The Torngats seven.

• Position: Torngat Mountains

On July 18, 2013, Matt Dyer lugged his fifty-pound pack into the Quality Hotel Dorval in Montreal. He'd taken a twelve-hour overnight bus from Lewiston, Maine. The afternoon sun was hot, and he was tired. Larry Rodman walked in at the same time, fresh off the airport shuttle bus after a quick flight from New York City. The two men started talking. Rodman, the big-city law partner, and Dyer, the legal aid attorney with the scraggly gray ponytail, hit it off immediately. For Dyer, especially, that was a relief. He'd been less concerned about the arduous journey than about the people he'd be trapped with in the wilderness. When you're paying for the trip of a lifetime, you want to enjoy the company.

Rich Gross and Marta Chase had flown in a day earlier to buy supplies and make last-minute arrangements. Chase's husband, Kicab Castaneda-Mendez, was there, too. Chase had introduced him to hiking when they were in their thirties, and since then the sixty-four-year-old management consultant had joined all of her Sierra Club trips, playing the default role of group photographer. On this trip his presence was even more reassuring. In June, Chase had been diagnosed with breast cancer and was told she needed a mastectomy. At first, she assumed she'd have to cancel the trip. But when her doctor assured her that postponing the surgery for a few weeks wouldn't matter, she decided to go.

Chase and Gross had developed a division of labor for their trips. Chase handled logistics like transportation and meals. Gross handled park permits and the route, poring over maps and plotting out the various options for getting from A to B. Each year they alternated who took the lead on research and outreach. For the Torngats trip, it was Chase's turn.

Chase studied the website for the Torngat Mountains Base Camp & Research Station, which Canada's parks department opened in 2006 in the Nunatsiavut region. This autonomous area in Labrador includes five small Inuit communities and the Torngat Mountains National Park. The camp sits just outside the southern limit of the park, on Saglek Fjord.

Chase sent an email to Base Camp, thinking it might serve as their entry point to the park. When she didn't get a response, she contacted Vicki Storey, an adventure travel agent in Alberta. Storey sent Chase to Alain Lagace, who operated two camps that offered guided tours, fishing expeditions, and wildlife safaris. Lagace knew the area well, Storey assured Chase. He'd been arranging trips into the Torngats for decades. In emails and phone calls over the course of months, Chase and Lagace shaped the Sierra Club trip. The group would fly from Montreal to Kuujjuaq, the largest Inuit community in Nunavik, the Inuit region of Quebec. Then they'd take a small charter plane to Lagace's Bamoin River Camp and spend the night. The next morning a floatplane would deposit them in the Torngats, where they'd be on their own for eleven days. "The thought of polar bears is still a concern to me," Chase said in one of her emails to Lagace. "I have experience with black and brown bears but not with polar."

Guns are generally prohibited in Canada's national parks, but in 2011 Parks Canada broadened the rules for parks with polar bears, allowing researchers, guides licensed by Parks Canada, and local, native Canadians to apply for gun permits for those parks. Guns could also be carried by Inuit bear guards who have taken a polar bear safety course and been licensed by Parks Canada. In its explanation for loosening the restrictions, Parks Canada cited an "increased risk of dangerous human-bear encounters" due to the impact of climate change on sea ice.

Parks Canada's website "strongly encourages" visitors to the Torngats to hire bear guards, but they are not required. When Chase asked Lagace whether they'd need a bear guard, she said he told her that no one who traveled through his camp used them. And none of the Parks Canada employees mentioned bear guards. Chase and Gross decided that flare guns, bear spray, and electric fences would offer them the protection they needed. The guides arranged to rent two flare guns from Lagace, each with four shells. Gross picked up two electric fences from the Sierra Club--one to encircle their campsite, the other to protect the area where they would cook and store their food. Each fence stood about three feet high and consisted of three parallel wires suspended from four-foot posts. Although the wires looked flimsy, they carried five to seven kilovolts of charge--not enough to seriously injure a bear, but supposedly enough to send it running.

Gross emailed a picture of the fence to Castaneda-Mendez. "What's the polar bear supposed to do? Die of laughter?" Castaneda-Mendez wrote back. They'd also take bear spray, which they had carried on previous trips to ward off grizzly bears. Should something go wrong, they would have a satellite phone to call for help.

