18 September 2000
Himalayan Ice Reveals Climate Warming
by Kate Melville
Ice cores drilled through a glacier more than four miles up in the Himalayan Mountains have yielded a highly detailed record of the last 1,000 years of earth's climate in the high Tibetan Plateau. Based on an analysis of the ice, both the last decade and the last 50 years were the warmest in 1,000 years.
The core also showed a clear record of at least eight major droughts caused by a failure of the South Asian Monsoon, the worst of these a catastrophic seven-year-long dry spell that cost the lives of more than 600,000 people.
The new findings, published in this week's issue of the journal Science, outline data recovered from three cores drilled through the Dasuopu Glacier, a two-kilometer-wide ice field that straddles a flat area on the flank of Xixabangma, a 26,293-foot (8,014-meter) peak on the southern rim of the Tibetan Plateau. The international team, including American, Chinese, Peruvian, Russian and Nepalese members, retrieved the cores during a 10-week, 1997 expedition to the region. The expedition was supported by the National Science Foundation.
"This is the highest climate record ever retrieved," explained Lonnie Thompson, professor of geological sciences at Ohio State University and leader of the expedition, "and it clearly shows a serious warming during the late 20th Century, one that was caused, at least in part, by human activity. This is a very compelling story."
For the last 25 years, he and his colleagues have drilled cores from glaciers and ice caps in some of the most remote parts of the planet in an effort to recover records of ancient climate. Most current predictions of global climate change suggest that early signs of warming will be seen at high elevations where these ice caps exist. So far, Thompson's work has borne this out.
Researchers at Ohio State's Byrd Polar Research Center and the Chinese Lanzhou Institute of Glaciology and Geocryology divided the three cores and were able to identify annual layers for the last 557 years. Samples from these layers were analyzed for dust concentrations, chemical composition and oxygen- and hydrogen-isotope ratios.
The isotope ratios let researchers extrapolate the air temperatures present when the ice was formed. Dust concentrations give an indication of dryness or wetness in the region, and the analysis of chlorides, sulfates and nitrates provide clues about volcanic activity, fossil fuel burning and desertification.
"We now have a record from 23,500 feet in the atmosphere (about as high as instruments are carried in a weather balloon), one that has been preserved naturally, that shows the last 50 years were warmer than any other equivalent period in the last 1,000 years," Thompson said.
The real surprise came with the monsoon records the core revealed.
The South Asian Monsoon is a major climate event that cycles annually across India, Pakistan, the southern Himalayan region, the Far East and reaches as far west as Africa. In the summer, when the Eurasian continent is warmed by solar radiation, prevailing winds flow offshore, leaving the region moisture-poor so that drying intensifies. When the cycle reverses, the winds flow onshore, heavily laden with moisture from the oceans, and bringing the heavy monsoon rains that drench the regions. Changes in the monsoon cycle can bring catastrophic flooding or droughts.
The core data showed that in 1790, the cycle changed, the rains lessened and drought took hold in the region, a condition that continued for seven years until 1796 when the monsoons returned.
"That event was major," Thompson said. "It killed more than 600,000 people in one region of India alone. And that was at a time when global populations were much less than they are today." (Estimates place the world population in 1800 at 980 million.) "If a similar event occurred today, the social and economic disruptions would be horrendous," he said. Current world population is just over 6 billion people.
The ice core record showed other serious monsoon failures and ensuing droughts in 1876-77, and around 1640, 1590, 1530, 1330, 1280 and 1230, though none was as devastating as the 1790 event. Thompson's paper offered no indications of what might have triggered the monsoon failures.
The data, however, do seem to point to the impact human activities have had on changing climate in the region. Core samples covering the last century reveal a four-fold increase in dust trapped in the ice and a doubling of chloride concentrations, suggesting an increase in both drying and desertification in the region.
"There is no question in my mind," he said, "that the warming is in part, if not totally, driven by human activity. I think the evidence for that is so clear - not only from this site but also from Kilimanjaro in Africa." Thompson led an expedition to the ice fields atop the highest mountain in Africa earlier this year. At least 75 percent of the ice there has disappeared since 1912, caused in part, he said, by global warming.