"Melting ice drives polar bear mothers to land"

WASHINGTON (Reuters) - Melting sea ice is driving mother polar bears onto dry land to give birth in northern Alaska, U.S. Geological Survey scientists reported on Thursday.

They found that just 37 percent of polar bear dens were built on sea ice between 1998 and 2004, compared to 62 percent between 1985 and 1994.

"Right now, pregnant females foraging offshore in summer must wait up to a month longer than they did even 10 years ago for new sea ice to form so they can travel to denning areas on land," USGS researcher Steve Amstrup said in a statement.

"Alternatively, they must swim ever greater expanses of open water to reach suitable land denning habitat or they must den on ice that may not be stable enough to survive the winter."

Eventually, the researchers predicted, the ice will freeze so late that bears will be stranded at sea too far away to reach land safely. Pregnant polar bears must create dens to protect new cubs from the Arctic winter.

Arctic seas are warming and the ice is melting as global temperatures rise.

Amstrup's team used satellite telemetry to see where the big white bears were building their dens.

They said the edge of the sea ice remains as far as 125 miles offshore in late September and early October. Only a decade ago, the water was frozen almost to the shore by that time, they reported in the journal Polar Biology...