Soot gets everywhere. Even into the world's highest mountains

THE Himalayas and the adjacent Tibetan plateau are sometimes referred to as the Earth's third pole, because of the amount of ice they host. They are also known as Asia's water tower. Their glaciers feed the continent's largest rivers—and those, in turn, sustain some 1.5 billion people. Many studies suggest, though, that the Himalayan glaciers have been shrinking over the past few decades. This has usually been attributed to rising air temperatures, but climate researchers have now come to realise that tiny airborne particles of soot and dust are also to blame. Being dark, they absorb sunlight. And that warms their surroundings.

Near cities, and in regions like South-East Asia, where people are clearing vegetation by burning it, soot is expected. But as Angela Marinoni of the Institute of Atmospheric Sciences and Climate in Bologna explained to an audience at the 2nd Third Pole Environment Workshop in Kathmandu on October 27th, the high Himalayas are also under an onslaught from this sort of pollution. Even at altitudes above 5,000 metres (16,400 feet), soot is widespread. And when it lands on glaciers it accelerates their melting.

Dr Marinoni and her colleagues have been examining Himalayan soot since 2006. In that year the Nepal Climate Observatory - Pyramid, in the Khumbu valley, began a full-time study of aerosol particles, soot among them. The researchers' initial intention was to take advantage of what they assumed would be the pristine conditions found at such high altitude (the observatory is 5,079 metres above sea level) to measure typical background conditions of the atmosphere. Instead, they were surprised to find a thick haze, loaded with soot, smothering the mountain slope. In the rainless pre-monsoon months between January and May, about one day in five saw the Khumbu valley blanketed in a dense brown cloud.

By analysing atmospheric circulation patterns, Dr Marinoni and her colleagues found that winds could bring soot and dust from as far away as Europe, the Middle East and North Africa. And if that were not bad enough, the Himalayan valleys act as chimneys, pumping pollutants from the Indian plains to the mountain peaks. Dr Marinoni estimates that the combined effect of this crud could reduce the glaciers' ability to reflect light by 2-5% and increase the amount of melting by 12-34%.

Those suggestions are corroborated by a study led by Xu Baiqing of the Chinese Academy of Sciences' Institute of Tibetan Plateau Research, in Beijing. His team drilled cores from the ice of five Tibetan glaciers in order to examine the past few decades' worth of pollution. These cores show that the level of pollution, especially soot, in Himalayan glaciers correlates with emissions in Europe and South Asia.

In the north-west and centre of the Tibetan plateau Dr Xu and his team found that particularly high levels of soot had fallen on the glaciers during the 1950s. These regions are under the influence of westerly winds, suggesting that the soot in question originated in Europe. That hypothesis is supported by the observation that soot levels fell during the 1970s, a period when many European countries enacted clean-air regulations.

The glaciers in the south-eastern part of the plateau, by contrast, are downwind of the Indian subcontinent—and Dr Xu found that the concentration of soot in those glaciers went up by 30% between 1990 and 2003, coinciding with a period of rapid industrial growth in India.

The worry is not that the Himalayan glaciers will disappear. Despite a foolish mistake in a report by the International Panel on Climate Change suggesting that this might happen quickly, no one believes that to be the case. But accelerated melting induced by this soot could cause flooding. That would be bad enough.