Antarctica’s seasonal sea ice extent reached a new high in September even though the Southern Ocean continues to warm. Credit: NOAA.
NOAA said in a news release Tuesday that “as counterintuitive as expanding winter Antarctic sea ice may appear on a warming planet, it may actually be a manifestation of recent warming.”
The most important thing to know about Antarctica and ice is that a large part of the South Pole’s great sheet of land ice is close to or at a point of no return for irreversible collapse. The rate of loss of that ice has reached record levels, tripling in the last five years alone. Only immediate action to sharply reverse carbon pollution could stop or significantly slow that.
And that really matters since 90 percent of Earth’s ice is in the Antarctic ice sheet, and even its partial collapse could raise sea levels by tens of feet (over a period of centuries) and force coastal cities to be abandoned.
So you can imagine why the people who don’t want to take any action on climate change focus on floating seasonal Antarctic sea ice, whose winter maximum has been increasing (unlike Arctic sea ice, which has sharply declined). In September, the extent of seasonal Antarctic sea ice reached a new record.
For the dwindling number of people who seriously deny the objective reality of man-made warming, this is “proof” that their anti-scientific views are right. For the 97 percent of climate scientists (and world governments and others) who understand the reality of human-caused climate change, this is an intriguing puzzle to be solved.
In the reality camp, Skeptical Science reviews the scientific literature (here), explaining that “Antarctic sea ice has been growing over the last few decades but it certainly is not due to cooling — the Southern Ocean has shown warming over same period.”
So why the increased sea ice growth? The National Snow and Ice Data Center (NSIDC) explained this week that the best explanation from NSIDC scientists is that it “might be caused by changing wind patterns or recent ice sheet melt from warmer, deep ocean water reaching the coastline … The melt water freshens and cools the deep ocean layer, and it contributes to a cold surface layer surrounding Antarctica, creating conditions that favor ice growth.”
In its release, NOAA goes into more detail on why scientists think that. NOAA first points out that “much of this year’s sea ice growth occurred late in the winter season, and weather records indicate that strong southerly winds blew over the Weddell Sea in mid-September 2014.”
Antarctic sea ice concentration (via NOAA).
NOAA goes on to explain:
Winds probably did not act alone to spur so much sea ice growth; melting land ice may have played a role. Most of Antarctica’s ice lies in the ice sheets that cover the continent, and in recent decades, that ice has been melting. Along the coastline, ice shelves float on the ocean surface, and much of the recent melt may be driven by warm water from the deep ocean rising and making contact with ice shelf undersides.
How does the melting of land ice matter to sea ice formation? The resulting meltwater is fresher than the seawater. As it mixes with the seawater, the meltwater makes the nearby seawater slightly less dense, and slightly closer to the freezing point than the ocean water below. This less dense seawater spreads out across the ocean surface surrounding the continent, forming a stable pool of surface water that is close to the freezing point, and close to the ice onto which it could freeze.
That’s why NOAA concludes, “as counterintuitive as expanding winter Antarctic sea ice may appear on a warming planet, it may actually be a manifestation of recent warming.”
Certainly Antarctic seasonal sea ice trends are an intriguing scientific puzzle, and scientists are only starting to have a fuller understanding of the causes in a warming world. But we must keep our eyes on the prize: Sharp reductions on carbon pollution to slow and, if possible, stop Antarctic (and Greenland) land ice trends before hundreds of millions of people are forced from their homes worldwide by rising seas — and hundreds of millions more are threatened with Sandy-level storm surges on a regular basis.
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