Researchers are forecasting a one-in-three chance that the extent of sea ice covering the Arctic will reach an all-time record low this year.

Sea ice forms as ocean water freezes. Sea-ice extent is the area of an ocean covered by at least 15 percent ice. Arctic sea ice has been declining since the late 1970s, and scientists believe this is caused by rising winter temperatures in the region fueled by global warming.

Arctic sea ice reaches its maximum extent in March and its minimum in September. The maximum extent of Arctic sea ice for March of this year was the second-lowest maximum on record, according to the National Snow and Ice Data Center.

The researchers, from the University of Colorado at Boulder, also say there is a 57 percent chance that the sea ice minimum this year will be lower than last year’s, which was only 2.27 million square miles (currently the second lowest on record).

There is a 70 percent chance that the sea-ice minimum this year will rank within the lowest five years on record, say the scientists.

While declines in sea ice are opening up shipping lanes for many northern countries, they are also endangering species like polar bears that live on the ice.