Figure 1. Arctic sea ice reached its minimum on September 16, 2012, and was at its lowest extent since satellite records began in 1979. Image credit: National Snow and Ice Data Center (NSIDC).
When was the last time the Arctic was this ice-free?
We can be confident that the Arctic did not see the kind of melting observed in 2012 going back over a century, as we have detailed ice edge records from ships (Walsh & Chapman, 2001). It is very unlikely the Northwest Passage was open between 1497 and 1900, since this spanned a cold period in the northern latitudes known as "The Little Ice Age." Ships periodically attempted the Passage and were foiled during this period. Research by Kinnard et al. (2011) shows that the Arctic ice melt in the past few decades is unprecedented for at least the past 1,450 years. We may have to go back to at least 4000 B.C. to find the last time so little summer ice was present in the Arctic. Funder and Kjaer (2007) found extensive systems of wave generated beach ridges along the North Greenland coast, which suggested the Arctic Ocean was ice-free in the summer for over 1,000 years between 6,000-8,500 years ago, when Earth's orbital variations brought more sunlight to the Arctic in summer than at present. Prior to that, the next likely time was during the last inter-glacial period, 120,000 years ago. Arctic temperatures then were 2-3 °C higher than present-day temperatures, and sea levels were 4-6 meters higher.
Figure 2. Year-averaged and 3-month averaged Northern Hemisphere sea ice extent from Chapman and Walsh (2001), as updated by the University of Illinois Cryosphere Today. I've updated their graph to include 2011 plus the first 9 months of 2012.
Figure 3. Late summer Arctic sea ice extent over the past 1,450 years reconstructed from proxy data by Kinnard et al.'s 2011 paper "Reconstructed changes in Arctic sea ice over the past 1,450 years." The solid pink line is a smoothed 40-year average, and the light pink areas shows a 95% confidence interval. Note that the modern observational data in this figure extend through 2008, though the extent is not as low as the current annual data due to the 40-year smoothing. More commentary on this graph is available at skepticalscience.com.
When will the Arctic be ice-free in summer?
So, when will Santa's Workshop need to be retrofitted with pontoons to avoid sinking to the bottom of the Arctic Ocean in summer? It's hard to say, since there is a large amount of natural variability in Arctic weather patterns. Day et al. (2012) found that 5-31% of the changes in Arctic sea ice could be due to natural causes. However, the sea ice at the summer minimum has been declining at a rate of 12% per decade, far in excess of the worst-case scenario predicted in the 2007 IPCC report. Forecasts of an ice-free Arctic range from 20-30 years from now to much sooner. Just this week, Dr. Peter Wadhams of Cambridge University predicted that the Arctic will be ice-free in summer within four years. A study by Stroeve et al. (2012), using the updated models being run for the 2014 IPCC report, found that "a seasonally ice-free Arctic Ocean within the next few decades is a distinct possibility." Of the 21 models considered, 2022 was the earliest date that complete Arctic sea ice occurred in September.
Video 1. A powerful storm wreaked havoc on the Arctic sea ice cover in August 2012. This visualization shows the strength and direction of the winds and their impact on the ice: the red vectors represent the fastest winds, while blue vectors stand for slower winds. According to NSIDC, the storm sped up the loss of the thin ice that appears to have been already on the verge of melting completely. Video credit: NASA.
But Antarctic sea ice is growing!
Figure 4. Surface air temperature over the ice-covered areas of the Southern Ocean surrounding Antarctica (top) and sea ice extent, observed by satellite (bottom). Image credit: (Zhang, 2007).
Commentary: Earth's attic is on fire
Funder, S. and K. H. Kjaer, 2007. A sea-ice free Arctic Ocean?, Geophys. Res. Abstr., 9 (2007) 07815.
Kinnard et al., 2011. Reconstructed changes in Arctic sea ice over the past 1,450 years, Nature.
Walsh, J. E and W. L.Chapman, 2001. Twentieth-century sea ice variations from observational data, Annals of Glaciology, 33(1) (January 2001) 444-448.
Half of the polar ice cap is missing: Arctic sea ice hits a new record low. September 6, 2012, blog post
Wunderground's Sea Ice page.