A Parks Canada employee told Chase that anyone entering the park was required to watch a DVD on polar bear safety. Parks Canada agreed to send the video to Lagace's camp, so they could watch it right before they entered the park. The employee assured them they were in good hands with Alain Lagace.

Parks Canada also discussed options for their backpacking route. Chase says bear guards were never mentioned during these conversations, nor were Gross and Chase warned that the area is a "polar bear highway," as one Parks Canada official later described it. The group decided to start at Nachvak Fjord and move inland, packing up camp most mornings and working their way toward Komaktorvik Fjord, where they'd be met by a plane from Lagace's camp. It would be a tough trip. They'd be carrying their fifty-plus-pound bags for about six hours each day, stopping in midafternoon to find a place to camp. Halfway through the trip, Lagace would send a plane to drop off the rest of their food.

The conversation about climate change and its consequences often revolves around abstract concepts--sea-level rise, ocean acidification--but that's not the case with the melting of the sea ice. In the Arctic, the consequences are more tangible, more immediate. Using yearly averages taken throughout the Arctic, NASA scientist Claire Parkinson has reported that about 695,000 square miles of sea ice have been lost since 1979. That's roughly the same as if the western portion of the United States--California, Nevada, Oregon, Washington, Arizona, Utah, and most of Idaho--had disappeared.

Parkinson explains that sea ice has a symbiotic relationship with climate change. It's not just that the ice is melting, but also that its disappearance is exposing the dark ocean below. A surface that once reflected the sun's radiation is being replaced by a surface that absorbs it, further warming the ocean and leading to even more sea ice melt. This process, called ice albedo feedback, contributes to a phenomenon called polar amplification, which refers to the increased rate of warming near the poles in response to rising temperatures, which are precipitated by greenhouse gas emissions.

In a study in the journal Geophysical Research Letters, published in June 2014, Parkinson analyzed the satellite record and found that since 1979 there has been an average of at least five fewer days of sea ice per decade in areas with seasonal ice. In some areas, the decline is much steeper. Parkinson looked at the number of ice-free days in the Davis Strait, which is part of the ecoregion that includes the Torngat Mountains National Park. She found a decrease of about fifteen days per decade--or roughly fifty days since 1979.

Reports by the Intergovernmental Panel on Climate Change earlier this year also quantified the sea ice loss. The IPCC said that Arctic sea ice has disappeared at a mean rate of between 173,000 and 196,000 square miles per decade since 1979--a loss larger than the state of California every ten years. The ice is disappearing even faster in the more southern areas of the Arctic--between 280,000 square miles (California plus Arizona) and 410,000 square miles (California, Arizona, and Colorado) per decade.

It wasn't long ago that scientists who came out with such alarming findings faced doubt and ridicule. In 2006, Cecilia Bitz, a physicist who studies sea ice and does climate modeling at the University of Washington, coauthored an article in Geophysical Research Letters that projected the Arctic would have its first completely ice-free period by the end of the summer of 2040. The findings were greeted with skepticism, even a touch of derision. The Village Voice ran a cartoon mocking them. Even Bitz had trouble internalizing the magnitude of what she had learned.

When the 2007 readings came out, she, like many of her colleagues, was caught by surprise. By mid-August of that year, the sea ice minimum had broken every existing record--and there was still a month to go before it hit the annual low point. By the time the ice melted to its minimum that year, it was almost 40 percent below the 1979-2000 average. "To be that fooled by what came to pass was really shocking to me and a big wakeup call," Bitz said. "I think the whole community felt that way. We were startled." When the next record-breaking low came around, in 2012, Bitz and other experts were less shocked. Another 300,000 square miles of sea ice had been lost--more than the area of the state of Texas.

As the melting continues, a ripple of change reverberates through the flora and fauna that rely on sea ice as their habitat--the capelin and other fish that harvest the plankton along the ice's edge, the narwhals and beluga whales that swim below, and, of course, the polar bear, the king of the Arctic, sitting patiently on the shrinking ice, waiting for its prey.

On the afternoon of July 19, the nineteen-seat Air Inuit Twin Otter descended over a steep waterfall and bumped down onto the gravel landing strip at Barnoin River Camp, some 900 miles north of Montreal...