We have precise data for Arctic sea ice extent from satellite observations over about the last 30+ years, and a pretty good estimate for the last century or more from ship and aircraft observations. The data show clearly that Arctic sea ice has declined dramatically over the last few decades, a decline the like of which has certainly not been seen for over 100 years prior to that. In addition, a recent survey of available proxy information — not a reconstruction, but a thorough review of the evidence — tells us that the modern decline in Arctic sea ice is “unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.”
Now there’s even more evidence of the shocking state of the Arctic sea ice. New research reports an actual reconstruction of the extent of Arctic sea ice over the last 1,450 years (Kinnard et al. 2011, Nature, 479, 509-512, doi:10.1038/nature10581).
Kinnard and his coworkers collected 69 proxy data records from the Arctic region, mostly from ice cores (using oxygen isotope ratios, percentage of infiltration ice, and sea-salt ions) but also including tree rings, lake sediments, and historical data of sea ice observations. Many of the proxies have information about physical properties other than sea ice extent — especially temperature — but the analysis method chosen (partial least squares) enabled the authors to identify modes of variability in the proxy data which are distinct from the temperature signal, and which correlate with observed sea ice extent. This method also helps overcome the “collinearity problem,” in which multiple proxies are so similar to each other that they provide overlapping information.
The regression correlated proxy data with late summer (August) sea ice extent, both from hemispheric data and from additional data for the Russian Arctic. Kinnard et al. found that their reconstruction had skill from the year 561 onward, i.e., over the last 1,450 years. The “hockey-stick” shape of the history of Arctic sea ice is remarkable (smoothed with a 40-year lowpass filter):
Uncertainty levels are greater the farther back in time one goes, because there are fewer proxy records to use for reconstruction. Nonetheless, the remarkable decline in the late 20th century is far beyond anything seen before it, both in magnitude and duration.
Prior to the recent decline, there were periods of sustained greater sea ice (about 1250 to 1450, and 1800 to 1920) and periods of sustained lesser sea ice (before about 1200). The minimum of sea ice before the industrial revolution was even earlier, around the year 640. There were also two later episodes of sea ice decline, in the late 16th and early 17th centuries, but none of these other declines even comes close to the “falling off a cliff” of sea ice we’re seeing today.
Surprisingly, sea ice shows signs of decline during the “little ice age” when it might naively be expected to increase. The authors suggest that transport of heat into the high Arctic may have been responsible, so this episode would represent one version of “heat piracy” in which the cooling in Europe and elsewhere was accompanied by warming of the Arctic. Likewise, the pre-industrial minimum in the year 640 coincides with what they refer to as the “dark ages cold period,” and may similarly represent the diversion of heat (perhaps by ocean currents) to the Arctic.
But the modern crash of sea ice extent corresponds to a warm period, both hemispherically and globally, so it’s not a case of heat migrating from one region to another, but of — how shall we say? — global warming. The authors conclude:
In the present state of knowledge, anthropogenically forced (‘greenhouse gases’) warming stands out as a very plausible cause of the record atmospheric and oceanic warmth of the recent decades, which may soon lead to an ice-free Arctic Ocean in summer.