The pace of glacier melting has sped up over the last two decades, according to an analysis of hundreds of thousands of glaciers that researchers called the most accurate of its kind.

In the study, published Wednesday in Nature, the researchers used NASA satellite images to estimate ice loss of 217,175 glaciers between 2000 and 2019, findings that they say are important for local-scale water management and understanding sea level rise.

Glaciers are rivers or domes of ice that change over time, growing as snow hardens and shrinking as ice melts. These icy masses, which are not part of the vast Greenland and Antarctic ice sheets, are key parts of the planet's water cycle as their meltwater contributes to river flow, but climate change is accelerating the rate of glacier melt, altering water supplies and raising sea levels.

"Glaciers are an icon of climate change because they are quite beautiful and they are a big source of tourism in many regions of the world. And they are going to disappear in those regions, which is sad," said study lead author Romain Hugonnet, a Ph.D. student in geophysics and oceanography from space at the University of Toulouse and in hydraulics, hydrology and glaciology at ETH Zürich.

Previous estimates of global glacier loss had mostly used gravitational measures from satellites to calculate ice loss, but this technique is limited to rough estimates that have a lot of uncertainty, according to Hugonnet. Although some individual glaciers are precisely monitored by scientists, this is only done for about 10% of glaciers on Earth.

"We didn't have complete mapping of glacier thinning," Hugonnet said. "It was incomplete and the picture was really fragmented."

To build a more complete picture of glacier loss, Hugonnet and his colleagues used images from NASA's Terra satellite, which were made freely available in 2016. The satellite's instrument snaps pictures of the planet at two different angles, allowing the team to reconstruct 3D maps of the Earth's surface.

"If I take a picture of your face on the left side and the right side, I can reconstruct the 3D profile. So we do this for the surface of the Earth," Hugonnet explained. "Now we have this entire archive that we can use to look back at the changes of the surface of the Earth for the past 20 years."

The analyses showed that glacier mass loss has been accelerating at a rate of 48 gigatons per year each decade since 2000. On average, the Earth's glaciers lost 267 gigatons of water, an amount that would cover the state of Pennsylvania at a height of more than seven feet, each year between 2000 and 2019. 

The researchers also estimated that, during those two decades, water from melting glaciers contributed to around 21% of sea level rise, a global problem that could threaten the homes of at least 200 million people by 2100.

The study's estimates of glacier loss and sea level rise are in the same range as those of previous reports, according to Hugonnet. But, he added, "Because we have this global picture at high resolution, our advantage is that we really reduce the uncertainties."

The researchers also found that glacier loss was not consistent across the planet.

"What was quite surprising was that there was an acceleration in a lot of regions but some also decelerated — and by quite a bit. For instance, the rate of thinning in Iceland almost halved between the first and the second decade of our study," he explained. "So the message is that we need global-scale observations because we can't rely on observations in one region to infer at the scale of the planet."

Hugonnet hopes that the fine-scale analysis of individual glaciers will help inform future management of water resources. 

"This could maybe help us improve the prediction of floods or the opposite, water scarcity, and plan in advance for the risks that some of the population will be confronted with," he said.

The researchers found that patterns of glacier loss were linked with both elevated temperature and low precipitation, supporting other evidence that climate change is speeding up ice melting.

"If there was a message about these findings, it would be that we really need to act now about climate change," Hugonnet said. "At the scale of each person's everyday habits and consumption, but also society in general. We need to limit our [greenhouse gas] emissions in the long term." 

The study, "Accelerated global glacier mass loss in the early twenty-first century," published April 28 in Nature, was authored by Romain Hugonnet, Université de Toulouse, ETH Zürich and Swiss Federal Institute for Forest, Snow and Landscape Research; Robert McNabb, Ulster University and University of Oslo; Etienne Berthier, Université de Toulouse; Brian Menounos, University of Northern British Columbia and Hakai Institute; Christopher Nuth, University of Oslo and The Norwegian Defense Research Establishment; Luc Girod and Andreas Kääb, University of Oslo; Daniel Farinotti, ETH Zürich and Swiss Federal Institute for Forest, Snow and Landscape Research; Matthias Huss, ETH Zürich, Swiss Federal Institute for Forest, Snow and Landscape Research and University of Fribourg; Ines Dussaillant, Université de Toulouse and University of Zurich; and Fanny Brun, Université Grenoble Alpes.