Image caption The tropical glaciers are melting at their fastest rate in 300 years

Glaciers in the tropical Andes have shrunk by 30-50% since the 1970s, according to a study.

The glaciers, which provide fresh water for tens of millions in South America, are retreating at their fastest rate in the past 300 years.

The study included data on about half of all Andean glaciers and blamed the melting on an average temperature rise of 0.7C from 1950-1994.

Details appear in the academic journal The Cryosphere.

The authors report that glaciers are retreating everywhere in the tropical Andes, but the melting is more pronounced for small glaciers at low altitudes.

Glaciers at altitudes below 5,400m have lost about 1.35m in ice thickness per year since the late 1970s, twice the rate of the larger, high-altitude glaciers.

"Because the maximum thickness of these small, low-altitude glaciers rarely exceeds 40 metres, with such an annual loss they will probably completely disappear within the coming decades," said lead author Antoine Rabatel, from the Laboratory for Glaciology and Environmental Geophysics in Grenoble, France.

Water shortages

The researchers also say there was little change in the amount of rainfall in the region over the last few decades and so could not account for changes in glacier retreat.

Without changes in rainfall, the region could face water shortages in the future, the scientists say.

The Santa River valley in Peru could be most affected; its hundreds of thousands of inhabitants rely heavily on glacier water for agriculture, domestic consumption, and hydropower.

Large cities, such as La Paz in Bolivia, could also face problems. "Glaciers provide about 15% of the La Paz water supply throughout the year, increasing to about 27% during the dry season," said co-author Alvaro Soruco from the Institute of Geological and Environmental Investigations in Bolivia.

The Intergovernmental Panel on Climate Change (IPCC) has pointed to the importance of mountain glaciers as sensitive indicators of climate change.

Globally, glaciers have been retreating since the early 20th Century, with a few exceptions. Himalayan glaciers are relatively poorly studied and there are suggestions that some are actually putting on mass.

Some scientists say the Chacaltaya glacier in Bolivia, which used to be the world's highest ski run, has already nearly disappeared.