The ice fields of Mount Kilimanjaro in Tanzania have given up remarkable new information about the African climate stretching back more than 11,000 years.

Some time between 2015 and 2020 [the] ice will be gone - along with the archive of climate history recorded in those glaciers

Prof Lonnie Thompson

Cores drilled into the glaciers high up on the peak support earlier evidence that there were three catastrophic droughts on the continent in the intervening period.

The research, published in the journal Science, also reinforces predictions made last year that rising temperatures - if they persist - could clear the mountain's ice completely within two decades.

This could cause difficulties for local people whose economies depend in part on the melt waters coming from the mountain and who also benefit from the influx of tourists drawn to the beauty of the white-capped tropical peak.

Wet and dry

Professor Lonnie Thompson, from Ohio State University, US, collected six cores from the mountain.

The ice columns were investigated for deposits trapped in the yearly snowfalls that built up the glaciers.

Its ice is an important climate archive for Africa

By checking these markers against other historical records, Thompson and colleagues were able to construct a climate "history book".

Included in the record are radioactive markers related to the fall-out of nuclear bomb tests, which accurately date some of the ice sample; and specific types of oxygen and hydrogen atoms that can be used to infer past temperatures. Dust layers give an idea of yearly precipitation.

The cores show much of the past 11,000 years to have been generally wetter and warmer than present, but they also show evidence for three major droughts - 8,300, 5,200 and 4,000 years ago - the last of which went on for 300 years.

Ice retreat

By using global positioning from satellites, aerial maps and an array of stakes placed on the ice fields, the researchers have been able to confirm that Kilimanjaro's white cap is retreating in extent and volume.

In February 2001, Professor Thompson said the rate of retreat could see the mountain completely ice free within 20 years. He said the latest work had not changed that assessment.

One of the last remnants of Kilimanjaro's eastern ice field: A six-metre spire

He told the BBC: "We have a series of maps - the first made in 1912.

"Then there was about 12.1 square kilometres of ice on the mountain.

"Since then, there have been five maps, the latest by us produced from aerial photographs taken on 16 February, 2000. That showed only 2.2 sq km of ice remained on the mountain - so we've lost about 80% of the ice since 1912.

"If you look at the area on the maps in between you have a series of dots that line up.

"If you project those into the future, some time between 2015 and 2020 that ice will be gone - along with the archive of climate history recorded in those glaciers."

Global changes

But colleague Dr Douglas Hardy, from the University of Massachusetts at Amherst, also US, cautioned against jumping to conclusions about global warming.

Thompson says the Furtwangler ice wall on Kilimanjaro has undergone a massive retreat in recent years

"Without diagnostic evidence, a definitive link to global warming is on thin ice," he said.

"Evidence is mounting that human influences on climate are causing glaciers to retreat dramatically around the world, and especially at high elevations in the tropics.

"But Kilimanjaro's glaciers have little in common with mid-latitude Alpine glaciers, and we must accept that simple explanations are not always possible.

"Kilimanjaro is a mountain that defies expectations and shatters assumptions."