Friday, October 28, 2016

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

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) Click to Enlarge.
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 right).  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
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.
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 left).

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.”

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.

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

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