Extent of Arctic Sea Ice at Record Low
Actually "record low" depends on the time period under consideration. In this case, the National Snow and Ice Data Center (NSIDC) reports:
Arctic sea ice appears to have broken the 2007 record daily extent and is now the lowest in the satellite era. With two to three more weeks left in the melt season, sea ice continues to track below 2007 daily extents…
Arctic sea ice extent fell to 4.10 million square kilometers (1.58 million square miles) on August 26, 2012. This was 70,000 square kilometers (27,000 square miles) below the September 18, 2007 daily extent of 4.17 million square kilometers (1.61 million square miles).
Including this year, the six lowest ice extents in the satellite record have occurred in the last six years (2007 to 2012).
In November, 2011 researchers reported in Nature that summer Arctic sea ice cover in 2007 was the lowest in the past 1,500 years. From the abstract:
…we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that—although extensive uncertainties remain, especially before the sixteenth century—both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic6 seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.
However, another set of researchers analyzed driftwood on Arctic beaches to create a 10,000 year record of Arctic sea ice variability, [sub required] published in the August 5, 2011 issue of Science. Combining their data with climate models, the researchers report:
In this exercise, our records would correspond in the model to an Arctic Ocean sea-ice cover in summer at 8 ky B.P. that was less than half of the record low 2007 level [emphasis added].The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA [little ice age], attained its present (~2000) extent at ~ 4 ky B.P.
During the Holocene Temperature Maximum (~8,000 years ago), Arctic temperatures are estimated to have been about 2 degrees celsius higher than at present. Those higher HTM temperatures are thought to have been the result of orbital shifts that increase the amount of sunlight hitting the Northern Hemisphere. The current temperature increases are thought to be the result of increased concentrations of greenhouse gases in the atmosphere.