Thirst threatens as tropical glaciers melt away

Mountain glaciers around the world are melting faster now than at any time in the past 5000 years because of an unprecedented period of global warming, a study has found.

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

Scientists have warned that human activities over the past 100 years may have nudged the climate beyond a critical threshold which could see most of the highest ice caps disappearing in the near future.

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 plant material which has 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 5200 years," the scientists write in the journal 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," they say.

Professor Thompson said the research was based on nearly 50 expeditions over three decades to seven mountain glaciers, including the Huascaran and Quelccaya ice caps of Peru, the Sajama ice cap of Bolivia and the Dunde and Puruogangri ice caps of China.

"We have a record going back 2000 years and when you plot it out, you can see the medieval warm period [from AD1000 to AD1300] 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 - whether you look at individual cores or the composite of all seven - is how unusually warm the last 50 years have been."

The most dramatic evidence of change comes from 28 sites where the retreating ice has exposed ancient plants that have been frozen and preserved beneath glaciers for between 5000 and 6000 years.

Sceptics have suggested that the glaciers are retreating because of a decline in precipitation, rather than rising temperatures.

However the study found that in all but one of the sites covered by the research, precipitation had increased over the past century.

"That means that the retreat of the ice is driven mainly by rising temperatures," said Professor Thompson.

"What this is really telling us is that our climate system is sensitive ... If what happened 5000 years ago were to happen today, it would have far-reaching social and economic implications for the entire planet.

"The take-home message is that global climate can change abruptly, and with 6.5 billion people inhabiting the planet, that's serious."

- INDEPENDENT