The Himalayas hold the planet's largest body of ice outside polar caps
The BBC's Chris Morris travels to the main source of the Ganges river to find out why the glaciers are melting.
As the first light of dawn lit up the snow-covered mountain peaks, we trekked through a barren landscape 4,000 metres up in the Indian Himalayas, heading for the Gangotri glacier, the main source of the River Ganges.
About 2km from our destination, we passed a rock inscribed with the date 1891. This was where the snout of the glacier stood just over 100 years ago. And the retreat continues.
"All the glaciers in this region are melting," said Dr Rajesh Kumar, a glaciologist who accompanied us to Gangotri. "And we have to find out why."
The Himalayas hold the planet's largest body of ice outside the polar caps - an estimated 12,000 cubic kilometres of water. They feed many of the world's great rivers - the Ganges, the Indus, the Brahmaputra - on which hundreds of millions of people depend.
But climate scientists are worried.
"There's the possibility that if we don't do something about stabilising the earth's climate then these glaciers could easily vanish in the next few decades," said RK Pachauri, the Nobel Prize-winning Indian scientist who heads the Intergovernmental Panel on Climate Change (IPCC).
"We need to understand the dynamics of these glaciers better and we need much more precise information and measurements."
That's what scientists like Dr Kumar are beginning to provide.
There are a variety of instruments tracking daily temperatures to try to establish patterns over time.
Less snow means the glacier melts even faster
"We only started doing this a few years ago," he admitted, "I don't think anyone took it seriously before that. There wasn't any money for research."
The glacier itself was grey and discoloured - a huge wall of ice stretching back up the mountain. Water dripped from icicles glinting in the early morning sun.
From one side a stream of fast flowing water emerged - the exact point where the Ganges begins its long journey from the Himalayas across the north Indian plains to the Bay of Bengal.
Dr Kumar's previous visit had been just over five months earlier.
His GPS told him that in that time the glacier had retreated by some 15 metres.
"It's a big change," he admitted.
The Indian government has said the rate of retreat in many glaciers has decreased in recent years. But there is evidence to suggest that it's picking up again.
And there are other signs of a climate in transition.
A few miles downstream from Gangotri, in the village of Harsil, we visited Basanthi Negi's house, where she showed us how high the snowfall used to be.
She reached up over her head to touch the wooden floor of her balcony - about two metres above the ground.
"But we haven't had snow like that in at least 10 years," she explained.
"We can all feel things changing."
Less snow means the glacier melts even faster as the ice is more exposed to the heat of the sun - as well as receding, the glacier is getting thinner.
"Current trends and patterns of melting clearly spell a very difficult situation for most of South Asia in the future," RK Pachauri said.
India says the rate of retreat in many glaciers has decreased in recent years
That is why the IPCC has been so outspoken in campaigning for action, which will help mitigate the rise in the earth's temperature.
"It's a global challenge and this is where countries like India... have to raise their voices and make it known to the global community that what you're really going to create is social chaos, which will spill across borders," he warned.
India is taking action: the prime minister has created a new high-level body to assess the Himalayan glaciers, and there is talk of much greater co-operation across the border with China. But even they can't act alone.
Back at the glacier pilgrims prayed at a small shrine to the goddess of the river.
This is a place of huge religious significance. But now, it's all about the science - and the politics - of climate change.