Faced with a dearth of logistical support and challenges related to climate change, human-powered trips to the North Pole may be on the brink of extinction.

“North Pole expeditions are going the way of the passenger pigeon,” says Eric Larsen, a Colorado-based polar explorer who has completed three North Pole expeditions.

According to Tom Sjogren from adventurestats.com, the record-keeper of Arctic feats, a true North Pole expedition must travel from the coastline of Alaska, Greenland, Canada, or Russia over the polar ice mass to the North Pole, which sits at a latitude of 90 degrees north. Once the journey to the North Pole has been completed, it’s acceptable to get picked up by helicopter or plane.

According to purists, the preferred expedition style is completely human powered, unsupported, and unassisted, meaning no air-dropped supplies or external aids such as dogs, kites, or motorized vehicles, which increase speed and lessen the load. Since Admiral Robert E. Peary purportedly completed the first expedition to the North Pole in 1909 (subsequent analysis has cast doubt on whether he made it), only 47 of the 247 treks completed to 90 degrees north have been unsupported and unassisted.

In an unsupported expedition, North Pole travelers must ski, snowshoe, swim, and climb, all while towing a 300-pound sled of supplies approximately 480 miles, which takes about 50 to 70 days. There are mounds of ice as big as houses to get over and stretches of 30-degree water to traverse, which travelers swim across wearing a full-body rubber suit. Air temperatures often hover around 40 degrees below zero.

“It’s the most difficult expedition on the planet that nobody really knows about,” says Larsen.

The window to join this exclusive—if painful—club may soon be closed. In the last five years, only one unsupported, unassisted expedition has completed the journey to the North Pole, compared to seven from 2005 to 2010.

“They’re done,” says Richard Weber, an Arctic pioneer from Canada who has skied to the North Pole six times, more than anyone in history. “The future of skiing to the North Pole is dim.”

Thinning, Drifting Ice Increases Danger

That’s largely due to changing ice conditions in the Arctic caused by climate change. The Arctic sea ice extent in March 2015 was the lowest for that month since satellites began tracking the ice in 1981, according to the National Snow & Ice Data Center. Multiyear ice, the oldest ice, which typically survives the summer melt cycle, is disappearing at a rate of 15.1 percent per decade.

Thinning ice has a multitude of consequences. “Historically, you would have ice that was five, six feet thick and that’s relatively stable,” says Larsen. “Now, the ice is thinner [and] breaks up more often and much more irregularly. As a result, you have a more rough surface area, which is more difficult to cross.”

Richard Weber doesn’t agree that the new ice is tougher to cross, but he does think thinner ice poses new problems for Arctic adventurers.

“If you don’t have multiyear ice to camp on, you’re camping on thin ice and to me that’s dangerous. It’s thin, and it moves in the middle of the night. Nothing’s going to stop it from cracking under your tent.”

Also, there’s more open water now. Weber didn’t start bringing dry suits on his expeditions until 2000.

“A floating field of ice can drift miles in any direction, throwing explorers off course.”

Plus there are more ice formations. As the ice breaks away from the polar cap, it floats toward Canada. When it collides with the coast, it begins to buckle, creating a zone of ice blocks and miniature ice-mountain ranges, or pressure ridges, which makes travel very arduous. During the first 18 days of Larsen’s 2014 expedition, his two-person team traveled an average of 2.78 miles over eight hours a day.

Furthermore, the melting of multiyear ice has affected the Arctic drift—the general direction that the ice floats in the Arctic Ocean. Ice tends to move southward toward Canada. But because the ice is thinner and more broken up now, it’s more affected by the wind, which means a floating field of ice on which an explorer is traveling can drift miles in any direction, throwing him off course, adding significant distance to an already difficult journey that is constrained by time.

There has always been drift, but now it is now more irregular and dramatic. Says Ron Kwok, a senior research scientist from NASA, “Ice is moving faster compared to 20, 30 years ago. Because it’s thinner, it’s moving faster and it’s more responsive to the wind.”

During a 2007 expedition, Weber walked for 10 to 12 hours a day, but because the ice he was on was drifting south, he stayed in virtually the same spot. That’s “way worse” than it was in previous years, he says.

Further complicating the matter, the weather window for reaching the North Pole is short—and getting shorter every year—lasting from early March to early May, when the harshest temperatures of winter abate and the summer melt cycle is just beginning.

Air Charter Ends North Pole Flights

The newest and perhaps biggest impediment to North Pole expeditions is the lack of air service. The only air charter operating in the North Pole stopped supporting private expeditions this year. In November, Kenn Borek Air announced it would no longer support private expeditions to the Arctic because of the challenging economics involved in these operations.

“It takes a lot of logistics to get it done and keep it organized and set it up for just a little bit of work. Why bother when you can do something else with that same aircraft?” says Wally Dubchuk, a pilot who has been flying for Kenn Borek Air for ten years.

In 1970, the Calgary-based charter airline began flying to the Arctic to support oil exploration in the region. Using Twin Otter planes outfitted with skis or special wheels to land on snow and ice, Kenn Borek Air expanded to North Pole expeditions—flying Arctic travelers to Ward Hunt Island, northwest of Greenland, where they would start the journey, and then picking them up from the North Pole at the end of the trip. Their planes would sometimes drop supplies to travelers along their journey and perform emergency evacuations.

Dubchuk agrees that the ice conditions have changed, but insists they aren’t the reason Kenn Borek stopped servicing the Arctic. “The ice conditions have changed, but it’s not some insurmountable job to go up to the North Pole, or somewhere in between, from the tip of North America,” he says. “It’s just that the amount of work we do up there has basically dwindled down to nothing.”

With the air service unavailable, would-be North Pole adventurers must rethink several aspects of their expeditions. “The logistics are becoming almost impossible,” says Larsen. “You don’t have a way to get to the starting line anymore,” he says, referring to the lack of air service to Ward Hunt Island.

“With Kenn Borek out of the picture, it’s like if you weren’t allowed to use oxygen any more on Everest,” said Thomas Ulrich, a Swiss adventurer who set out on April 15 to ski from the North Pole to Greenland but abandoned his attempt two days later.

Ulrich says his decision had nothing to do with changing ice conditions. (He believes that the younger ice is actually easier to travel over.) Rather he was unwilling to accept the level of risk involved in his expedition.

“I knew there could be no call to Kenn Borek Air if I was tired or if there was an emergency and that was the kind of pressure I had. I needed more of a safety net if I was going to do it in any kind of safe way.”

Ulrich isn’t convinced that Arctic expeditions are over but agrees that the future for unsupported trips is changing. “If you want to do an unsupported expedition, you’re going to have to plan on sailing.”

Perhaps we’ll soon be anointing the first adventurer to take a boat to the North Pole.