Sea ice extent and volume


in Science, The environment

Over at RealClimate, Dirk Notz has written a good post about climate change and Arctic sea ice. In it, he highlights the importance of aggregated data, pointing out how, despite variations in regional weather, the mean temperature of the entire globe during the last 12 months is the highest it has been since the beginning of the records 130 years ago.

The figure that is most watched, when it comes to Arctic sea ice, is the extent of ocean covered by ice. This isn’t a spectacularly good measure, however, as Notz explains:

The reason for this is mostly that sea ice in the Arctic has become very thin. Hence, in contrast to the much thicker ice of past decades, the ice now reacts very quickly and very sensitively to the weather patterns that are predominant during a certain summer. This currently limits the predictability of sea-ice extent significantly.

A better measure is ice volume, which has been falling consistently as the planet warms.

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peer reviewed science October 13, 2011 at 8:51 pm

Why Climate Models Underestimated Arctic Sea Ice Retreat: No Arctic Sea Ice in Summer by End of Century?

ScienceDaily (Oct. 12, 2011) — In recent decades, Arctic sea ice has suffered a dramatic decline that exceeds climate model predictions. The unexpected rate of ice shrinkage has now been explained by researchers at CNRS, Université Joseph Fourier and Massachusetts Institute of Technology. They argue that climate models underestimate the rate of ice thinning, which is actually about four times faster than calculations. This model bias is due to the poor representation of the sea ice southward drift out of the Arctic basin through the Fram Strait. When this mechanism was taken into account to correct the discrepancy between simulations and observations, results from the new model suggested that there will be no Arctic sea ice in summer by the end of the century.

. October 8, 2012 at 1:10 pm

Though the extent of the September sea ice did bounce back a little from 2007’s nadir, in every year since then the minimum has been lower than it was in every year before 2007. And this year 2007’s record has not been merely broken, it has been smashed.

This is all the more surprising because 2012 has in other ways been a pretty ordinary year in the Arctic. In 2007 the summer weather was particularly inimical to the persistence of ice, with lots of warm southerly winds and clear skies that allowed the sunshine to do its worst. This year has seen far less in the way of special circumstances. It is true that a powerful cyclone chewed up a lot of ice in the East Siberian and Chukchi seas in early August—but the rate of ice loss outstripped that seen in 2007 both before the storm and after it.

It is still possible that changes in wind patterns and longer-term natural climate shifts may slow the currently tumultuous process of decline. But according to Mark Serreze of NSIDC the system has entered a “new regime” in which, eventually, most of the ice will come and go every year, with little lasting the whole summer. September ice cover of less than 1m km2 could be normal within decades. That’s just France and Germany.

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