Sea Ice Extent Is Near Record Lows--South as Well as North
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By Bob Henson
Published: 16:03 GMT le 26 octobre 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.”
Figure 1. The extent of Arctic sea ice has moved into record-low territory this month compared to all other Octobers since satellite monitoring of the Arctic began in 1979. This year surpassed its nearest rival, 2007, in mid-October. Image credit: National Snow and Ice Data Center.
Figure 2. The extent of Antarctic sea ice decreased rapidly in October 2016 compared to all other Octobers since satellite monitoring of the Antarctic began in 1979. The only year with a lower Antarctic extent as of October 24 was 1986 (gold line). Image credit: National Snow and Ice Data Center.
The big north-vs-south difference in sea ice The stark difference between yearly patterns of sea ice in the Arctic versus the Antarctic is mainly a function of where the land sits. Northern sea ice melts and freezes within the Arctic Ocean, which surrounds and includes the North Pole. Southern ice melts and freezes in a ring around the continent of Antarctica, which keeps it well away from the South Pole and at lower latitudes than Arctic sea ice. As a result, southern sea ice covers a larger area than northern sea ice each winter, yet more than 80% of it disappears each summer. From winter max to summer min, a typical yearly drop in sea-ice extent in recent years would be from around 15 to 5 million sq km in the Arctic and from around 19 to 3 million sq km in the Antarctic.
Unfortunately, the longer-term, year-round decline in Arctic sea ice extent over the last couple of decades makes all too much sense. Temperatures at high northern latitudes have been soaring, this year in particular. (One example: the statewide temperature average in Alaska for the first nine months of 2016 is nearly 3°F warmer than for any Jan-Sep period since records began in 1925.) The Arctic is still more than cold enough each winter to re-cover the Arctic Ocean with sea ice, but the quality and thickness of that return ice has been declining, and the amount that survives as multiyear ice has plummeted.
Scientists long expected the Antarctic’s sea ice to decline as well. Instead, it’s actually expanded to record-high extents at times over the last few years. Even top computer models have been flummoxed by this trend. Among the simulations of Antarctic climate carried out in support of the most recent IPCC report, a majority predicted that ice should have declined between 1979 and 2013. In a review paper published in Nature Climate Change in September, a group of Antarctic experts surveyed what we know about high-latitude southern climate. 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.”
Figure 3. Maximum extent of Antarctic sea ice in 2013 (left) and 1979 (right) as observed by satellite. October is typically the global maximum for sea ice, largely because of the vast extent of Antarctic ice at that time. Image credit: Jesse Allen and Joshua Stevens/NASA Earth Observatory.
What the tropics tell us about the future of Antarctic sea ice I asked David Schneider (National Center for Atmospheric Research), a coauthor on the paper above, for his thoughts on recent changes in the Antarctic and what they might portend. Schneider has carried out extensive research on the intriguing links between tropical and Antarctic climate. “The Amundsen Sea Low [ASL] is strongly influenced by tropical variability, and in particular it is deeper during La Niña years,” he told me. As it turns out, La Niña events were more frequent than El Niño events from 1999 to 2014, in sync with a negative phase of the Pacific Decadal Oscillation (PDO). “Around the end of 2014, the PDO transitioned to its positive phase, and then the 2015-16 El Nino event occurred,” Schneider noted. “I would hypothesize that this transition of tropical climate is important for Antarctic sea ice and probably marks the end of the record high Antarctic ice extents that were observed through 2014.” Schneider added a word of caution: “The ASL exhibits the largest large-scale variability of any atmospheric circulation system on Earth, and it can vary independently of La Nina and El Nino. Thus, anomalously deep ASL years and relatively large Antarctic ice extents, particularly in the Ross Sea, are still possible.”
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.
Figure 5. Temperatures across parts of the Arctic ran more than 30°F above average during the first two weeks of October 2016, based on an NCEP/NCAR reanalysis technique that involves observations across the region. Image credit: University of Maine and NOAA/ESRL/PSD.
The Arctic this autumn: Warm air, warm water, thin ice The Arctic sea ice entered record-low territory this month after a faster-than-usual refreeze in late September and early October. “The thinned and broken ice across the central Arctic basin was able to refreeze rapidly as temperatures cooled after the sea ice minimum,” noted Zachary Labe (University of California, Irvine), a doctoral student recently profiled by Climate Central. Labe thinks the slower-than-usual growth of sea ice over the last few days is in large part due to very warm sea surface temperatures (SSTs) for the time of year. The record-warm air temperatures currently slathering the Arctic are not only associated with an atmospheric blocking pattern, but also related to the warm oceans themselves, Labe noted. “The air-ocean heat exchange from the open water is helping to modify the cold air near the surface, in addition to the release of latent heat as sea ice refreezes. The warm SSTs are really acting to prevent the sea ice to expand, especially into the Beaufort, Chukchi, and East Siberian Seas.”
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.”
Figure 6. The Barrow Sea Ice Webcam, operated by the University of Alaska Geophysical Institute, showed ice-free conditions on the Arctic Ocean coast on October 25, 2016. Barrow’s average temperature for the period October 1 - 25, 2016, was 30.6°F, which is 11.5°F above average for the period. In records going back to 1922, Barrow’s warmest October was in 2012, which averaged 27.5°F. Forecasts through the end of the month indicate that Barrow is very likely to break that October record. Image credit: Barrow Sea Ice Webcam.
Antarctic ice claims a polar researcher Dr. Gordon Hamilton, a glaciologist based at the University of Maine’s Climate Change Institute, died on Saturday, October 22, in an accident about 25 miles from Antarctica’s McMurdo Station. Hamilton was killed when his snow machine plunged some 100 feet into a crevasse located in an area known as the Shear Zone. He and colleagues affiliated with the U.S. Antarctic Program, managed by the National Science Foundation, had been working to identify and remediate crevasses that had appeared in the past year. “The death of one of our colleagues is a tragic reminder of the risks we all face--no matter how hard we work at mitigating those risks--in field research,” said Kelly Falkner (NSF Division of Polar Programs) in a statement on Facebook.
Hamilton’s research focused on mass balance and other aspects of polar ice sheets, including the stability of ice shelves in Antarctica, one of the most crucial uncertainties in the future of global sea level rise.
Justin Gillis (New York Times) wrote in a tribute to Hamilton: “He died doing a job whose urgency and importance, whose implications for the fate of all humanity, he understood as well as anyone. Yet he had carried out his work with a sense of wonder. Can you believe, he said to me in one of our conversations, that some of us get to spend our lives exploring places like Greenland and Antarctica?”
The four-minute film below, produced by the UM Climate Change Institute features Gordon Hamilton discussing his work. “I can’t think of another job I’d rather be doing,” Hamilton says.
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