Note that the Y axis on this chart and the one further down the page does not start at zero.
The National Snow & Ice Data Center
has published its assessment of the extent of Arctic and Antarctic sea ice for April. The average extent of the ice was the second lowest in the satellite record for the month:
Arctic sea ice extent for April 2015 averaged 14.0 million square kilometers (5.4 million square miles), the second lowest April ice extent in the satellite record. It is 810,000 square kilometers (313,000 square miles) below the 1981 to 2010 long-term average of 15.0 million square kilometers (6.0 million square miles) and 80,000 square kilometers (31,000 square miles) above the previous record low for the month observed in 2007.
Ice extent remained below average in the Barents Sea, the Sea of Okhotsk, and the Bering Sea. Sea ice was slightly more extensive than average off Newfoundland, in the Davis Strait, and in the Labrador Sea. The Labrador Sea is an important breeding area for harp and hooded seals in early spring. More extensive ice in this region favors more seal cubs being fully weaned before the ice breaks up, increasing their chance of survival.
This month, the center is using new sensors along with older sources to provide a more complete picture of changes in the thickness of Arctic ice. R. Lindsay and A. Schweiger have published a paper—
Arctic sea ice thickness loss determined using subsurface, aircraft, and satellite observations—that shows a sharp drop in ice thickness over the Arctic Ocean. From 1975 to 2012, the average has fallen by 3.59 meters (11.78 feet) to only 1.25 meters (4.10 feet). That's down 65 percent.
More on this can be found below the orange ice floe.
The news came on the heels of a report from the National Oceanic and Atmospheric Administration that the extent of winter ice in the Arctic was the lowest ever. That is consistent with the view of scientists who have predicted that by 2040, the Arctic will be completely ice-free:
Ed Farley, a scientist with NOAA’s Alaska Fisheries Science Center, said that studies over the last 15 years showed that ice melting faster year-on-year led to a drastic loss in the fat contained in zooplankten – a fish food crucial for the entire area’s ecosystem.
“We all know that we are what we eat,” Farley said, echoing lines more commonly heard on reality television show The Biggest Loser. But his words were not aimed at educating teenagers on the virtues of broccoli and blueberries, or at educating adults in getting into shape beyond the treadmill.
Meanwhile:
Antarctic sea ice extent averaged 9.06 million square kilometers (3.5 million square miles) for the month and is now the highest April extent in the satellite record. April extent was 300,000 square kilometers (116,000 square miles) higher than the previous record observed in 2014, and 1.70 million square kilometers (656,000 square miles) above the 1981 to 2010 long-term average.
The growth in sea ice around Antarctica is not because of cooling, however. The Southern Ocean has been warming at the same time ice is extending its coverage. Why is that? The website Skeptical Science
notes:
There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). A side-effect is a strengthening of the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas leads to increased sea ice production (Turner 2009).
Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).