Asia has been undergoing its own industrial revolution over the last 50 years. New factories and power stations open every day, while cars and lorries gridlock the once empty roads. Cities are choking under the clouds of pollution and vast plumes of black smoke rise and disperse into the air.

Some of this smoke billows over the Himalayas, depositing soot onto the pristine white glaciers. In order to measure how dirty Himalayan glaciers have been getting, Jing Ming, from the China National Climate Center in Beijing, and colleagues studied satellite data going back to 2000.

They measured the change in colour of eleven representative glaciers between 2000 and the present and found a clear darkening trend (Environmental Research Letters). And this darkening is likely to be contributing to the melting process.

"During the spring – when melting is strongest on glaciers on the northern slopes – the deposited soot can be accumulated and concentrated in the surface layers of the glaciers and enhance the radiation absorption by as much as 20%," Ming told environmentalresearchweb. Add in the effect of rising temperatures and it really does seem that Himalayan glaciers are in trouble. "The melting of the glaciers is very severe," said Ming.

A related paper in Environmental Research Letters by Baiqing Xu from the Chinese Academy of Sciences in Beijing and colleagues has shown just how enriched the glacier soot can become. The team dug snow pits on a glacier in the Tien Shan mountains, in central Asia, and measured soot concentration every month over the course of one year. They found that soot concentrations increased by at least an order of magnitude over the course of one season.

In the summer surface-snow black-carbon concentrations were as high as 400 nanograms per gram, while in the snowpack – the snow that survives more than one season – the researchers recorded levels of up to 3000 nanograms per gram. This enhancement at greater snow depths could become significant if warming raises the snowline and exposes older snow. Previous research has shown that soot concentrations of around 500 nanograms per gram decrease albedo (surface reflectivity) between 5 and 17%, which translates to a 50% increase in snow melting.

This mechanism of soot enrichment may also explain the increased melting seen on some Himalayan glaciers. Currently these glaciers are extremely important freshwater reservoirs, serving the needs of more than 1 billion people. In addition they modulate water flow, holding rainwater and releasing it slowly. Without the storing properties of the glaciers, flash floods are likely to become far more frequent.

How long before the glaciers melt? No-one can be sure. But on the positive side, cleaning up pollution is possible. If Asian countries scrub up their factories and cut down their smoke then Himalayan glaciers could revert to shiny white, perhaps buying them some more time.