A shipload of visitors arrived in the fjord overnight, so Ingibjorg Gisladottir dressed like a Viking and headed out to work in the ruins scattered along the northern edge of this tiny farming village. Qassiarsuk is tiny (population: 56), remote, and short on amenities (no store, public restrooms, or roads to the outside world), but some 3,000 visitors come here each year to see the remains of Brattahlid, the medieval farming village founded here by Erik the Red around the year 985.

When they arrive, Ms. Gisladottir, an employee of the museum, is there to greet them in an authentic hooded smock and not-so-authentic rubber boots. "There were more visitors this year than last," she says. "People want to know what happened to the Norse."

The Greenland Norse colonized North America 500 years before Christopher Columbus "discovered" it, establishing farms in the sheltered fjords of southern Greenland, exploring Labrador and the Canadian Arctic, and setting up a short-lived outpost in Newfoundland. But by 1450, they were gone, posing one of history's most intriguing mysteries: What happened to the Greenland Norse?

There are many theories: They were starved off by a cooling climate, wiped out by pirates or Inuit hunters, or perhaps blended into Inuit society as their own came unglued.

Now scientists are pretty sure they have the answer: They simply up and left.

"When the climate deteriorated, and their way of life became more difficult, they did what people have done throughout the ages: They looked for a more opportune place to live," says Niels Lynnerup, a forensic anthropologist at the University of Copenhagen in Denmark who studies the Norse.

Climate change was clearly driving the Norse, with their sheep- and cattle-farming traditions, to the edge of survival. With the onset of the Little Ice Age (from 1300 to 1850), conditions deteriorated across the Norse lands, particularly for people living on marginal farmland in Iceland, northern Norway, and Greenland.

Today, Greenland is warming up, with residents witnessing dramatic changes over the past five years. Winter sea ice, which the indigenous Inuit people in north Greenland traditionally relied on for sled dog transportation and seal hunting, has stopped forming reliably and robustly. Meanwhile, farmers in southerly communities like Qassiarsuk have enjoyed a markedly expanded growing area and season. Potatoes, previously confined to the far south, now grow as far north as the capital, Nuuk, 185 miles south of the Arctic Circle.