An isolated remnant of an ice spire in the crater of Mount Kilimanjaro in Tanzania. (Lonnie Thompson, Ohio State University) The ice fields that top Mount Kilimanjaro, Africa's highest peak, could disappear within 25 years because of rising global temperatures, scientists say.

Scientists at Ohio State University collected data on the thickness and extent of the African glaciers by examining core samples and historical documentation.

In research published this week in Proceedings of the National Academy of Sciences, the scientists report that 85 per cent of the ice that covered Mount Kilimanjaro in 1912 was gone by 2007, and 26 per cent of the ice there in 2000 is now gone.

The glaciers aren't just shrinking in size, as seen from above in aerial photographs, but are losing thickness, too. The researchers found the loss of glacier ice from thinning is now about equal to the loss from shrinking.

"This is the first time researchers have calculated the volume of ice lost from the mountain's ice fields," Lonnie Thompson, a paleoclimatologist with Ohio State's Byrd Polar Research Center, said in a statement.

In 2000, Thompson and his colleagues found a layer of ice 1.6 metres below the surface of Kilimanjaro's Northern Ice Field with a radioactive marker corresponding to the Operation Ivy nuclear weapons tests in the Marshall Islands in 1952. The marker has been found in glaciers around the world.

That layer is now gone at Kilimanjaro, the ice field having lost 2.5 metres of thickness between 2000 and 2007.

'Lost half of its thickness'

The tops of both the Northern and Southern Ice Fields have thinned, by 1.8 metres and 5.1 metres, respectively. A smaller glacier, called Furtwangler, has thinning about 50 per cent between 2000 and 2009.

Kilimanjaro's glaciers are retreating at their edges and thinning as the surface ice melts, researchers say. (Lonnie Thompson, Ohio State University) "It has lost half of its thickness. In the future, there will be a year when Furtwangler is present and by the next year, it will have disappeared," said Thompson.

Thompson and his colleagues found the major cause of the loss of ice is likely the rise in global temperatures, although changes in cloud cover and precipitation may also play a role.

The current climate around Mount Kilimanjaro and the recent, sustained melting of ice is unprecedented in the 11,700-year history of the glaciers, the researchers found.

They found elongated air bubbles trapped in the ice at the top of one of the glacier cores, suggesting the surface ice melted and refroze.

They found no other evidence of such melting in the column of ice drilled out of the glacier. A 300-year drought about 4,200 years ago left a two-centimetre layer of dust in the core, but no evidence of sustained melting.