The moment Duncan Quincey arrived at the base of Khumbu Glacier, the massive river of ice draped across Mount Everest’s western side, a pang went through him.

“I looked over the edge and thought, ‘Wow, these changes are huge,'” he said in an interview with The Washington Post.

Even now, speaking from the warmth of his office at the University of Leeds, he can still see the marks in the rock where ice has dropped away, like the ring of grime around a bathtub left by draining water. He can visualize the glacier’s shifting proportions. And he can recall the lakes that have formed on the surface of the ice, vast pools of meltwater where he once only saw puddles.

“These glaciers are changing. They’re changing very rapidly … and our best evidence supports the fact that it’s climatically driven,” said Quincey, a professor of geomorphology. The question now, he continued, is, “What does that mean for the glacier and the people living downstream?”

Quincey and a team of other British geologists just returned from three weeks on the Khumbu Glacier — which ranges from 15,000 feet to nearly 25,000 feet above sea level — aiming to investigate that question. They say that for the first time supraglacial ponds on the ice river’s surface have coalesced into lakes the length of several football fields. Many are deep and broad enough to paddle across in boats.

The appearance of the lakes is perhaps the “best visual representation of how fast these glaciers are changing,” Quincey said.

If the lakes continue to develop, some researchers say they could pose a risk to people living downstream. In the past, glacial lakes have grown so large they overwhelmed the natural dams that contained them, sending thousands of gallons of meltwater into the valley below.

“The worrisome thing is that if the process continues, then there will be a new glacial lake to worry about in a populated and heavily used valley,” University of Arizona hydrologist Jeffrey Kargel told the BBC.

But that danger is distant, Quincey said; it will be years or even decades before the glacial lake on the Khumbu is large enough to pose an imminent threat. The reality is that researchers don’t really know what will happen as meltwater pools on the glacier’s surface — which is exactly the problem.

“That’s one of the driving questions of our research,” said Quincey.

What is clear is that the formation of these pools is accelerating. Water is less reflective than ice — rather than throwing the sun’s energy back, it absorbs it, warming both the ponds and the ice around them. That causes a positive feedback cycle: “Sun can heat up water faster, water transmits heat into surrounding ice, the ice melts a little bit faster, that means there’s more water, and that heats the ice,” Quincey explained. “And the cycle continues.”

The shift is especially dramatic for the Khumbu Glacier, which is typically pocked with rocks and other accumulated detritus. Rock heats more slowly than both ice and water, so the layer of debris usually shields the ice river from the some of the effects that rising temperatures have on other “smooth” glaciers. But the appearance of pools on the surface of the ice mitigates those benefits.

Though the pools on the Khumbu are relatively small, glacial lakes elsewhere in the Himalayas offer a hint at what they might become. Just west of Everest, the glacial lake Tsho Rolpa is two miles long and contains nearly 100 million cubic meters of water. Half a century ago, according to National Geographic, it extended just a tenth of a mile.

This year Tsho Rolpa was outfitted with a new warning system, given the risk of a glacial lake outburst flood, the Kathmandu Post reported. If it bursts, 6,000 people would be affected.

Back on Everest, the growth of the pools could have any of a number of impacts. They might overwhelm the natural barriers that contain them and flood, as Kargel warned, or they might act as a natural reservoir and provide a healthy amount of water to the valley below. In the short term, the lakes hold meltwater back — but if they hold too much, the villages below lose access to a crucial water supply.

“One problem is that we don’t have really objective methods by which to assess the danger that these lakes pose,” Quincey said. 

Even though it’s important work, he said, especially in light of the climate conference taking place in Paris this week, such research is also difficult and dangerous — scientists are no less at risk amid Khumbu’s treacherously shifting ice fields than mountain climbers.

Quincey and his colleagues will return to the mountain’s steep slopes in May to reassess the glacier and see how it changed.