Arctic sea ice extent hit a new January low last month, as warm temperatures enveloped the region and low air pressure allowed cold air to escape to mid-latitudes.

Arctic sea ice extent during January averaged 5.2 million square miles (13.53 million square kilometers), which is 402,000 square miles (1.04 million square kilometers) below the 1981 to 2010 average for the month and 35,000 square miles (90,000 square kilometers) below the previous record January low in the satellite era, which occurred in 2011. This was largely driven by unusually low ice coverage in the Barents Sea, Kara Sea, and the East Greenland Sea on the Atlantic side, and below average conditions in the Bering Sea and Sea of Okhotsk.

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Temperatures across most of the Arctic Ocean during January were a remarkable 13 degrees F (6 degrees C) above average. This was to a large extent a result of the Arctic Oscillation — a variation in pressure patterns in the Northern Hemisphere — entering a strong negative phase during the first three weeks of the month, according to the National Snow and Ice Data Center (NSIDC), which released the figures. That meant that lower air pressures allowed warmer air to push north, and cold air to head the other way.

However, NSIDC underlined that, even allowing for such extremes as a result of natural fluctuations, the overall trend is clear: January sea ice extent in the Arctic is declining by an average of 3.2 percent per decade. Since January 2005, that extent has not once been above 5.5 million square miles (14.25 million square kilometers) during the month; from the start of the satellite record through 2004, it had never been below that level.

Antarctic sea ice extent was also below average for the month, just one year after it reached record high levels. Indeed, on average Antarctic sea ice continues to increase in extent (albeit at a far lower level than Arctic sea ice is decreasing) despite the fact that the ocean around it is warming.

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Several theories have been offered to explain this, including changes in wind or water circulation patterns, or increases in snowfall. Another theory is that melting glaciers and ice shelves — which, unlike sea ice, are freshwater, which freezes at a higher temperature than saltwater — are causing a freshening of the sea surface along the edges of the Antarctic continent.

It’s certainly the case that numerous Antarctic glaciers and ice shelves are melting, including some that had previously been considered stable. In some cases, this has led to outright collapse: For example, Larsen-A in the Antarctic Peninsula collapsed in 1995, followed by Larsen-B in 2002, and now their larger neighbor, Larsen-C, twice the size of Wales, may well join them.

Unlike sea ice — which, as its name reflects, is frozen ocean — ice shelves are floating on the sea surface while still anchored to the shore; as they melt or collapse, they add to the total volume of the ocean. Of particular long-term concern is the fact that ice shelves that fringe Antarctica also act as “dams” for the massive ice sheet that covers the continental landmass; were those plugs to be removed, it is postulated, it could ultimately result in the loss of that ice sheet, with globally catastrophic levels of sea level rise.

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A new study in the journal Nature Climate Change attempted to quantify how much of such ice shelves could “safely” be lost without starting to affect the dynamics of the ice sheet behind them. The authors combined satellite radar data with airborne observations of ice thickness to create models to calculate the buttressing effect of ice shelves.

They found that, on average, 13 percent of total ice shelf area is “passive shelf ice:” it’s floating ice that provides no further buttressing. That is to say, the loss of such passive shelf ice would not in itself affect the ice sheet, although it would expose a “safety band” of ice behind it.

“Once ice loss through the calving of icebergs goes beyond the passive shelf ice and cuts into the safety band, ice flow toward the ocean will accelerate, which might well entail an elevated contribution to sea-level rise for decades and centuries to come,” explained study co-author Dr Johannes Fürst, from the University of Erlangen-Nuremberg’s Institute of Geography. Unfortunately, some ice shelves have less passive ice than others.

“The Amundsen and Bellingshausen Seas (in the West Antarctic) have limited or almost no passive ice shelf, which implies that further retreat of current ice-shelf fronts will have serious dynamic consequences,” Furst added. ”This region is particularly vulnerable as ice shelves have already been thinning at high rates for two decades.”