The amount of sea ice in the Arctic during the northern summer was the fourth lowest on record, says Nasa.

An analysis of satellite data showed that the melting sea ice this year had reached its minimum extent of 4.41 million km² on September 11. This is 1.81 million km² lower than the 1981 to 2010 average.

And the low extent of the sea ice this year was not exacerbated by any major weather, as was the case in 2012, the lowest on record, when a powerful cyclone fractured the ice cover and sped up its melting.

The Arctic sea ice, made of frozen seawater that floats on the ocean, shrinks in the summer and grows in the winter.

However, because of global warming, the extent of the summer sea ice has been in decline since the late 1970s. This has accelerated since 1996.

The 10 lowest summer sea ice extents in the satellite record all occurred in the last 11 years.

Nasa said although it appeared the 2015 minimum extent of sea ice had been reached, there was a chance that changing winds could reduce the sea ice even further in the next few days.

Walt Meier, Nasa sea ice specialist, said the Arctic sea ice was becoming less and less resilient.

“It does not take as much to melt it as it used to. The sea ice cap, which used to be a solid sheet of ice, now is fragmented into smaller floes that are more exposed to warm ocean waters. In the past Arctic sea ice was like a fortress. The ocean could attack it only from the sides. Now it’s as if the invaders have tunnelled in from underneath and the ice pack melts from within,” Meier said.

Nasa said in some years the melting of the sea ice in summer had been exacerbated in part by the weather.

“This year is the fourth lowest, and yet we haven’t seen any major weather event, or persistent weather pattern, in the Arctic this summer that helped push the extent lower, as often happens,” Meier said.

Nasa will start its Operation IceBridge next week, an airborne survey of polar ice. These flights will help validate satellite readings.