Sunday :: Mar 1, 2009

if you've seen one sea ice floe...

by Christina Hulbe

Sea ice was in the opinion news recently (as noted here) and appears again today in the Washington Post Ombudsman's column. I'll leave professional football comparisons to others but I would like to clarify a few things regarding the science.

Post Ombudsman Andrew Alexander writes The editors who checked the Arctic Research Climate Center Web site believe it did not, on balance, run counter to Will's assertion that global sea ice levels "now equal those of 1979." To his credit, Alexander identifies the distinction between global and northern hemisphere sea ice and notes that it would have been a good idea to seek clarity on this point before the column ran. Indeed. Arctic sea ice extent (and thickness) is shrinking and has been for some time, while around the Antarctic, sea ice extent is increasing, although with quite large variability.

Both the shrinking Arctic and expanding Antarctic sea ice cover can be understood in the context of global warming and variability in atmospheric circulation. I've written about these topics before and don't want to belabor them now but a few key points and references seem in order.

A couple of years ago, I wrote an introduction to sea ice, archived here. In a nutshell, sea ice forms as the ocean surface cools below its (salinity dependent) melt temperature and small crystals begin to grow. The crystals knit together over time to produce a more or less continuous cover that matures over time into ice floes a few centimeters to a few meters thick. Sea ice drifts according to the ocean and wind circulation. When the surface warms above the melt temperature (either seasonally or due to drift into a warmer area), the ice begins to melt.

The differing geographies of the Arctic and Antarctic oceans result in very different sea ice characteristics in the northern and southern hemispheres. In the relatively confined Arctic, sea ice can drift for years, slowly thickening as it does. This is not the case in the Antarctic, where offshore drift tends to move sea ice away from the continent (and into warmer conditions) and thick multi-year ice is relatively rare.

Right now in the Arctic, the combination of atmospheric warming and a pattern of atmospheric variability called the northern annular mode is working to rapidly reduce sea ice cover. I wrote about this here. The most recent rate calculated by sea ice folks at the US National Snow and Ice Data Center (NSIDC) is -3.1 +/-0.7% per decade, a rate that is faster than climate models project. As the sea ice extent declines, the amount of solar radiation reflected away from the surface also declines and air-sea heat exchange increases, producing strong global warming feedbacks.

In the Antarctic, changes in sea ice extent are linked to a mode of atmospheric variability that causes westerly winds over the circumpolar ocean to either strengthen and shift southward or weaken and shift northward. As the wind pattern changes, the amount of sea ice "exported" away from the continent changes. This is important because as sea ice moves away, new ocean surface is exposed and subject to freezing. The effects of variations in the SAM are complicated by a strong dipole pattern around the continent (sea ice extent may be expanding in one quadrant while shrinking in another) and variability is significant but there is a modest positive trend in sea ice extent around the continent as a whole. The most recent trend calculated at NSIDC is +2.6 +/-5.0% per decade. The large uncertainty on the trend gives you a feel for the variability.

The decline in Arctic sea ice is clearly connected to global warming. Changes in Antarctic sea ice are as well, although the pattern there is more complicated than in the north. When George Will cites a statistic about "global sea ice" in the context of global warming, the most favorable interpretation is that he's ignorant regarding the science. Fortunately, there is a cure for this condition. The NSIDC web site is a fantastic source if you want to read more, either about sea ice itself or about how it is observed. Try here and here.

(Note to Peter: a sensor failure earlier this year on one of the satellites used in sea ice monitoring was recognized and corrected. Read all about it here.)

Christina Hulbe :: 6:33 PM :: Comments (15) :: Digg It!