Mountain glaciers are melting faster now than at any time in the past 5,000 years because of an unprecedented period of global warming, a study has found.

Ice cores taken from mountains as far apart as the Andes in South America and the Himalayas in Asia have revealed how climate change is leading to a full-scale retreat of the world's tropical glaciers.

These photos released by the Glacier National Park Archives show the dramatic recession of the Grinnell Glacier as seen from the Grinnell Glacier Overlook off the Highline Trail in Glacier National Park, Montana. The first view taken in circa 1940 shows the early formation of Upper Grinnell Lake, a proglacier lake visible at the terminus of the glacier. The 2004 photo shows a dramatic increase in the size of the lake as a result of melting ice. (AP Photo /1940 c. Unknown; 2004 Karen Holzer, USGS/Courtesy of Glacier National Park Archives)

Scientists have warned that human activities over the past 100 years may have nudged the global climate beyond a critical threshold which could see most of the highest ice caps disappearing within the near future. Melting glaciers in South America and Asia not only contribute to rising sea levels, they are also vital sources of freshwater for many millions of people who live within their range at lower altitudes, the scientists said.

The scientists, led by Lonnie Thompson of Ohio State University, present three lines of evidence pointing to a dramatic melting of glaciers in both the Andes and the Himalayas: a change in the chemical isotopes of the ice cores, the widespread retreat of glaciers and the uncovering of frozen plants that had been buried for thousands of years.

"These three lines of evidence argue that the present warming and associated glacier retreat are unprecedented in some areas for at least 5,200 years," the scientists wrote in the Proceedings of the National Academy of Science. "The ongoing global-scale, rapid retreat of mountain glaciers is not only contributing to global sea-level rise but also threatening freshwater supplies in many of the world's most populous regions."

Professor Thompson said the research was based on nearly 50 scientific expeditions to seven mountain glaciers over the past three decades, including the Huascaran and Quelccaya ice caps in Peru, the Sajama ice cap in Bolivia and the Dunde and Puruogangri ice caps in China. He said: "We have a record going back 2,000 years and when you plot it out, you can see the medieval warm period [from 1000 to 1300] and the little ice age [from 1600 to 1850]. And in that same record, you can clearly see the 20th century and the thing that stands out is how unusually warm the last 50 years have been. There hasn't been anything like it, not even in the medieval warm period.

"The fact that the isotope values in the last 50 years have been so unusual means that things are dramatically changing."

The most dramatic evidence comes from 28 sites where the retreating ice has exposed plants that have been frozen and preserved for between 5,000 and 6,000 years by the glacier's base.

"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," Professor Thompson said. "If it had been, then the plants would have decayed."

© 2006 Independent News and Media Limited

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