Climate change has also made for warmer and longer summers in Svalbard, helping to increase the reindeer populations. That leads to greater competition among the animals for limited food resources, putting them at greater risk of starvation.

Estimates for Norway’s total reindeer population are around 220,000. Reindeer herding is a way of life for the Sami, an indigenous people of fewer than 140,000 who inhabit mostly the northern reaches of Sweden, Norway, Finland and Russia. The total Svalbard reindeer population is not known, according to the Norwegian Arctic Institute, but monitoring of certain areas has shown an exponential rise in the number of animals. In one valley, their number grew from 400 to 1,200 between 1979 and 2013.

Svalbard reindeer, unlike their cousins elsewhere, do not live in herds and tend to be stationary, Ms. Pedersen said. Using GPS signals, researchers found that the reindeer have been venturing far from their normal grazing areas. They have descended on the shores for seaweed and have hiked up into the rocky Arctic desert, much like mountain goats, in pursuit of the vegetation there.

In the early 20th century, the Svalbard reindeer population was severely depleted by hunting. The animals have been a protected species since 1925 in Norway, helping the population recover. Over 40 years of monitoring has allowed researchers to observe changes precisely, including those resulting from climate change.

The Arctic has been warmer over the past five years than at any time since records began being kept in 1900, and the region has been warming at twice the rate as the rest of the planet, a report last year by the National Oceanic and Atmospheric Administration in the United States said.