The draft — which is yet to be approved — from the Inter-governmental Panel on Climate Change (IPCC) describes regional impacts of rising temperatures, widely blamed on emissions of greenhouse gases from burning fossil fuels.

“If current warming rates are maintained, Himalayan glaciers could decay at very rapid rates, shrinking from the present 500,000 sq km to 100,000 sq km by the 2030s,” the draft’s technical summary said.

The annual per capita availability of freshwater in India is expected to drop from around 1,900 cubic metres to 1,000 cubic metres by 2025 due to population growth and climate change, according to the draft.

Scientists and officials from more than 100 countries will meet in Brussels from tomorrow to review and approve a 21-page summary amid disputes on some findings, including how far the rising temperatures may contribute to spreading disease.

The chairman of the IPCC, India’s Rajendra Pachauri, declined comment on the contents of the report today.

“It’s still in a draft stage. This is not an authorised release,” Pachauri told The Telegraph on phone from Europe.

But a senior Indian scientist who has been tracking the fate of Himalayan glaciers for several years said some “dramatic impacts” within the next 20 or 30 years would be consistent with trends already visible in the Himalayas.

“Some glaciers are clearly at an extremely critical juncture,” said Anil Kulkarni, coordinator with the snow and glacier project at the Space Applications Centre, Ahmedabad. “Even a half-a-degree increase in average temperature could lead to a situation in which there’s no formation of fresh ice on some glaciers.”

The loss of glaciers will change water flow patterns in streams and rivers — an initial increase as more snow melts, followed by a steady decrease within a few decades as not enough snow is retained, Kulkarni said. “This can threaten our water security.”

His studies have already shown a rise in winter-time stream water flow in the Baspa basin in Kinnaur district of Himachal Pradesh. “The changes in stream runoff patterns are substantial,” Kulkarni said.

The IPCC draft also predicts a fall in crop yields in parts of Asia.

An Indian scientist at the Brussels meeting told The Telegraph that studies indicate that with progressive warming grain yields are likely to decline, although initially there may be a mixed signal — increase in some areas and decrease in others.

According to the IPCC draft, disruptions from climate change are likely to be felt hardest in poor nations such as in sub-Saharan Africa and Asia where millions could go hungry because of damage to farming and water supplies.

But some countries may benefit from global warming, according to the draft report which says that, as temperatures rise, crops might grow better in nations far from the tropics such as Canada, Russia or New Zealand.

Negotiations on a global treaty to restrict emission of greenhouse gases after 2012 are stalled. Among the world’s top emitters — the US, China, Russia and India — only Russia is bound by caps under the Kyoto protocol.