Wednesday, November 02, 2016

Sea Ice Extent Is Near Record Lows--South as Well as North

By: Bob Henson , 4:03 PM GMT on October 26, 2016

It’s been a banner year for global sea ice, and not in a good way. After a record-smashing mild winter, the Arctic’s summer sea-ice melt culminated in a tie with 2007 for the second-lowest extent since satellite measurements began in 1979. The drama intensified this month, with Arctic sea ice extent now at a clear record low for late October as calculated by the National Snow and Ice Data Center (see Figure 1 below). This behavior isn’t really such a shock, given that Arctic sea ice has been declining for decades in the midst of sharp high-latitude warming. What’s more startling is the huge extent loss this year in the Antarctic, where sea ice extent had actually been increasing in recent years. This year’s Antarctic extent peaked very early, on August 31, and it’s now at its second-lowest value on record for late October, beaten only by 1986 (see Figure 2 below).

Together, these simultaneous drops have sent global sea ice extent--Arctic plus Antarctic--to its lowest level by far for this time of year since regular satellite monitoring began in 1979. The global extent as of October 25 was more than 1 million square kilometers below this date in 2011, the previous record-holder. In fact, it appears that the last few days are the first time we’ve seen a global departure from average in sea ice extent of more than 3 million sq km—which is more than four times the area occupied by Texas.

We shouldn’t pin too much on this record, because global sea ice extent is a much-abused and somewhat artificial metric. The Arctic and Antarctic have vastly different climate regimes, and what happens at one pole is far more important to its own regional climate than what’s occurring at the other pole. Still, the dramatic dip in global ice extent is worth noting if only because climate-change skeptics and deniers have pointed to global sea ice for years, and especially the Antarctic’s unexpected evolution, in an attempt to discount other evidence of a planet being warmed by increasing amounts of human-produced greenhouse gases. As Jeff Masters put it in this blog in 2010: “Diminishing the importance of Arctic sea ice loss by calling attention to Antarctic sea ice gain is like telling someone to ignore the fire smoldering in their attic, and instead go appreciate the coolness of the basement, because there is no fire there.”


It appears that a set of interlocking, difficult-to-model factors over the last few years has fostered the increasing trend in Antarctic sea ice, especially in the Ross Sea area. These include:

--A strengthening of the midlatitude westerlies that encircle Antarctica. These have fostered upwelling of cold subsurface waters across the Southern Ocean, which allows sea ice to expand more readily.

--Increased meltwater flowing from Antarctica into the Southern Ocean. This reduces the salinity of waters near the coast, thus allowing the surface to freeze at a warmer temperature.

--Strengthening of a prevailing low in the Amundsen Sea off West Antarctica. The flow around the Amundsen Sea Low pulls cold air off the continent and into the Ross Sea. The resulting increase in sea ice over that region has made up for ice reductions in the Amundsen-Bellingshausen region, where the circulation around the prevailing low tends to bring relatively mild air onshore.

The same study also noted unmistakable signs of climate change in the Antarctic, including warming of the subsurface ocean, thinning of ice shelves, and the acceleration of outlet glaciers that ring the ice sheet. According to an essay in The Conversation by three of the study’s authors, the cooling of surface waters around Antarctica has been masking “a much more ominous change deeper down in the ocean, particularly near the West Antarctic Ice Sheet and the Totten glacier in East Antarctica. In these regions, worrying rates of subsurface ocean warming have been detected up against the base of ice sheets. There are real fears that subsurface melting could destabilise ice sheets, accelerating future global sea level rise.”


It should also be emphasized that the globe’s overall sea-ice budget has been in the red for quite some time. In 2015, NASA’s Claire Parkinson showed that the losses in Arctic ice were already outweighing the lesser increases in Antarctic ice. “I think that the expectation is that, if anything, in the long term the Antarctic sea ice growth is more likely to slow down or even reverse,” Parkinson said.


Labe is even more impressed by the very low ice thickness and volume, as estimated by computer models that reproduce the ice in three dimensions. “I think this will become a bigger story,” he said. “The Arctic ice is looking incredibly thin, which is to be expected from such warmth in both the air and ocean.”


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