With about three weeks left in the Arctic summer, this year could wind up breaking the previous record, scientists said.
Arctic ice always melts in summer and refreezes in winter. But over the years, more of the ice is lost to the sea and with less of it recovered in winter. While ice reflects the sun's heat, the open ocean absorbs more heat and the melting accelerates warming in other parts of the world.
Sea ice also serves as primary habitat for threatened polar bears.
"We could very well be in that quick slide downward in terms of passing a tipping point," said center senior scientist Mark Serreze. "It's tipping now. We're seeing it happen now."
Within a few years - "five to less than 10 years" - the Arctic could be free of sea ice in the summer, said NASA ice scientist Jay Zwally.
"It also means that climate warming is also coming larger and faster than the models are predicting and nobody's really taken into account that change yet," he said.
Other scientists, including James Hansen, director of NASA's Goddard Institute of Space Studies in New York, agreed. Hansen in a Wednesday e-mail said the sea ice "is the best current example of a tipping point."
Last year was an unusual year when wind currents and other weather conditions coincided with global warming to worsen sea ice melt, Serreze said. Scientists wondered if last year's melt was an unusual event or the start of a new and disturbing trend.
This year's results suggest the latter because the ice had recovered a bit more than usual thanks to a somewhat cooler winter, Serreze said. Then this month, when the melting rate usually slows, it sped up, he said.
And the melt in sea ice has kicked in another effect, long predicted, called "Arctic amplification," Serreze said.
That's when the warming up north is increased in a feedback mechanism and the effects spill southward starting in autumn, he said. Over the last few years, the bigger melt has meant more warm water that releases more heat into the air during fall cooling, making the atmosphere warmer than normal.
Science Writer Seth Borenstein reported from Washington and Dan Joling reported from Anchorage, Alaska.