After a brief time-out while its servers were protected from the flooding in Colorado, the National Snow and Ice Data Center's (NSIDC) website was back this week, just in time for its scientists to declare that the 2013 melt season had come to a close. If you're a fan of ice, it was a good year for you. After a truly staggering ice loss last year, this summer's melt was nowhere near as severe, and in fact clusters in with the conditions that were typical at the end of last decade.
Enlarge / This year's melt, shown in light blue, is about the same as the one seen in 2009, making for a sharp contrast with last year's record.
If you've been following this for several years, you should note that the grey area, which shows two standard deviations from the average, is broader than it has been in years past. That's because the NSIDC has changed its baseline, extending it a decade to cover 1981-2010. That means it now extends into the era where ice has been shrinking rapidly. That broadens the distribution of the data, thereby increasing the standard deviation. This has the effect of making everything look a bit less severe, or a bit closer to "normal" conditions.
The reason for the greater retention of ice is pretty simple: cooler weather. In contrast to past years, when warm air parked over parts of the Arctic, this year saw a blob of cool air protecting the ice. Just about anything other than a near-total wipeout would have looked good compared to 2012, but the cool air has pushed 2013 down to the sixth-lowest ice extent on record. That's not to say that the downward trend in Arctic ice has been reversed (as you can see in the graph below), but it does suggest that the huge loss in 2012 hasn't pushed the Arctic into a completely different behavior pattern.
Enlarge / Trends in sea ice for the last few decades, as measured in July.
Meanwhile, at the other pole, sea ice is reaching its annual maximum. The dynamics of Antarctic ice are completely different, as it's partly fed by the ice sheets of the continent's interior and very exposed to the often violent Southern Ocean. Because of these different dynamics, there has been little change in the seasonal changes since we've been monitoring, with all years clustered tightly around the average. Nevertheless, this year saw a weaker melt lead to a new record high for this winter's freeze, barely edging out last year.
We've seen at least two different explanations for why Antarctic ice may be growing even as the continent nearby warms. If the scientific community reaches a consensus, we'll let you know ( via arstechnica.com ).