The melting of Himalaya’s glaciers: fact or fiction?
What’s going on with the largest swath of glaciers outside Greenland and Antarctica? We all know that an Intergovernmental Panel on Climate Change report from 2007 erroneously stated that Himalayan glaciers would disappear by 2035. But the ice fields in these giant mountains are indescribably inaccessible: remote, steep and often enmeshed in politics.
Records of glaciers, weather and river flow in the region are imperfect — if they even exist.
Now comes a report that Himalayan glaciers, which cover about 60,000 square kilometers, are “changing at the same rate as the global average,” says Andreas Kääb, a professor of geosciences at the University of Oslo (Norway).
According to Kääb’s new paper in Nature, these glaciers thinned by about 21 centimeters per year between 2003 and 2008, feeding 13 billion extra tons of meltwater each year into rivers that water such nations as China, India and Vietnam.
Despite their importance, these glaciers are difficult to study, Kääb says. “These are the highest mountains in the world, the range is 2,000 to 3,000 kilometers long, and some areas are extremely hard to reach. The major glacier in Bhutan requires a two-week walk from the capital.”
Eye from the sky
Kääb and colleagues used data from space to study Himalayan ice, which presented its own complications. The ICESat satellite, which operated from 2003 to 2009, was built to measure the height of glaciers by timing the reflection of a laser beam. Although the satellite was accurate to within a few centimeters, it was designed for horizontal ice in the Antarctic and Greenland.
In the mountains, the 70-meter circle bathed by ICESat’s laser could contain any number of elevations. However, in tests in Svalbard, a mountainous island north of Norway, Kääb figured out how to adapt ICESat readings to steep terrain.
Kääb says ground truthing — the comparison of satellite data to conditions on the ground — continued in the Himalayas, where ICESat measured stable ground and found no movement. “This gives us some confidence that this is a true signal.”
Because ICESat only started operating in 2003, the researchers got baseline data from a Space Shuttle mission that measured the height of ice with radar in 2000.
An average bunch of glaciers
The Nature results showed an overall thinning in the Himalayas that matched the global average. Curiously, the thinning was equally fast in glaciers covered by rocky debris, which supposedly insulates the ice.
“It was puzzling at first,” says Kääb. “It’s true that the debris cover insulates the ice and protects against solar radiation, but that’s only valid for intact debris.” However, the many meltwater pools on Himalayan glaciers accelerate the melting through a positive feedback that can become “a runaway process.”
The thinning also showed regional variation, with a bit of thickening seen in Karakoram mountain glaciers in the northwestern Himalayas. These glaciers may be getting more snow due to increased water vapor in the atmosphere, a side effect of global warming.
The larger picture in the Himalayas, however, “is very similar to the global average,” says Kääb. “This means that most of these Himalayan glaciers are not the exceptions that they were believed to be for a long time.”
River of worries
More than a billion people live in watersheds fed by the glaciers in the Himalayas and Tibet, which source giant rivers like the Ganges, Brahmaputra, Yangtze and Mekong. According to the Kaab’s calculations, glacier melt supplied, on average, 2 to 4 percent of the extra water flow as these rivers enter the lowlands. “This means that, on average, a 2 to 4 percent surplus is in these rivers right now.”
In general, glaciers store water in winter and release it in summer, says Kaab. “One can imagine shrinking glaciers as a water storage container that slowly empties over the years, even if the seasonal water level in the river goes up an down.”
The decline of the Himalayan glaciers does not prove climate change, Kääb says. “What we did was measure glacier thickness over five or six years; no more, no less. Climate is defined as weather over 30 years, so our study did not talk about climate-induced change.”
Further, five or six years is “not a long time,” Kääb says. “It could be that they were very good or very bad years for the glaciers, but there are almost no sufficient meteorological measurements on the ground.”
As scientists debate the condition of the Himalayan glaciers (a estimate from earlier this year put the annual mass loss at a much lower 5 billion tons), a measurement that aligns with numbers from other parts of the world at least passes the sniff test.
“What is clear is that the results are consistent with glacier decline seen elsewhere, and with the ongoing warming of the climate,” Kääb says.
— David J. Tenenbaum
- Contrasting patterns of early twenty-first-century glacier mass change in the Himalayas, Andreas Kääb, et al, Nature, 23 August 2012. ↩
- Feast your eyes on Himalayan peaks! ↩
- Documenting the demise of glaciers ↩
- Why understanding glacier melt and growth is important ↩
- Tibetan reporting on Himalayan glacial melting ↩