The Andes’ glaciers shouldn’t be free of snow so early this time of year, but some are now bare.

Warm conditions in January, including a scorching heat wave with temperatures exceeding 100 degrees Fahrenheit in some locations, melted almost all snow cover on some of Chile’s Olivares Glaciers and Volcan Overo in Argentina. With around eight weeks left in the melt season, the exposed glacial ice could disappear faster now without a blanket of snow.

“We’re seeing snow-free glaciers at unusual times, and that means midsummer in the Andes,” said Mauri Pelto, a glaciologist at Nichols College. “Those are all related to just high temperatures.”

As global temperatures rise, mountain glaciers around the world are sweating. This could affect nearly 1.9 billion people living in and downstream of mountainous areas, who depend on melting ice and snow for drinking, agriculture and hydroelectric power. In the tropical Andes, for example, glaciers provide almost one-third of the water that millions of people in major cities use during the dry season.

A new study published Monday in the journal Nature Geoscience shows the decline could be more calamitous than previously thought. Earth’s mountain glaciers may have less ice than previously estimated – meaning they could be tapped dry sooner than expected, especially as climate change hastens their melt.

The researchers also found the potential sea level rise contribution from the glaciers would decrease by about 20%, from 13 to 10 inches. But since mountain glaciers contribute around only one-third of global sea level rise, this has only a modest impact on future projections.

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“This study is not good news because we have less freshwater for people if we have less ice,” said Romain Millan, lead author of the study and postdoctoral candidate at the Institute of Environmental Geosciences in France. “For sea level rise, it does not change anything to the big picture … because Greenland and Antarctica are the major drivers of sea level rise.”

In the first worldwide survey of its kind, Millan and his colleagues determined the movement and thickness of more than 250,000 mountain glaciers, which account for 98% of glaciers outside of the Greenland and Antarctic ice sheets.

“You would be surprised by how little we still know about mountain glaciers. While we know their surface area pretty well, we have direct measurements of ice thickness for less than 1% of the 250,000 glaciers worldwide,” Mathieu Morlighem, co-author and professor at Dartmouth College, wrote in an email. “Their future is notoriously difficult to predict because we are still lacking basic characteristics, such as their geometry.”

For the new study, researchers from France, Denmark and the United States analyzed 800,000 pairs of satellite images — including small glaciers that have never been studied, such as in New Zealand and the southern cordilleras of South America, as well as large ones in Patagonia and the Arctic. The team tracked the movement of their physical features, like crevasses, over more than a year to assess their ice flow. Using the glacier’s velocity and slope, they were able to estimate the ice’s thickness and volume.

The scientists concluded global glacial ice content was lower than previously thought, but it varied by region. In the tropical Andes mountains, the researchers found nearly a quarter less glacial ice, which translates to about 23% less freshwater.

Extreme heat waves in the region, like last month’s, make these smaller stores of ice dwindle faster and leave even less for future generations.

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“When you lose your snowpack early in the summer, [the glacier] melts a lot faster because the ice is dark, so it strips volume away quicker,” said Pelto, who was not involved in Monday’s study.

Mountain glaciers in other locations, researchers found, had more ice than previously estimated. The Himalayas’ glaciers have 37% more ice, the researchers estimate, but some of these ice fields are under intense pressure as temperatures rise.

A new study published Thursday in the Nature Portfolio Journal Climate and Atmospheric Research shows the world’s highest glacier, on top of Mount Everest, is melting faster than ever. The Himalayas’ South Col Glacier is losing decades worth of ice every year because of climate change, according to researchers who retrieved an ice core there.

Ice that took nearly 2,000 years to form on the glacier melted in about 25 years, declining at a rate about 80 times as fast as it took to form. South Col Glacier may disappear by the middle of this century, the authors wrote.

“We show that even a small increase in temperature can result in increased melting and significantly more sublimation particularly if the glacier surface is not covered with snow all year,” study co-author Paul Andrew Mayewski, a professor at the Climate Change Institute at the University of Maine, said in an email.

Researchers in different parts of the world have cautioned that mountain glaciers could disappear in the near future unless the world reduces greenhouse gas emissions faster. The U.N. climate agency said the last three mountain glaciers in eastern Africa are shrinking so fast they could vanish in two decades. And Austrian scientists studying ice caves in the eastern Alps say that at the current rate, those glaciers will be gone, too.

Morlighem said his team’s new comprehensive data set could improve existing projections of glacier behavior, although direct field measurements would help reduce the team’s overall uncertainty. On-the-ground measurements could also help inform the communities depending on their water supply.

“We can’t predict how they will continue to respond to climate change if we don’t know how thick they are,” he said of glaciers. “We hope that more data will be collected over the coming years to improve our ability to predict how much time these glaciers have left, as many communities rely on them for hydropower production, agriculture or even drinking water.”