At opposite ends of the world, trends in sea ice coverage are in stark opposition. Arctic sea ice coverage remains depressed, while Antarctic levels have achieved a record high for the second straight year.
Due to warming of the Arctic, the depleted state of Arctic sea ice is expected. The ever-growing Antarctic sea ice levels are more of a surprise.
As sea ice waxes and wanes during its annual cycle, coverage reaches a maximum in the Antarctic in mid-September – the end of winter in the Southern Hemisphere, and a minimum in the Arctic – the end of summer in the Northern Hemisphere.
Any day now, Arctic sea ice is expected to bottom out at its sixth lowest extent on record reports the National Snow and Ice Data Center (NSIDC). Its 2014 extent is roughly 440,000 square miles below the 1981-2000 average. This year’s levels are a hair above 2013’s, by approximately 11,600 miles. The Arctic has lost about 11,100 square miles of ice per day so far this September, nearly the twice the average 1981-2010 rate.
Meanwhile, Tuesday’s Antarctic sea ice coverage expanded to 7.63 million square miles, exceeding record high levels set in September 2013.
In both the Arctic and Antarctic, sea ice records date back to the late 1970s.
If Arctic sea ice is declining, why was 2014’s minimum extent above the last two years?
There are year to year fluctuations in both Arctic and Antarctic sea ice extent due to randomly varying weather patterns. The fact that the 2014 sea ice minimum in the Arctic contained 640,000 more square miles of ice cover than the record minimum in 2012 have led some to conclude the Arctic sea ice is recovering. But the trend in Arctic sea ice is in an unmistakable long-term decline, as Climate Central reports:
The average annual sea ice extent has been declining by 4.52 percent per decade, according to NSIDC statistics — that amounts to a loss of about 20,500 square miles of ice per year. The September minimum has seen an even steeper decline of 13.7 percent per decade, or about 34,000 square miles per year.
Why is Antarctic sea ice setting record highs in a warming world?
The behavior of Antarctic sea ice is a bit of an enigma. As I wrote last year, when Antarctic sea ice set a record high, scientists have developed numerous theories for why the ice is growing – but its dynamics remain poorly understood. The Intergovernmental Panel on Climate Change weighed in on this complicated matter in its assessment published earlier this year:
For Antarctic sea ice extent, the shortness of the observed record and differences in simulated and observed variability preclude an assessment of whether or not the observed increase since 1979 is inconsistent with internal variability. Untangling the processes involved with trends and variability in Antarctica and surrounding waters remains complex and several studies are contradictory. In conclusion there is low confidence in the scientific understanding of the observed increase in Antarctic sea ice extent since 1979…
This lack of understanding notwithstanding, increasing Antarctic sea ice does not in any way disprove global warming. Despite the increase in winter sea ice in the Antarctic, the water in the Southern Ocean is warming while satellite measurements have shown that the Antarctic continent is – on balance – losing ice.
Whereas there is an apparent straight forward relationship between rising temperatures and loss of ice during the summer in the Arctic; warming temperatures in the Antarctic in winter (when background temperatures are really cold) simply do not have the same effect.