http://news.yahoo.com/s/ap/20071211/...S9OhzJpFas0NUE

Melting of sea ice and Greenland's ice sheets also alarms scientists
because they become part of a troubling spiral.

White sea ice reflects about 80 percent of the sun's heat off Earth,
NASA's Zwally said. When there is no sea ice, about 90 percent of the
heat goes into the ocean which then warms everything else up. Warmer
oceans then lead to more melting.

"That feedback is the key to why the models predict that the Arctic
warming is going to be faster," Zwally said. "It's getting even worse
than the models predicted."

NASA scientist James Hansen, the lone-wolf researcher often called the
godfather of global warming, on Thursday was to tell scientists and
others at the American Geophysical Union scientific in San Francisco
that in some ways Earth has hit one of his so-called tipping points,
based on Greenland melt data.

"We have passed that and some other tipping points in the way that I
will define them," Hansen said in an e-mail. "We have not passed a point
of no return. We can still roll things back in time — but it is going to
require a quick turn in direction."