Pregnant polar bears in Alaska, which spend most of their lives on sea ice, are increasingly giving birth on land, according to researchers who say global warming is probably to blame. The study by three scientists for the U.S. Geological Survey suggests the State's bear population could be harmed if the climate continues to grow warmer. Though bears are powerful swimmers, at some point they might have to cross vast stretches of open water to reach habitat on shore suitable for building dens in which to give birth. From 1985 to 1994, 62 per cent of the female polar bears studied dug dens in snow on sea ice. From 1998 to 2004, just 37 per cent made dens on ice. The rest dug snow dens on land, according to the study. Researchers "hypothesised that the sea ice changes may have reduced the availability or degraded the quality of offshore denning habits," said wildlife biologist Anthony Fischbach, lead author of the study.