What's going on with Arctic sea ice? First I heard that it's all melting away, and that this was an early warning sign of global warming on the march. Now I'm hearing that Arctic ice levels have miraculously rebounded. Which is it? And if it's true that sea ice is growing, does that mean we can stop worrying about global warming?
For the past month or so, news has been circulating around the Internet that global levels of sea ice—i.e., the floating ice that forms on top of ocean water—are back to where they were in 1979. In particular, Arctic sea ice, which was supposed to be melting rapidly, reportedly "rebounded" in 2008. This argument, which originated on the Website Daily Tech, rests in large part on the reported "rebounding" of Arctic sea ice in 2008 and is being held up by climate-change contrarians as a "gotcha" to Al Gore-ish Chicken Littles. Scientists who study the cryosphere, however, say that the latest data on sea ice does nothing to refute global warming—unless you willfully misread it.
Before we go on, let's start with Sea Ice 101. Millions of square miles of the stuff blanket both the Arctic Ocean and Southern (aka Antarctic) Ocean, with large swathes of it melting away each summer and refreezing in the winter. Sea ice can be measured in terms of its thickness and its extent—the total area of ocean covered by at least a 15 percent concentration of ice.
Arctic sea ice behaves very differently from its Antarctic brethren, largely because the two regions' geographies are so different. Arctic sea ice tends to get most of the attention from pundits and scientists because its levels are changing more rapidly, particularly in the summer. Over the past 30 years, the ice cover that remains at the end of the melt season has dropped 11.1 percent per decade—or about 915,000 square miles every 10 years—relative to seasonal averages from 1979 to 2000.As you may recall from various news reports, the summer of 2007 was a particularly dire one, as Arctic sea ice reached the lowest minimum extent ever recorded. Last year wasn't far behind.
Antarctic sea ice, on the other hand, hasn't exhibited much change—in fact, its annual extent has actually been increasing a little, by about 0.8 percent per decade. (As Brendan I. Koerner stated in this previous Green Lantern, no one really knows why this is the case—although it may actually be a surprising effect of rising temperatures in the area.)
Now, let's go back to the Daily Tech article. It states: "Earlier this year, predictions were rife that the North Pole could melt entirely in 2008. Instead, the Arctic ice saw a substantial recovery." First of all, the predictions that the article refers to were in regard to summer sea ice—no one is claiming that the Arctic will see ice-free Christmases anytime soon. Also, the scientific community isn't nearly as unified as the article suggests; predictions as to when those watery Arctic summers might commence range anywhere from 2013 to 2100. Some scientists said it was possible that the summer of 2008 would be ice-free, but those statements weren't made as decisively as the Daily Tech piece asserts.
As for the "substantial recovery" claim—well, sea ice always "recovers" in the winter, in the sense that it grows back after it melts. And, yes, September 2008 did show more ice than September 2007—but the Lantern would argue that going from the worst summer on record to the second-worst is nothing to crow about.
The period between September and December 2008—the first months of the freezing season—did see more rapid growth than usual. The Daily Tech piece is correct in chalking that growth up, in part, to the fact that this new ice had less insulating snow cover. (Less snow cover means more exposure to cold air temperatures.) Recent data suggest, however, that this growth has slowed. January 2009 showed the sixth-lowest Arctic extent on record for any January since 1979, and right now, with about a month left in the 2008-2009 freezing season, Arctic ice extent is lagging well below 1979-2000 seasonal averages. Plus, the Arctic ice pack as a whole is much younger and thinner than it was decades ago, meaning large areas are vulnerable to melting out in the summer.
The "miraculous recovery" argument makes the classic mistake of confusing short-term changes with long-term trends. The rate at which sea ice melts or freezes is determined by a complex mix of variables: not just atmospheric temperature but also wind patterns, ocean currents, saline levels, and the amount of open water surrounding the ice. So looking at a single data point is bound to skew your analysis if you ignore the clear and persistent long-term changes, as this blog post wittily demonstrates.
The Daily Tech piece also sows confusion about the meaning of global ice levels. In a global-warming scenario, it's possible that Antarctic sea ice might rise as Arctic sea ice plummets. Looking at the combined ice area of both regions doesn't tell us much about the effects of greenhouse-gas emissions, as this response to the Daily Tech item—written by the scientists whose data the piece cites—explains.