The Arctic is on course for an ice-free summer within the next few decades, as scientists on Friday declared that sea ice in the region had fallen to one of the lowest annual minimums on record.

On 13 September, the expanse of frozen water in the Arctic fell to 5.10 million square kilometres (1.97 million square miles), the sixth-lowest such measurement on record, the National Snow and Ice Data Center (NSIDC) said in a statement. The annual minimum was not as extreme as the collapse of sea ice last year, which smashed through all records. But it was still well below the average of the last three decades.

"The overall trend is still decidedly downwards," the NSIDC director, Mark Serreze, said in a statement. "The pattern we've seen so far is an overall downward trend in summer ice extent, punctuated by ups and downs due to natural variability in weather patterns and ocean conditions."

He went on: "We could be looking at summers with essentially no sea ice on the Arctic Ocean only a few decades from now."

In the run-up to the release next week of the United Nations' blockbuster global warming report, climate doubters have seized on the apparent "recovery" of sea ice, compared to last year's death spiral, as evidence that there is no need for concern about the melting of the Arctic sea ice. But satellite images going back to 1980 and records compiled by scientists using ice cores and tree ring data going back as far as 1870 show a continued and dramatic long-term decline in summer sea ice.

The so-called recovery of the sea ice this year does not even begin to reverse that decline, scientists said on Friday. "Last year was so outrageously below the trend line that it was really no surprise that it would not be quite so low this year," said Jennifer Francis, a research scientist at Rutgers University. "Clearly we are on this very definite downward trend."

Julienne Stroeve, another NSIDC researcher, noted that this year's low minimum was reached amid cooler temperatures than the last several summers, which helped to slow melting. "We had a pretty cold summer in general for the time period we're looking at and yet the sea-ice cover didn't recover to the extent that we had in the 1970s and 1980s," she said.

The annual sea-ice minimum, based on a five-day average, is seen as an important indicator of climate change. Overall, the Arctic has lost 40% of its sea-ice cover since 1980, and 75% of its volume. Most scientists believe the ocean at the north pole could be entirely ice-free in the summer by the middle of the century – if not sooner.

"We really are heading towards an ice-free Arctic in the summer," said Andreas Münchow, a scientist at the University of Delaware who studies the Arctic. "It just takes a freak event eventually, in the next five or 10 or even 20 years. The next year there will be a huge Arctic cover, but it is all going to be thin on top, and the long-term trend is that the ice is disappearing in the summer in the Arctic."

Jennifer Francis agreed. "People keep asking: 'When are we going to see the last of the summer sea ice? What year? But practically speaking it is not going to take that much more for it to be a seasonal ice, or ice-free in the summer."

The loss of sea ice will be a key part of the findings released next week by the UN's climate science panel, the Intergovernmental Panel on Climate Change. The melting of the Arctic has wide-reaching effects on the climate system beyond the northern latitudes.

The melting of the sea ice and the warming of waters around Greenland could accelerate the melting of that vast ice sheet, adding to sea-level rise. The prospect of an ice-free Arctic in the summer has spurred oil and gas exploration and shipping in the north, which threaten fragile ecosystems, and scientists are also beginning to tie the disappearing ice to extreme weather events in Europe.

The ice cover looks likely to continue its rapid decline because of thinning of the ice, which makes it more vulnerable to melting, the scientists said. Observations from the European Space Agency's CryoSat mission, released last week, showed the volume of sea ice in the Arctic falling to a new low last winter.

Münchow said: "There is very little thick multi-year ice left covering these great areas. It is really thin, so if you get a little weather the next year, it's all gone."