Himalayan glaciers getting smaller every year: Study

Himalayan glaciers getting smaller every year: Study

Himalayan glaciers across India, China, Nepal and Bhutanhave been losing the equivalent of more than a vertical foot and half of iceeach year since 2000, a new comprehensive international study said onWednesday.

The analysis, spanning 40 years of satellite observations,indicates that melting of the Himalayan glaciers caused by rising temperatureshas accelerated dramatically since the start of the 21st century — almostdouble the amount of melting that took place from 1975 to 2000.

The study is the latest and perhaps most convincingindication that climate change is eating the Himalayas' glaciers, potentiallythreatening water supplies for hundreds of millions of people downstream acrossmuch of Asia.

"This is the clearest picture yet of how fast Himalayanglaciers are melting over this time interval, and why," said lead authorJoshua Maurer, a Ph.D. candidate at Columbia University's Lamont-Doherty EarthObservatory.

While not specifically calculated in the study, the glaciersmay have lost as much as a quarter of their enormous mass over the last fourdecades, said Maurer.

The study appears this week in the journal Science Advances.

Currently harboring some 600 billion tonnes of ice, theHimalayas are sometimes called the earth's "Third Pole".

Many other recent studies have suggested that the glaciersare wasting, including one this year projecting that up to two-thirds of thecurrent ice cover could be gone by 2100.

But up to now, observations have been somewhat fragmented,zeroing in on shorter time periods, or only individual glaciers or certainregions.

These studies have produced sometimes contradictory results,both regarding the degree of ice loss and the causes.

The new study synthesises data from across the region,stretching from early satellite observations to the present.

The synthesis indicates that the melting is consistent intime and space, and that rising temperatures are to blame.

Temperatures vary from place to place, but from 2000 to 2016they have averaged one degree Centigrade (1.8 degrees Fahrenheit) higher thanthose from 1975 to 2000.

Maurer and his colleagues analysed repeat satellite imagesof some 650 glaciers spanning 2,000 kms from west to east. Many of the20th-century observations came from recently declassified photographic imagestaken by US spy satellites.

The researchers created an automated system to turn theseinto 3D models that could show the changing elevations of glaciers over time.

They then compared these images with post-2000 optical datafrom more sophisticated satellites, which more directly convey elevationchanges.

They found that from 1975 to 2000, glaciers across theregion lost an average of about 0.25 metres (10 inches) of ice each year in theface of slight warming.

Following a more pronounced warming trend starting in the1990s, starting in 2000 the loss accelerated to about half a meter (20 inches)annually.

Recent yearly losses have averaged about 8 billion tonnes ofwater, or the equivalent 3.2 million Olympic-size swimming pools, said Maurer.

Most individual glaciers are not wasting uniformly overtheir entire surfaces, he noted; melting has been concentrated mainly at lowerelevations, where some ice surfaces are losing as much as five metres (16 feet)a year.

Some researchers have argued that factors other thantemperature are affecting the glaciers. These include changes in precipitation,which seems to be declining in some areas (which would tend to reduce the ice),but increasing in others (which would tend to build it).

Another factor: Asian nations are burning ever-greater loadsof fossil fuels and biomass, sending soot into the sky.

Much of it eventually lands on snowy glacier surfaces, whereit absorbs solar energy and hastens melting.

Maurer agrees that both soot and precipitation are factors,but due to the region's huge size and extreme topography, the effects arehighly variable from place to place.

Overall, he says, temperature is the overarching force. Toconfirm this, he and his colleagues compiled temperature data during the studyperiod from ground stations and then calculated the amount of melting thatobserved temperature increases would be expected to produce.

They then compared those figures with what actuallyhappened. They matched.

"It looks just like what we would expect if warmingwere the dominant driver of ice loss," he said.

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