The National Snow and Ice Data Center (NSIDC) confirms that the Northwest Passage and the Northern Sea Route are largely free of ice, allowing the potential for a circumnavigation of the Arctic Ocean.

The minimum ice extent for the year will probably occur in the next two weeks.

At the end of August, ice extent had fallen to the fourth lowest in the satellite record, behind the seasonal minima recorded for 2007, 2008, and 2009. On September 3, ice extent fell below the seasonal minimum for 2009 to claim third lowest on record.

At Climate Progress, Joseph Romm reports that a major analysis, titled “History of sea ice in the Arctic”, finds that there is less ice covering the Arctic today than at any time in recent geologic history.

This is from the abstract:

[E]pisodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.

In the past, the Arctic thawed because of changes in Earth’s orbit. This time, it’s due to human emissions.

Romm quotes lead author Leonid Polyak’s response to the question, when was the last time the Arctic was ice free?

The paleo data we have so far is very scant, so we can’t know for sure when the Arctic was ice free in the summer last time. To be conservative, the closest candidate is the early Holocene (roughly ~10 kyr ago), when the insolation in the Arctic was high due to the beneficial orbital configuration; however, the more data I see, the stronger is my impression that there was not that little ice at that time. The next best (actually, better) candidate is the Last Interglacial, about 125kyr ago, again due to orbitally-driven high insolation: the ice was likely very low, but we can’t say whether it was completely ice free in summer or not. There are also a few other major interglacials, which may have had a similar picture, in particular Marine Isotopic Stage 11, about 450 kyr ago. In any case we are talking about very rare events controlled by a forcing very different from today. If none of those intervals was really ice free, then a million year assessment would be correct.

External forcings – primarily orbital in the past and primarily greenhouse gases now – are further boosted by Arctic amplification, primarily positive feedbacks from the retreat of ice and snow. The Arctic warming threatens to set off a cascade of effects, including speeding up the melting of the Greenland ice sheet, accelerating the rise in sea levels; and even more serious, melting of the permafrost. Release of even a fraction of the methane stored in the permafrost (the East Siberian Arctic Shelf, for example) could trigger abrupt and catastrophic climate warming.

The evidence is undeniable: human-caused greenhouse gas emissions are overwhelming the system.