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Tropical Ice Cores Show Two Abrupt Global Climate Shifts

by:    29 June 2006

Sensitive climate system can change abruptly due to natural or human forces

Washington - For the first time, glaciologists have combined and compared sets of ancient climate records trapped in ice cores from the South American Andes and the Asian Himalayas to see how climate has changed - and is still changing - in the tropics.

The U.S. National Science Foundation, the National Oceanic and Atmospheric Administration and Ohio State University (OSU) funded the research, according to a June 26 OSU press release.

The conclusions indicate a massive climate shift to a cooler regime that occurred just over 5,000 years ago, and a more recent reversal to much warmer temperatures in the last 50 years.

The evidence suggests that most high-altitude glaciers in the planet's tropical regions will disappear in the near future, and shows that in most of the world, glaciers and ice caps are rapidly retreating, even in areas where precipitation increases. This implicates rising temperatures, not decreasing precipitation, as the most likely culprit.

The researchers from OSU's Byrd Polar Research Center and three other universities combined chronological climate records retrieved from seven remote locations north and south of the equator. Cores drilled through ice caps and glaciers have captured a climate history of each region, in some cases providing annual records and in others decadal averages.

"Approximately 70 percent of the world's population now lives in the tropics," said OSU geological sciences professor Lonnie Thompson, "so when climate changes there, the impacts are likely to be enormous."

In the past 30 years, Thompson has led nearly 50 expeditions to remote ice caps and glaciers to gather climate records. The current study includes cores taken from the Huascaran and Quelccaya ice caps in Peru, the Sajama ice cap in Bolivia, and the Dunde, Guliya, Puruogangri and Dasuopu ice caps in China.

For each of these cores, the team extracted chronological measurements of the ratio of two chemical forms of oxygen, called isotopes. The ratio is an indicator of air temperature at the time the ice formed.

All seven cores gave clear annual records of the isotope ratios for the past 400 years and decadal averages dating back 2,000 years.

"We have a record going back 2,000 years and when you plot it out, you can see the Medieval Warm Period and the Little Ice Age," Thompson said.

In the Medieval Warm Period, from about 1000 to 1400, global temperatures are thought to have been a few degrees warmer than those of the preceding and following periods. Its climatic effects were confined mainly to Europe and North America.

Following this period, the Little Ice Age, from 1400 to 1800, was characterized by the expansion of mountain glaciers and the cooling of global temperatures, especially in the Alps, Scandinavia, Iceland and Alaska.

"In that same record," Thompson said, "you can clearly see the 20th century and the thing that stands out - whether you look at individual cores or the composite of all seven - is how unusually warm the last 50 years have been."

He added, "There hasn't been anything in the record like it - not even the [Medieval Warm Period]."

The real story, he said, is that the unusual oxygen isotope values in the past 50 years means things are dramatically changing.

The isotope evidence is clear throughout all the cores, but Thompson said the more dramatic evidence is the emergence of unfossilized wetland plants around the margin of the Quelccaya ice cap, uncovered as the ice retreated in recent years.

Since their discovery in 2002, the researchers have identified 28 sites near the ice cap's margin where these ancient plants have been exposed. Carbon dating showed that the plants range from 5,000 to 6,500 years old.

"This means that the climate at the ice cap hasn't been warmer than it is today in the last 5,000 years or more," Thompson said. "If it had been, then the plants would have decayed."

The researchers say a major climate shift around 5,000 years ago in the tropics likely cooled the region since the ice cap quickly expanded and covered the plants. The fact that they are now being exposed indicates that the opposite has occurred - the region has warmed dramatically, causing the ice cap to quickly melt.

Tropical glaciers are a warning system for the global climate system," Thompson said, because they integrate and respond to most of the key climatological variables - temperature, precipitation, cloudiness, humidity and radiation.

"What this is really telling us is that our climate system is sensitive, it can change abruptly due to either natural or to human forces," he added. "If what happened 5,000 years ago were to happen today, it would have far-reaching social and economic implications for the entire planet."

Text of the press release and graphics are available at the OSU Web site.

For additional information, see Climate Change.

(Distributed by the Bureau of International Information Programs, U.S. Department of State. Web site: