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New research shows the world’s glaciers have less ice than scientists previously thought.

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The study used data on glacier velocity to find South America's tropical Andes mountains have less ice, which impacts drinking water for communities in the area. In this depiction of Peru's Cordillera Blanca range, darker colors mean faster glacier speed.

To know how climate change will impact glaciers – and the communities that rely on them for drinking water – we need to know how much ice all of the world’s glaciers have right now, says Mathieu Morlighem, a glacier expert and earth sciences professor at Dartmouth.

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But it’s hard to measure the thickness of a glacier.

Morlighem helped create the first atlas that measures the thickness and movement of mountain glaciers, using satellite data to make better estimates of how much ice they hold.

And through his research, which covers 98% of the world’s glaciers, his team found that many glaciers are shallower than scientists previously thought.

“We know from other instruments that all the glaciers are melting very, very rapidly around the world. If anything, this study shows us that in many regions around the world, we have less time than we may have thought before they're gone,” Morlighem said.

At first, scientists estimated the depth of a glacier based on the surface area of the ice. That’s kind of like trying to figure out how deep a lake is just by looking at it, Morlighem said.

“We have very deep lakes that are pretty small in terms of surface area,” he said. “We may have gigantic lakes that are very shallow. It's the same with glaciers.”

Scientists developed some more sophisticated ways of measuring the thickness of glaciers, but have struggled to calibrate the data. And some previous studies have struggled to count mountain glaciers, leading to overcounting.

Using new information from satellites and simple physics, Morlighem and his team were able to determine the thickness of glaciers by measuring how steep they are, and how fast they’re flowing.

“This is something people don’t realize: glaciers move,” he said. “It flows, like a thick syrup.”

Morlighem says the information his team collected could help people decide how to adapt to climate change, especially in the South American Andes, where his team found a quarter less ice than previous estimates showed.

“Policymakers have to take this new information into account and plan for the future,” he said.

That could mean diverting drinking water from other sources, or relocating populations if conditions become unlivable. About 2 billion people rely on glaciers or snowpack for drinking water, Morlighem said.

In the Himalayas, however, Morlighem’s team found one-third more ice than previously thought, which could buy communities in that part of the world a bit more time.

The research also shows the reduction in the estimated size of the world’s glaciers could mean their contribution to sea-level rise is slightly less than previously predicted. The melting of mountain glaciers does contribute to sea-level rise, but not as much as the ice sheets in Greenland and Antarctica, which have a much larger potential to cause rising sea levels, Morlighem said.

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