The Arctic has lost more sea ice this year than at any time since satellite records began in 1979, Nasa says.
Scientists involved in the calculations say it is part of a fundamental change.
What is more, sea ice normally reaches its low point in September so it is thought likely that this year’s melt will continue to grow.
Nasa says the extent of sea ice was 1.58m sq miles (4.1m sq km) compared with a previous low of 1.61m sq miles (4.17m sq km) on 18 September 2007.
The sea ice cap grows during the cold Arctic winters and shrinks when temperatures climb again, but over the last three decades, satellites have observed a 13% decline per decade in the summertime minimum.
The thickness of the sea ice is also declining, so overall the ice volume has fallen far – although estimates vary about the actual figure.
Joey Comiso, senior research scientist at Nasa’s Goddard Space Flight Center, said this year’s ice retreat was caused by previous warm years reducing the amount of perennial ice – which is more resistant to melting. It’s created a self-reinforcing trend.
“Unlike 2007, temperatures were not unusually warm in the Arctic this summer. [But] we are losing the thick component of the ice cover,” he said. “And if you lose [that], the ice in the summer becomes very vulnerable.”