Ice cores from a Himalayan glacier confirm global warming

More evidence that the Earth is warmer than at any time in the past 1,000 years has come from ice cores in a glacier on the "roof" of the world.

More evidence that the Earth is warmer than at any time in the past 1,000 years has come from ice cores in a glacier on the "roof" of the world.

Himalayan ice cores provide convincing evidence that the past 50 years - and the 1990s in particular - have been the warmest of the past millennium, says a study published today in the journal Science.

An international team of scientists drilled three cores each of about 150 metres (500ft) into the Dasuopu glacier, an ice field on the flank of Xixabangma, a peak that rises to 26,293ft on the southern rim of the Tibetan plateau. They analysed the cores for dust particles and several chemical isotopes, which can be used to estimate air temperatures. Most of the ice in this region is deposited by monsoon rains, when warm, moisture-laden winds blow off the Indian Ocean each year.

Lonnie Thompson, a professor of geological sciences at Ohio State University and leader of the expedition, which included Peruvian, Russian and Chinese researchers, said the study provided a unique insight into global temperatures over many centuries.

"This is the highest climate record ever retrieved 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," he said.

The scientists were able to identify layers in the ice cores that corresponded to the passing years. Isotopes of hydrogen and oxygen could be used to estimate air temperatures while dust concentrations corresponded to dryness.

"We now have a record from 23,500ft in the atmosphere - about as high as instruments are carried on 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," Professor Thompson said.

The ice cores show years when the monsoon rains failed to arrive, such as the six years beginning in 1790 when the resulting drought killed more than 600,000 people in one Indian territory alone.

The amount of dust trapped in the ice cores has quadrupled in the 20th century, with concentrations of chloride doubling for the same period. That suggests an increase in the dryness of the air and the rate at which land was becoming desert, the scientists said.

"The warming is in part, if not totally, driven by human activity," Professor Thompson said. "The evidence for that is so clear, not only from this site but also from Kilimanjaro in Africa."

A study earlier this year of the glaciers on Africa's highest mountain revealed that at least 75 per cent of the ice has disappeared from Kilimanjaro since 1912.

A diverse array of studies of polar ice cores and tree rings from around the world is now pointing to a genuine global warming phenomenon, which has become more pronounced in the past 10 years.

Meteorologists have found that temperature records of the past two centuries show that the 1990s were the warmest decade on record. Tree rings and ice cores taken from the polar regions alsoindicate that the world is now warmer than at any time since AD1,000.