Some Himalayan glaciers are advancing despite an overall retreat, according to a study into how climate change affects vital river flows from China to India.
The researchers believe a blanket of dust and rock debris was apparently shielding some glaciers in the world's highest mountain range from a thaw, a factor omitted from past global warming reports. And varying wind patterns might explain why some were defying a melt.
"Our study shows there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover," scientists at universities in Germany and the United States wrote in the study of 286 glaciers.
The findings underscore that experts in the UN's Intergovernmental Panel on Climate Change (IPCC) were wrong to say in a 2007 report that Himalayan glaciers could vanish by 2035 in a headlong thaw. The panel corrected the error in 2010.
The report says 58 per cent of glaciers examined in the westerly Karakoram range of the Himalayas were stable or advancing, perhaps because they were influenced by cool westerly winds than the monsoon from the Indian Ocean.
Elsewhere in the Himalayas "more than 65 per cent of the monsoon-influenced glaciers ... are retreating," they write in the journal Nature Geoscience of the satellite study from 2000 to 2008. Some glaciers that were stable in length were covered by a thick layer of rocky debris.
"Overall in the Himalayas, the glaciers are retreating," says lead author Dirk Scherler of the University of Potsdam in Germany.
From the Alps to the Andes
Scherler says the findings did not allow the experts to make any new estimates of water losses from Himalayan glaciers, whose seasonal melt helps keep up flows in the dry season in rivers from the Ganges to the Yangtze. More study was needed, he says.
"Glaciers are important to water supply to many people living in lowlands, not only for food and drinking water but also for hydropower," says Scherler. "It's essential to know what's going on."
Worldwide, most glaciers are shrinking from the Alps to the Andes in a trend blamed by the IPCC on greenhouse gases from human activities, led by the burning of fossil fuels.
Debris in the Himalayas - darker than ice and so soaking up more of the Sun's energy - tended to quicken a thaw if it was less than 2 centimetres thick. But a thicker layer on some Himalayan glaciers acted as insulation, slowing the melt.
Among complexities, some debris-covered glaciers that are stable in length might be getting thinner and so losing water overall, he says. That trend had been shown by past studies of the Khumbu glacier on Mount Everest, for instance.
After the Himalayan error, the IPCC has reaffirmed its key conclusion that it is more than 90 per cent likely that human activities are the main cause of climate change in the past 50 years, stoking more floods, droughts and rising sea levels.