonal precipitation from northern Chile's Atacama Desert," fuels a growing realization that the tropics play a more pivotal role in global climate change than previously thought. Their research is funded by the InterAmerican Institute, National Geographic and the National Science Foundation.

"It may seem paradoxical to try to reconstruct large-scale changes in tropical rainfall from the driest desert in the world," Betancourt said. But in places like the Amazon Basin, geologic evidence of climate change is either buried under jungle vegetation or has been washed away by meters of annual rainfall, he added.

"The Atacama Desert, in the rainshadow of the central Andes, is at the tail end of the tropical rainfall belt, where annual precipitation wanes to zero," said Betancourt, an adjunct research associate in the UA geosciences department. Betancourt and his colleagues are part of the USGS - UA Desert Laboratory, a unique facility for paleoenvironmental research on Tucson's Tumamoc Hill.

The researchers refer to the Atacama as "absolute" desert -- desert that more resembles the surface of Mars than the Sonoran Desert. It may not rain in the desert for years or decades. Yearly precipitation at some of their study sites averages 20 millimeters, less than an inch, less than falls during a single, one-hour monsoon storm in Tucson.

"It is desert that makes Death Valley look lush," said UA geoscientist Jay Quade.

But it is hyperaridity that makes the Atacama so useful for studying changes in tropical rainfall, Betancourt explained. Any extra moisture from the Amazon Basin to the Pacific slope of the Andes creates an amplified hydrological and biological response that the geoscientists can expertly measure using radiocarbon dated fossil rodent midden and wetland deposit techniques they honed in the American Southwest.

During wetter periods, more moisture from the Amazon Basin over the Pacific slope of the Andes increases the snowpack and raises the regional water table, which discharges in springs along the foothills. The spillover also brings more rains to the lower elevations, causing vegetation, rodents and other organisms to expand into what is now Absolute Desert.

Hyperaridity preserves rodent "middens," or rodent urine-hardened nests rich in plant remains, for 40,000 years or more. Midden records are like snapshots of flora through time, Betancourt said.

He and others have led the North American midden research effort, collecting and analyzing more than 2,000 packrat middens from western North America over the years.

The researchers identified many plant remains in rodent middens as summer-flowering grasses. "Because summer temperatures are not limiting at these elevations in the Atacama Desert, we can specify that any climatic changes are due to changes in monsoonal moisture. This may seem trivial, but we know of few geologic records anywhere in the world that specify the season of moisture and preclude temperature effects," he said.

Quade and Rech reconstructed water table heights from ancient spring deposits left high and dry in canyons and lowlands of the Atacama Desert.

"Think of them as bathtub rings," said Quade of the Desert Lab, an expert in analyzing such wetland deposits. "When the water table was high, it left a ring on the canyon wall. It's the same stuff you get on your pipes or the bottom of your water cooler."

The combined data from middens and paleospring deposits provide a detailed and replicated record of climatic change during the past 22,000 years, the researchers report in Science.

The record shows that the desert became increasingly wet between 16,200 to 10,500 years ago, with the wettest period during the past 22 millennia occurring from 11,800 to 10,500 years ago, at the end of the last Ice Age. Higher elevation shrubs and summer-flowering grasses grew as much as 1,000 meters lower than they do today.

Paradoxically, the late glacial/early Holocene pluvial period coincided with a time when the amount of solar radiation over this part of the Earth was at its minimum. (Variations in aspects of the Earth's orbit through time causes changes in the amount and distribution of sunlight reaching Earth's surface.) Scientists assumed decreased solar heating would have weakened monsoon storms over southern Amazonia and the central Andes.

"Instead, we find evidence for intensified monsoonal circulation that spills more moisture over the rainshadow of the central Andes onto the Pacific slope and the highlands of the Atacama Desert," Betancourt said.

In their paper, he and the others suggest that greater solar radiation over the Tibetan Plateau 10,000 to 12,000 years ago intensified the central Asian monsoon, and that, combined with atmospheric circulation involved in El Nino - La Nina oscillations, resulted in stronger easterlies (east-to-west winds) over the Pacific Ocean. La Nina-like circulation enhances convection and precipitation over the central Andes.

"The moral of the story is that insolation (solar radiation) may operate on large-scale circulation and regional climates through climatic teleconnections that are far removed from the region in question," Betancourt said.

The scientists found that after 10,500 years ago, climate in the Atacama dried abruptly. After 9,000 years ago, the water table dropped to near modern levels. From 9,000 to 8,000 years ago, and the past 3,000 years mark perhaps the driest climate over the last 22 millennia.

"The climate changes (at around 10,500 years ago) might have been happening, basically, in a few decades," Betancourt said. "This question bears on our future. Is it possible within the current regime to get such rapid changes in climate?"

The midden-paleospring record also documents a previously undetected wet phase from 8,000 to 3,000 years ago. Other researchers have documented that Lake Titicaca on the Bolivian Altiplano, the extensive high plateau of the central Andes, was 100 meters below its modern level during this period. Two possible reasons for the discrepancy have to do with sharp geographic variations in summertime climate in the central Andes and Atacama Desert or with complex responses of such a large, deep lake to long-term variations in climate, Betancourt said.

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