WASHINGTON // Critical ice in the Arctic Ocean melted to record low levels this sweltering summer and that can make weather more extreme far away from the poles, scientists say.
The National Snow and Ice Data Center reported on Monday that the extent of Arctic sea ice shrank to 4.09 million square kilometres and is likely to melt more in the coming weeks. That breaks the old record of 4.17 million square kilometres set in 2007.
The North Pole region is an ocean that mostly is crusted at the top with ice. In the winter, the frozen saltwater surface usually extends about15.54 million square kilometres, shrinking in summer and growing back in the fall. That's different from Antarctica, which is land covered by ice and snow and then surrounded by sea ice.
Normally sea ice in the Arctic reaches its minimum in mid-September and then starts refreezing. But levels on Sunday shrank 69,930 square kilometres beyond the old record.
Figures are based on satellite records dating back to 1979. The ice centre bases its figures on averages calculated over five days.
Ted Scambos, a scientist at the data centre, said the melt can be blamed mostly on global warming from man-made emissions of greenhouse gases. There are natural factors involved too, including a storm that chewed up a significant amount ice this month. But, he said, dramatic summer sea ice losses in all but one year since 2007, continuous thin ice and warm air temperatures show a pattern that can only be explained by climate change.
"It really does imply that the Arctic is moving to a new state," said Tom Wagner, a scientist at Nasa. "The Arctic is changing."
Mr Wagner and Mr Scambos said in 2007 some people thought it was just an odd year that caused the dramatic melt, but years like this one show something bigger is happening.
This milestone is a "substantial step" to the day when there will be no significant sea ice in the Arctic in the summer, said Nasa's chief scientist Waleed Abdalati.
"Why do we care? This ice has been an important factor in determining the climate and weather conditions under which modern civilisation has evolved," Mr Abdalati said.
Scientists sometimes call the Arctic the world's refrigerator and this is like leaving the fridge door open, Mr Scambos said.
"This is kind of a knob on global weather," Wagner said. "We don't know the effect yet" of fiddling with it.