The UN climate panel issued its strongest warning yet on Friday that human activities are heating the planet, forecasting that temperatures would rise by between 1.8 and 4.0 Celsius (3.2 and 7.8 Fahrenheit) in the 21st century.

In India, the signs already back up forecasts that as the mercury rises the Indian subcontinent, home to one-sixth of humanity, will be one of the worst-affected regions.

"We are already seeing glaciers are receding at a faster rate and islands have disappeared and then there is all this freak weather phenomena," said Shruti Shukla, climate change officer for WWF India.

Experts say the melting of Himalayan glaciers could have serious consequences as more than 500 million residents -- almost half of India's total population -- of the Indus, Ganges and Brahmaputra river basins rely on them for water supply.

Research about the Gangotri glacier -- which feeds the Ganges -- has found the average rate of retreat has almost doubled to 34 metres (110 ft) per year compared to 19 metres in 1971.

"Glaciers are like a frozen reservoir of water, so when glaciers recede ... proportionally, there will be a decrease in the water, which affects drinking water supply, irrigation, hydro-power," said glaciologist Jagdish Bahadur.

This is likely to exacerbate already widespread water shortages.

Rising temperatures will also hurt the annual June-September monsoon rains, which India is heavily dependent on for its crops.

It is estimated that a temperature rise of between 2 and 3.5 Celsius would result in a loss of between 9 and 25 percent of revenue from agriculture -- which makes up 22 percent of India's GDP and employs 70 percent of the workforce.

Besides, researchers say rising temperatures will mean vector-borne diseases such as malaria and dengue fever will spread to higher altitudes known for being free of mosquitoes.

In the Sunderbans, off India's east coast, scientists say two of the 104 islands have disappeared over the past decade partially due to rising sea levels.

"Both islands were inhabited and thousands of people were forced to relocate to some of the other islands," said Sugata Hazra, who teaches oceanography at Jadhavpur University in West Bengal, adding that 12 more islands were vulnerable.

In western India, freak torrential rains flooded the desert state of Rajasthan, displacing hundreds of thousands and killing 140 people last year.