جون 29, 2012
The most comprehensive survey of glaciers on the Tibetan Plateau and surrounding areas has found that the majority of glaciers there are in rapid retreat – and over the past 30 years the rate has been increasing.
Yao Tandong, a glaciologist at the Chinese Academy of Sciences’ Institute of Tibetan Research in Beijing, and his colleagues analysed satellite data and field measurements of 15 glaciers for decades and published their findings in Nature Climate Change this week. However, as previous studies have shown, there is significant regional variability, with glaciers in the Himalayas are retreating faster on average than those in the Karakoram and the Pamir.
The reason for the variation in glacier behaviour lies in local weather conditions: “Temperature rise is important,” says Yao Tandong. “But its effects on glaciers also depend on climate regimes.”
“In places dominated by the westerlies [the prevailing winds from Europe], such as the Karakoram and the Pamir plateau, glaciers gain their mass mostly from winter snow, and so are less affected by warming because temperatures in winter are still below zero. In the eastern and central Himalayas, however, it snows mainly during monsoon season, and a slight increase in summer temperatures can affect glaciers drastically,” explained an article published in Nature.
In the past few decades, the Indian monsoon has been getting weaker. By contrast, the westerlies are getting stronger. “This explains why most glaciers that are either stable or advancing are in the Karakoram or the Pamir plateau,” says Yao.
The study also raises questions over data published earlier this year by the Gravity Recovery and Climate Experiment (GRACE) satellite mission, which suggested that high-altitude Asian glaciers on the whole are losing ice only one-tenth as fast as previously estimated and that glaciers on the Tibetan plateau are actually growing.
Critics argue that the GRACE measurements taken were taken over too short a time to capture the impact of climate change – a period of seven years. Yao asserts that the satellite is not suited to studying ice changes in the third pole region: “As the GRACE satellites can only feel the gravitational pull and can’t tell the difference between ice and liquid water, they may have mistaken expanding glacial lakes for increases in glacier mass.” Glacial lakes on the plateau have increased by about 26% since the 1970s.
The study highlights the complexity of glacier responses in the region and the importance of on-the-ground measurements. Current estimates suggest there are about 12,000 to 15,000 glaciers in theHimalayasand about 5,000 in the Karakoram. But of these thousands of glaciers, only 15 have been measured on the ground to see if they are gaining or losing ice overall.