Rapidly declining sea ice in the Arctic is causing polar bears to spend an additional 30 days on land, fast for longer periods of time, and produce smaller litters, according to a new study published in the journal Ecological Applications that analyzed two decades of tracking data on polar bears.

The research, led by an international team of scientists from the United States, Canada, Greenland, and Norway, examined remote sensing data for 43 adult females in Baffin Bay from 1991 to 1997 and 38 females from 2009 to 2015. They found that the bears follow the seasonal progression of sea ice, ending up on Baffin Island in the fall when the ice is at its lowest extent, and waiting there until the ice forms again and it is safe to travel back to Greenland.

But sea ice concentrations in the Arctic have declined by 13 percent per decade since 1979, and ice is forming later in the fall and breaking up earlier in the spring. The study found that as a result, polar bears are spending, on average, an additional 30 days on land compared to the 1990s.

“That’s important because when the bears are on land, they do not hunt seals,” Kristin Laidre, an Arctic ecologist at the University of Washington and lead author of the new study, said in a statement. “They have the ability to fast, but if they don’t eat for longer periods, they get thinner. This can affect their overall health and reproductive success.”

Bears of “all sex, age, and reproductive classes” have become thinner since the 1990s, the study states. Longer ice-free periods have also led to smaller litter sizes, a trend that scientists say could continue for at least the next three polar bear generations. “Results indicate that two-cub litters, previously the norm, could largely disappear from Baffin Bay as sea ice loss continues,” the scientists write in Ecological Applications.

“Polar bears are a harbinger for the future,” said Laidre. “The changes we document here are going to affect everyone around the globe.”