Annual Antarctic sea extents in million square kilometers from 1979-2012 (each line represents a different year). The current level of 16.22 million square kilometers is the highest so late in the season and second highest on record. See interactive chart. (University of Illinois Polar Research Group)

At face value, it’s remarkable to see such contrasting extremes in sea ice cover on opposing sides of our planet. For those unconvinced the Earth is warming, the Antarctic ice boom provides a convenient retort to alarms sounded about the Arctic ice “death spiral.”

But, in reality, it is in­cred­ibly misleading to equate the two records. Statistically, the decline in Arctic sea ice is much more dramatic. Further, the increase in Antarctic sea ice is not directly related to local air temperatures, which are actually going up.

Let’s begin by examining at the state of Antarctic sea ice. There’s no disputing it’s in record territory.

Data from the University of Illinois Polar Research Group (see chart above) show Antarctic sea ice extent reached 16.22 million square kilometers this week (around September 25). That is not only the largest extent ever reached so late in Antarctica’s cold season (seasons are reversed in the southern hemisphere), but also the second largest extent logged at any time dating back to 1979, when records began. The largest extent on record - 16.23 million square kilometers - occurred around September 20, 2007 and could be eclipsed soon.

As impressive as these numbers are in the Antarctic, they’re nothing compared to the Arctic statistics.

Antarctic and Arctic sea ice extent on September 27 indicated by the blue and orange Xs following their respective blue curves. The black line is the 1979-2000 average and the gray shaded area bounds two standard deviations. (National Snow and Ice Data Center)

It’s not that we should dismiss the Antarctic difference from average. At some two standard deviations from (above) the mean, that signifies there’s just about a 5 percent chance of this happening (assuming a normal distribution). But the Arctic difference from average is in an entirely different universe statistically speaking, way, way beyond two standards deviations from (below) the mean.

Arctic versus Antarctic sea ice extents in August from 1979-2012. (National Snow and Ice Data Center)

How does warming lead to increasing ice in the Antarctic?

Even if the decrease in Arctic ice easily trumps the increase in Antarctic ice, we still haven’t resolved the paradox of why Antarctic sea ice is increasing if local and global temperatures are warming. Much has been written about this in recent weeks in different articles and blogs, so I’ll just excerpt the explanations most frequently mentioned:

* From the Houston Chronicle SciGuy, Eric Berger: “ Unlike the Arctic, Antarctica is surrounding by water, so its less responsive to changes in air temperatures. Its climate is more governed by wind and ocean currents.” See related explanation on the National Snow and Ice Data Center’s website.

* From Live Science: “Another reason why the sea-ice extent in the Antarctic is remaining fairly high is, interestingly, the ozone hole,” [Mark] Serreze [director of the National Snow and Ice Data Center] told Life’s Little Mysteries. ... “The ozone hole affects the circulation of the atmosphere down there. Because of the ozone hole, the stratosphere above Antarctica is quite cold. Ozone in the stratosphere absorbs UV light, and less absorption [by] ozone makes the stratosphere really cold. This cold air propagates down to the surface by influencing the atmospheric circulation in the Antarctic, and that keeps the sea ice extensive.”

* From John Cook at Skeptical Science: “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.”

In short, the behavior of Antarctic sea ice is complex. The general expectation is that it will take much longer for Antarctic sea ice to respond to climate warming compared to the Arctic.

As Climate Central’s Michael Lemonick explains: “By the second half of the century, however, climatologists say that the human warming signal will become more apparent, and Antarctic sea ice will begin to follow its Arctic cousin in a downward spiral.