Most Denali climbers follow the West Buttress route up the 72 kilometer (45 mile)-long Kahiltna glacier—one of five large glaciers on the mountain. “You could put an entire city on one of these glaciers, and the city would
look small compared to the size,” says Roger Robinson, a mountaineering ranger at Denali National Park and Preserve.

On the way to the summit, climbers typically stop at four or five camps. For years, climbers left their waste in holes in the ice there. They’d covered the latrines when they moved on, but that only hid the waste temporarily. “Surface snow would melt, and so within three weeks what you buried would be on the surface,” says Robinson.

Most Denali climbers follow the West Buttress route. It runs up the 72 kilometer (45 mile)-long Kahiltna glacier. That’s one of five large glaciers on the mountain. “You could put entire cities on one of these glaciers, and the city would look small compared to the size,” says Roger Robinson. He’s a mountaineering ranger at Denali National Park and Preserve.

Climbers usually stop at four or five camps on the way to the summit. For years, they left their waste in holes in the ice at camp. They covered the latrines when they moved on. But that only hid the waste for a while. “Surface snow would melt, and so within three weeks what you buried would be on the surface,” says Robinson.

Loso estimated that over the past several decades, Denali climbers had left behind 73 tons of human waste on the Kahiltna glacier. And he was sure that poop would reappear below the climbing route in a remote area sometimes used by backcountry travelers.

To predict when this would happen, Loso and his team needed to know how fast the glacier was moving. They trekked up Denali and measured how fast snow accumulated and melted at different elevations along the glacier (see Anatomy of a Glacier). They also drove metal stakes into the ice and used GPS to track how far the stakes moved over time.

Loso figured out how much human waste was on the Kahiltna glacier. Over several decades, Denali climbers had left behind 73 tons. And Loso was sure that poop would reappear in an area below the climbing route. Backcountry travelers sometimes use this remote part of Denali.

Loso and his team needed to know how fast the glacier was moving. Then they could predict when the waste would reappear. To learn how fast snow fell and melted, they hiked up Denali. They took measurements at different places along the glacier (see Anatomy of a Glacier). They also drove metal stakes into the ice. Then they used GPS to track the stakes’ movement over time.

According to the team’s calculations, waste would take between 71 and 206 years to reappear, depending on where and when it was buried. “Climbers first used the West Buttress route in the 1950s,” says Loso. “Sometime soon their poop ought to be emerging.” But the warming effects of climate change are melting glaciers at a faster rate. That means poop could resurface earlier than predicted.

The researchers also wanted to learn if poop buried in ice would eventually break down or if pathogens in it would die. The scientists froze some donated human feces samples in their lab. They buried others on the glacier and dug them up a year later. In both cases, the samples didn’t break down while frozen, and the pathogens remained active. “Poop’s not going to go away because you dumped it in the ice,” says Loso.

The team figured that waste would take between 71 and 206 years to reappear. This depends on where and when it was buried. “Climbers first used the West Buttress route in the 1950s,” says Loso. “Sometime soon their poop ought to be emerging.” But climate change is warming the planet. As a result, glaciers are melting faster. That means poop could reappear sooner than predicted.

The researchers also wanted to learn if poop buried in ice would change. Would it break down over time? Would the pathogens in it die? So people donated human feces samples, and the scientists froze some in their lab. They buried others on the glacier. A year later, they dug them up. All of the samples looked the same after being frozen, and the pathogens were still alive. “It’s not going to go away because you dumped it in the ice,” says Loso.

Loso’s research influenced the passage of stricter regulations on Denali. Starting last year, climbers could dump their waste in only one crevasse high on the mountain. Below that, they had to carry it out. But many climbers choose not to toss poop into the crevasse. Robinson expects about three-quarters of this year’s climbers to carry out all their waste. “I’m proud to see how clean the mountain has gotten,” he says.

As more and more climbers commit to keeping Denali clean, Robinson believes that use of equipment such as CMCs should stop new waste from piling up. But with decades’ worth of frozen poo still traveling downhill, the past will be coming back to haunt the mountain for a long time to come.

Loso’s research helped get stronger rules passed on Denali. Starting last year, climbers could dump their waste in only one place. It’s a crevasse high on the mountain. Below that, they had to carry it out. But many climbers don’t toss poo into the crevasse. Robinson thinks about three-quarters of climbers will carry out all their waste this year. “I’m proud to see how clean the mountain has gotten,” he says.

More and more climbers want to keep Denali clean. As they use gear like CMCs, Robinson believes that new waste will stop piling up. But decades’ worth of frozen poop is still traveling downhill. So the past will be coming back to haunt the mountain for a long time.