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Monday, June 17, 2013

Sea ice in the Arctic and Antarctica

Sou | 12:01 AM Go to the first of 2 comments. Add a comment

Arctic sea ice watching season is here again. I was prompted to look at what is happening overall when I read a post by Perennially Puzzled Bob Tisdale. He's busy trying to prove that "all the models are wrong".  Unlike real scientists, Bob isn't about to "fix" them.  (Not sure he'd want to even if he could.)

In the case of sea ice projections, all the models are wrong, though CMIP5 is something of an improvement over the previous generation (CMIP3) models in regard to the Arctic.  In the Antarctic, more sea ice doesn't mean it's getting colder, particularly when as it warms, more ice shelves and glaciers break up and fill up the sea.


Antarctic sea ice and models


As far as Antarctica goes, researchers from the British Antarctic Survey write in the abstract of their recent paper that most models overestimate the sea ice extent at the minimum in February and some have less than two thirds of the observed ice extent at the maximum in September.  Not only that, but the models don't model the trends from 1860 to 2005 well.   The abstract concludes with this:
The negative SIE trends in most of the model runs over 1979–2005 are a continuation of an earlier decline, suggesting that the processes responsible for the observed increase over the last 30 years are not being simulated correctly.
Turner, John, Thomas J. Bracegirdle, Tony Phillips, Gareth J. Marshall, J. Scott Hosking, 2013: An Initial Assessment of Antarctic Sea Ice Extent in the CMIP5 Models. J. Climate, 26, 1473–1484. doi: http://dx.doi.org/10.1175/JCLI-D-12-00068.1

Exploring the matter further, a US team of researchers suggests that the problem may be in part due to the fact that observed internal variability in the Antarctic region is large and that the observed shifts in winds are not well simulated in the CMIP5 models.   Here is an excerpt from the abstract (my bold) and the full paper is available here:
...whether these models can be dismissed as being wrong depends on more than just the sign of change compared to observations.
We show that internal sea ice variability is large in the Antarctic region, and both the observed and modeled trends may represent natural variations along with external forcing. While several models show a negative trend, only a few of them actually show a trend that is significant compared to their internal variability on the time scales of available observational data. Furthermore, the ability of the models to simulate the mean state of sea ice is also important. The representations of Antarctic sea ice in CMIP5 models have not improved compared to CMIP3 and show an unrealistic spread in the mean state that may influence future sea ice behavior.
Finally, Antarctic climate and sea ice area will be affected not only by ocean and air temperature changes but also by changes in the winds. The majority of the CMIP5 models simulate a shift that is too weak compared to observations. Thus, this study identifies several foci for consideration in evaluating and improving the modeling of climate and climate change in the Antarctic region.
Mahlstein, I., P. R. Gent, and S. Solomon (2013), Historical Antarctic mean sea ice area, sea ice trends, and winds in CMIP5 simulations, J. Geophys. Res. Atmos., 118, doi:10.1002/jgrd.50443


Watching the sea ice disappear


To finish up here is a chart.  It shows the average extent for the minimum months in the Arctic and Antarctic. (Click the chart to enlarge it.)

Source: NSIDC


You'll see I've taken a bit of a liberty and added the areas together to give a 'total', although it's not a 'total' in any temporal sense, because the Arctic monthly average is for September whereas the Antarctic monthly average is for February.

Just the same, the minimum month at each hemisphere is when the sun shines the most on the sea. So if there is less ice then more heat is absorbed by the ocean.  In winter when there is lots of ice cover, then the sun doesn't shine anyway.  It's dark.  So the sea ice doesn't reflect sunlight back to space at that time.

2 comments:

bob said...

"In winter when there is lots of ice cover, then the sun doesn't shine anyway. It's dark. So the sea ice doesn't reflect sunlight back to space at that time."

Doh I hadn't thought of that before

bob said...

I'll have to remember that next time some denier tries to claim arctic decline is countered by increase in antarctica. Not that that's the only problem with their claim.