Did you ever leave your freezer door slightly open on a humid day only to find chunks of new ice formed at the gap? When that happens, did you conclude “Oh, my freezer is colder than usual, I wonder how that happened?” No. You concluded that you had left the door slightly open, some cold got out, and vapor froze on your gasket.
Sea ice is hard to make. The sea is salt water, so it has a lower freezing point than fresh water. The sea has potentially large waves and lots of currents. This is just not a situation where ice can easily form. Yet, it does form on the oceans near the Earth’s poles because it is really cold there. But even within that context, more or less ice can form because of important details like how much fresh water is mixing in with the cold salt water, and exactly where currents of warmer or colder water are going. The formation of sea ice at the ends of the Earth is probably somewhat more complicated than the formation of frost and rind on your refrigerator.
(A quick note: Sea ice is ice that sits on, and therefore, essentially, in the sea. It is not glacial ice. Those are two very different things. I’m sure you knew that but just in case this is a good moment to point it out.)
In recent years, the amount of sea ice forming around Antarctica has bee going up. Global warming causes local warming but it also causes local cooling (like when the Arctic Vortex got knocked off center last winter and visited the middle of North America, an event that still causes a sense of fear and loathing among those of us who experienced it). So when we hear about expanding sea ice in the Antarctic, knowing that anthropogenic global warming is a real thing, we might assume that this is just one of those phenomena that runs counter to expectations but that is still part of the overall process of warming-driving climate change resulting from the addition of greenhouse gasses to the atmosphere.
And that is essentially correct, though the reasons may be a bit unclear and require further study.
So, thinking about our freezer, and the overall problem of making sea ice, there seem to be three things that can cause more of this ice. One might be the addition of fresh water to the system. That seems likely if the Antarctic glaciers are melting (which they are). Depending on where the fresh water goes, that could allow the formation of sea ice. Also, if precipitation increased in the area, that would add fresh water.
Second, the area where the sea ice is forming could be colder. That seems backwards in on a warming planet, but actually, that can happen too. Antarctica is, to a larger extent than the Arctic, a semi-closed system of air and sea currents, because it is a roundish continent surrounded by sea at one end of the planet. This means that cold air might be retained over the continent rather coherently. At the North Pole, “Winter (January) temperatures … can range from about ?43 °C (?45 °F) to ?26 °C (?15 °F), perhaps averaging around ?34 °C (?29 °F),” while at the South Pole, “In winter, the average temperature remains steady at around ?58 °C (?72 °F).” (source: Google). The north pole is sea, the south pole is land, and the south pole is at a higher elevation, but those differences are partly why the south pole is colder. Anyway, with all this cold air on the Southern Continent, perhaps one only needs to have air currents change a little to move that cold air over the sea a bit more to add to the chances of freezing water and making sea ice.
Third is the possibility that the disruptive effects of storms, waves, or surface currents could change, making for a calmer environment, allowing more ice formation.
Have any of these things happened?
Yes. Yes, they have.
Joe Romm has a writeup on some recent research that helps to explain the increase in Antarctic Sea ice (NOAA: Record Antarctic Sea Ice Growth Linked To Its Staggering Loss Of Land Ice).
The National Snow and Ice Data Center notes:
…sea ice surrounding the Antarctic continent reached its maximum extent on September 22 at 20.11 million square kilometers (7.76 million square miles). This is 1.54 million square kilometers (595,000 square miles) above the 1981 to 2010 average extent, which is nearly four standard deviations above average. Antarctic sea ice averaged 20.0 million square kilometers (7.72 million square miles) for the month of September. This new record extent follows consecutive record winter maximum extents in 2012 and 2013. The reasons for this recent rapid growth are not clear. Sea ice in Antarctica has remained at satellite-era record high daily levels for most of 2014.
“What we’re learning is, we have more to learn,” said Ted Scambos, lead scientist at NSIDC.
The unusual sea ice growth in Antarctica might be caused by changing wind patterns or recent ice sheet melt from warmer, deep ocean water reaching the coastline, according to scientists at NSIDC. 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.
From Skeptical Science:
The most common misconception regarding Antarctic sea ice is that sea ice is increasing because it’s cooling around Antarctica. The reality is the Southern Ocean surrounding Antarctica has shown strong warming over the same period that sea ice has been increasing. Globally from 1955 to 1995, oceans have been warming at 0.1°C per decade. In contrast, the Southern Ocean (specifically the region where Antarctic sea ice forms) has been warming at 0.17°C per decade. Not only is the Southern Ocean warming, it’s warming faster than the global trend. This warming trend is apparent in satellite measurements of temperature trends over Antarctica…
And, from NOAA:
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. Antarctica is a continent surrounded by open ocean. So unlike the Arctic, where surrounding landmasses constrain how much sea ice can expand, Antarctic sea ice can spread out over a bigger area. Winds blowing from the land toward the ocean encourage ice growth in the waters north of the continent.
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.
Added cold seems to be a factor. Added fresh water seems to be a factor. Changes in where cold air and relatively fresh water goes seems to be a factor. I don’t know about storminess and currents at the outer edge of ice formation.
The dramatic and steady increase in Antarctic Sea Ice is yet another example of the effects of climate change.