By Paul Rincon
BBC News science reporter
A fabled tropical ice field in Africa could disappear in two decades because of climate change, a study says.
The finding comes from the first survey in a decade of glaciers in the Rwenzori Mountains, East Africa, often referred to as the "Mountains of the Moon".
A British-Ugandan team says an increase in air temperature over the last four decades has contributed to a substantial reduction in glacial cover.
Details of the work appear in the journal Geophysical Research Letters.
The Rwenzori Mountains straddle the border between Democratic Republic of Congo and Uganda.
They are home to one of four remaining tropical ice fields outside the Andes and are renowned for their spectacular and rare plant and animal life.
Their legendary status may stretch back to a reference by the 2nd Century AD Greek geographer Ptolemy, who wrote of snow-capped equatorial peaks that fed the Nile: "The Mountains of the Moon whose snows feed the lakes, sources of the Nile".
Some researchers think conceptual maps prepared by Ptolemy are a good fit for the Rwenzori, which feeds Lake Albert, which in turn feeds the White Nile.
An analysis of data from field surveys and images from the LandSat satellites shows the combined area of the Rwenzori glaciers has halved from around two sq km to just under one sq km since 1987.
Weather data collected from field stations shows that increased air temperature is the main driver behind the loss of glacial cover.
Trends point to an air temperature rise of roughly half a degree Celsius per decade since the 1960s without any significant change in annual rainfall.
Dr Richard Taylor of University College London and colleagues extrapolated the data on glacial shrinkage since 1906 and found that the glaciers would disappear within 20 years if trends continued.
"The observed increases of about 0.5C per decade are much greater than you would expect," Dr Taylor told the BBC News website. "You would expect, consistent with warming trends for the East African region, about 0.1-0.2C per decade."
The snow-capped peaks of Rwenzori feed the Nile (Image: Andrew Wielochowyski)
Rainfall data for the region extends back 100 years, but temperature records go back no further than the 1960s. Consequently, the researchers were unable to say whether they are observing a long-term trend.
There were no field stations higher than 1,800m (6,000ft) above sea level, which meant the scientists had to infer temperature and rainfall data from information gathered at lower altitudes. However, this was relatively straightforward and reliable, Dr Taylor explained.
Loss of cover
The UCL scientist added: "Tropical glaciers are very sensitive indicators of tropical climate. They indicate quite clearly that the climate is changing."
Local biodiversity is not expected to suffer in the short term, but habitats for rare vegetation could be constrained in the longer term.
The largest conglomerations of equatorial ice are in the Andes in South America, representing over 90% of glacial cover in the tropics.
Of the four outside the Andes, about two sq km of ice remains on Irian Jaya, 0.4 sq km on Mount Kenya, three sq km on Kilimanjaro and 0.96 sq km in the Rwenzori.
The disappearance of the glaciers would have a negligible effect on Nile waters, to which the major contribution comes from rainfall at lower altitudes.