Just how fast glaciers are shrinking and why has been a subject of some debate, particularly in India.

Last week the country's environmental minister released a report questioning whether climate change was driving the retreat of some Himalayan glaciers and arguing that some are actually growing.

"There is no conclusive scientific evidence to link global warming with what is happening in the Himalayan glaciers," he told reporters.

India, he said, needs to do its own scientific work on glacier melting and not rely on the work of Western scientists.

Rajendra Pachauri, the New Delhi-based chairman of the U.N. Intergovernmental Panel on Climate change, dismissed the new report as "arrogant," saying the science was clear.

Glaciers in the Himalayas are retreating faster than anywhere else in the world, his body has said, putting millions of lives at risk as a major source of water for heavily populated South Asia gradually disappears, perhaps by 2035.

What will resolve the debate?

Perhaps a look at a useful new video from the Asia Society's "On Thinner Ice: Melting Glaciers at the Roof of the World" website.

The video follows top glacier scientists to Everest and other mountains in the Himalayas, comparing photos of glacier extent taken early last century to video and photos taken more recently.

In the images, the receding of Himalayan glaciers is striking. Interviews suggest what their loss might mean to those who depend on the water.

Climate modelling is always a subject of debate, with so many unknown variables that making precise forecasts is difficult. But these photos show strong evidence of what has already happened. That should raise an alarm for India's environmental minister, who last week called glacier retreat in the Himalayas not "historically alarming."

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