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Glacier water supplies could peak sooner than expected


The world’s glaciers may hold less water than previously thought, a new study suggests, suggesting freshwater supplies may peak sooner than expected for millions of people around the world who rely on melting glaciers for drinking water, crop irrigation and daily use.

The latest findings are based on satellite images taken in 2017 and 2018. They are a snapshot in time; scientists will need to do more work to link them to long-term trends. But they imply that further global warming could wipe out today’s ice in many places in a shorter time than previously thought.

In the tropical Andes, for example, the study estimated that the volume of glaciers was 27% lower than the scientific consensus of a few years ago. In parts of Russia and northern Asia, the volume of glaciers was 35% smaller, according to the study.

Worldwide, the study found 11% less ice in glaciers than previously estimated. In the high mountains of Asia, however, he found 37% more ice, and in Patagonia and the central Andes, 10% more.

The new estimates come from a more detailed and realistic digital reconstruction of Earth’s 215,000 glaciers than before, said Romain Millan, a geophysicist at the Institute of Environmental Geosciences in Grenoble, France, and lead author of the study. , which was published on Monday in the journal Nature Geoscience.

Even so, “we still have a lot of uncertainty in some areas,” Dr Millan said, mainly due to the paucity of field measurements, which help inform any numerical reconstruction. These regions, including the Andes and the Himalayas, “are where people depend on fresh water from glaciers,” he said.

Melting glaciers are threatening livelihoods and reshaping landscapes in North America, Europe, New Zealand and many places in between.

In the upper Indus basin of the Himalayas, which straddles Afghanistan, China, India and Pakistan, melting glaciers account for nearly half of river flow. Still, logistical and political challenges mean scientists can only monitor a small portion of the Himalayan glaciers, said Anjal Prakash, a water expert at the Indian School of Business in Hyderabad who has no worked on the new study.

“This is a data-poor region,” Dr. Prakash said. “Countries are not cooperating. They do not share information with each other.

With 1.5 billion people benefiting from the water and other resources of the Himalayas, while facing growing risks of severe flooding, the region is “just waiting for disaster to strike”, said Dr Prakash.

The melting of glaciers has contributed to global sea level rise. The new study suggests that, all together, they could add 10 inches to the oceans instead of the roughly foot that was estimated earlier. Either way, it’s small compared to what the melting of Greenland and Antarctica could add to sea levels in the distant future if the planet warms to catastrophic levels.

To produce their new estimates of glacier dimensions, Dr. Millan and his colleagues combined more than 811,000 satellite images to measure the speeds at which glacier surfaces are moving. Glaciers may look like solid, unchanging masses, but in fact, they are constantly in motion: sliding across the terrain; deform under their own weight; flowing like syrup in the valleys. This movement is a clue to the amount of ice that is locked inside.

“The thickness of the glacier controls the speed at which it moves,” said Daniel Farinotti, a glaciologist at the Swiss university ETH Zurich who did not work on the new study. “And so, vice versa, if you know how fast it’s moving, you can tell something about the thickness.”

The high resolution of satellite images allowed Dr Millan and his colleagues to capture fine variations in the thickness of glaciers, such as narrow depressions in the ground below. They were able to map small ice sheets in South America, Europe and New Zealand that had never been mapped before.

In some ways, scientists understand some of the world’s mountain-draped glaciers less than the much larger Greenland and Antarctic ice sheets, said Mathieu Morlighem, a Dartmouth College geologist who worked on the new study.

Only a few thousand glaciers in the world have been measured on site. In places like North America, the milder climate means more water pockets in glaciers, which can thwart radar measurements. Compared to giant ice caps, where fast-moving ice has smoothed the underlying bedrock over time, the terrain beneath mountain glaciers can be “so complex”, Dr Morlighem said, making it more difficult the evaluation of their dimensions.

“As recently as 10 or 15 years ago, we barely knew the surface area of ​​glaciers,” said Regine Hock, a geoscientist at the University of Oslo in Norway, who was not involved in the new research. Estimates of glacier volume were “very, very rough,” she said.

Today’s “data revolution” is helping scientists make better predictions about local and regional water resources, even if the big picture – that glaciers will thin significantly this century – probably won’t change much, Dr. Hock said. “There’s not a lot of ice,” she said, “and then there’s none.”