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03.11.2009 The melting snows of Kilimanjaro

One of the small summit glaciers, called the Furtwängler, lost about half its thickness — nearly five meters — between February 2000 and February 2009. At that rate, the 4.6-meter-thick Furtwängler will be gone in another decade, Thompson estimates. Other glaciers on Kilimanjaro will survive longer, but they too will disappear between 2022 and 2033, Thompson's latest estimates suggest. A second summit glacier, the southern ice field, thinned 24% between 2000 and 2007.

Thinning accounts for about half of the ice loss on the mountain, says Thompson, with retreat of the ice from the glaciers' edges accounting for the other half.

Thinning and drying

Since 1912, when aerial photographers documented Kilimanjaro's glaciers, the mountain's ice fields have shrunk around 85% in area, decreasing from 12 square kilometres in size to just 1.85 square kilometres. That pace seems to be accelerating, as the glaciers shrunk 26% in area between 2000 and 2007. Thompson and colleagues calculated the glaciers' coverage with aerial and satellite images, confirming their retreat with the stakes the researchers began placing around the glaciers in 2000.

Melting and sublimation both contribute to the ice loss, says study author Doug Hardy, a glaciologist from the University of Massachusetts at Amherst. The glaciers have been in retreat for more than a century, Hardy says, with a drying climate in East Africa one main culprit.

"The top [of the mountain] is very, very dry. It's a desert up there," Hardy says. The lack of new snowfall means the dark, dirty tops of glaciers absorb more solar radiation than they otherwise would. In addition, the nearby Indian Ocean has warmed, says Hardy, altering circulation patterns that used to bring more moisture to the mountain. But Hardy says there is too little data to blame the ice loss on increasing atmospheric temperatures. "It's entirely reasonable that, yes, the glaciers are going away on Kilimanjaro in response to global warming," but the link is via Indian Ocean-driven circulation patterns rather than via a warmer atmosphere, says Hardy. The study is published in Proceedings of the National Academy of Sciences of the USA¹.

Disappearing fast

Chris Larsen, a glaciologist at the University of Alaska-Fairbanks, called tropical glaciers like those on Kilimanjaro the "drama queens" of the glacier world: they respond dramatically to even slight insults. "When the climate changes, they can portray a bad situation in even worse light," says Larsen. "The fact that these ice fields are now on the verge of ending a 11,000 year existence is quite significant."

As other data from Thompson and colleagues confirms, the much larger tropical ice fields of the Andes Mountains are also shrinking, which within decades will leave tens of millions of people without drinking water. In contrast, the relatively small glaciers on Kilimanjaro provide little water for local residents.

However, the Kilimanjaro glaciers do attract some 25,000 visitors each year, a major revenue source for Tanzania. Hardy, who's made 12 trips up the mountain since 2000, says he's noticed the crowds of summiteers growing larger; he thinks the stampede began in response to the global attention the shrinking glaciers received in 2002, when he and Thompson published their first paper on the subject².

"The mountain is famous because of its ice fields at the Equator," says Thompson, who added that tourism may dry up when the glaciers do. "Hemingway knew that a long time ago." 

Author:  Brian Vastag
Link:  http://www.nature.com/news/2009/091102/full/news.2009.1055.html
Source:  Nature News
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