It represents almost 50% more ice than the spectacular
satellite-era record-minimum achieved this time last year - when floes
were reduced to just 3.41 million sq km (1.32 million sq mi).
The NSIDC describes this summer's cover as a "temporary reprieve".
Steadily warming conditions in the far north have seen the
annual mean ice extent since 1979 - the beginning of continuous
space-based observations - fall by about 4% per decade.
This year represents the sixth smallest cover recorded by the satellites. It is close in size to that seen in September 2009.
Dr Julienne Stroeve, a NSIDC research scientist, said the
larger extent this year compared with 2012 was evidence of the natural
variability in the Arctic system.
"This year, we've had pretty cold conditions in the Arctic compared to the last few summers," she told BBC News.
"So, for example, instead of having a high pressure centred
over the central Arctic and the Beaufort Sea that tended to bring in
warm air, we had a bunch of low-pressure systems. And that just kept
things quite cool. But I think it's interesting that even though it was
cold, we got an extent of 5.10 million sq km, which is still quite a
bit below the long-term mean."
Computer models forecast that on current climate trends, the Arctic will eventually become ice-free in summer months.
Most of the models indicate this will happen before 2060,
with the most aggressive simulations pointing to ice-free conditions
occurring perhaps before the end of this decade.
Dr Stroeve said it was likely that the variation from year to year would become more exaggerated as the ice retreated.
"Even in the overall long-term decline, there are simulations
that show you can have temporary recovery for a few years, even maybe
10 years. That's not outside the numbers," she said.
The NSIDC defines sea-ice extent as the total area covered by
at least 15% of ice (because floes in summer become broken up and
Extent is different from thickness/volume, which scientists
say is the better indicator of the status of the ice cover. This
measure, too, is in long-term decline.
Provisional data released by the European Space Agency (Esa)
last week indicated that floe volume likely hit a satellite-era
record-low this past winter.
Esa's Cryosat spacecraft found there were just under 15,000
cu km of ice in March/April - the time of year when marine ice is at its
The mission team says it will get a sense for how this
situation progressed into the summer melt season when the Autumn
re-freeze gathers pace in a month or so's time.
In the Antarctic, winter sea ice reached its maximum extent on 18 September, NSIDC data shows.
The coverage was a tie with last year's record high of 19.45 million sq km (7.51 million sq mi).
Antarctic winter extent has been growing by about 1-2% per decade.
Cyrosphere specialists say the northern and southern poles
have different marine-ice systems and so are not directly comparable,
but that the shrinkage in the Arctic floes represents the more dramatic
and significant behaviour.