Tuesday 28 August 2018

Nicholas Humphrey on the 2018 Arctic melt season

Arctic Sea Ice Extent Threatening a Top 5 Lowest Annual Minimum This September

Nicholas Humphrey,meteorologist

27 August, 2018

The sea ice extent in the Arctic may possibly be the Top 5 record lowest on record going back to 1979 based on current trends. Sea ice melt has been limited in August compared to July, but given what has already occurred, and the abnormally warm Arctic this summer (with the exception of the very interior Arctic north  of 80 N), a September seasonal minimum in the Top 5 is certainly possible at this point, depending on ongoing weather conditions. 
Above is Arctic Sea Ice Extent and Concentration for August 24, 2018. Note the severe recession on the Atlantic side as well as ice retreat north of Greenland's coast. What classifies as severe? Here's another map, with the 1981-2010 average extent for the date. 
You can also see how severely recessed it is on the Pacific side. Record low extents were noted earlier this year on the Pacific side for the Bering and Chukchi Seas. On the Atlantic side, the sea ice extent *is* the lowest on record for the combined Greenland, Barents and Kara Seas. August 20th, it dropped to the lowest extent on record, which was previously set in Sept 2017. 
The overall sea ice extent is currently following the *2010-2017* average...which much below the 1980s or 1981-2010 normals. 2018 extent is currently the 4th lowest on record for the most recent date, behind 2012, 2016 and 2007. The current Top 5 annual minimum years are 2012 (record), 2016, 2007, 2015, and 2011. The minimum is in September.


Arctic sea ice is on a path of major declines compared to not only the modern observational record but also paleorecords. The Kara and Barents are currently experiencing their lowest extents in nearly a millennium at least (1).
(Ice thickness, along with extent is undergoing significant decline...hence ice volume is also on major decline with 2017 having the lowest *annual average* extent on record, with the lowest seasonal minimum volume in 2012. The oldest ice, being the thickest, is declining the fastest).
(7-day mean temperature anomalies for the Arctic region ending 06 UTC September 2nd. The drawing in of abnormally warm temperatures from the North Atlantic and Siberian land mass will contribute to further sea ice melt. 1981-2010 climatology). 

What This Means for Climate Change and Extreme Weather

As the Arctic continues to tip over into a warm season ice-free state, the increasingly continuous open blue ocean means more and more heat accumulating into the climate system. Less ice means more dark ocean absorbing more heat from the sun in the summer, as opposed to white ice reflecting more solar radiation back into space, which would limit warming. The continuous ocean warming contributes to the regional and global rise in oceanic heat content, but also means when the polar night comes, more heat must be released back to the atmosphere to cool the oceans sufficiently to form new sea ice. This means progressively warmer Arctic winters, with later season new ice formation, and ice which remains thin through the winter. 
(The very abnormally warm peripheral seas of the Arctic Ocean. The anomalies shown here are relative to 1961-1990 climatology to better show the effect of ocean warming caused by climate change). 
(Global rise in oceanic heat content in the first 2000 meters/6550 ft of the global ocean. Approximately 93% of excess heat retained by the planet is going into warming the oceans).
This has been an increasing phenomenon with the later initial ice formations and later extents in the Bering and Chukchi Seas, including prominently in the 2017-18 winter (2). Research also suggests that the rapidly warming Arctic (a process known as Arctic Amplification) is contributing to a weakening of the polar jet stream which dominates mid-latitude weather, leading to a slower and much wavier jet stream. This means areas suffering much greater extremes and much greater swings between extremes (3)(4). 
(The Arctic has warmed 2-3 times faster than the globe as a whole, even faster in the "deep" Arctic...80 N+. This temperature anomaly graph is relative to 1981-2010 climatology. The past 5 years, the Arctic annual temperature has averaged 2.5 times warmer than the globe at large).
Again, the 2017-18 winter was quite prominent in displaying this increasingly powerful phenomenon with extremes such as the abnormally warm and dry Western US vs. the abnormally cold Great Plains. Meanwhile the Eastern US saw record heat in February, followed by multiple nor'easters in March (5)(6). Much of Europe had abnormally warm temperatures early this year, but in March were hit by extreme cold associated with the "Beast from the East", a prominent Arctic air mass from Siberia shunted out of the Arctic as the Arctic underwent a wintertime "heatwave" with above freezing temps near the North Pole(7). In fact, the Arctic had its warmest winter on record(8). Later in the season, the US Great Plains saw a major swing in pattern, with many areas going from one of their coldest Aprils on record to one of their hottest Mays. My city of Lincoln, NE had its 3rd coldest April on record, followed by the 4th warmest May. Records here go back to 1887. It was also the warmest May on record for the Continental US(9). There is evidence that the rapidly warming Arctic is also leading to influences in the summertime atmospheric circulation patterns in the Northern Hemisphere(10).
The overall effects as sea ice continues to decline? Further acceleration of global warming, Arctic amplification of regional warming, further weakening of the jet stream circulation leading to much greater extreme weather (temperature and rainfall vs. drought) as well as swings between extremes. So this is a very important phenomenon we must care about. Another impact? A rapidly warming Arctic means accelerating releases of carbon (carbon dioxide and methane) to the atmosphere, accelerating global warming even more(11). And loss of sea ice, with warmer atmosphere and warmer winds would mean an increased acceleration of ice sheet melt from Greenland, accelerating sea level rise globally(12). Much for humanity to face in the coming years ahead. 
--Meteorologist Nick Humphrey

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