Saturday, August 2, 2014

Antarctic sea ice cover is increasing. Is Global Warming over?

sea ice Ross Sea. Photo © Bruce Luyendyk
Sea ice, Ross Sea, Antarctica, 1996. Photo © Bruce Luyendyk

Recent news items have drawn attention to Antarctic sea ice, the floating ice a meter or so thick that forms from freezing of the oceans surrounding Antarctica. Sea ice coverage is seasonal, more in the austral winter (June-September) and less in the summer (December-March), a difference of six-fold. A way to think of the scale is that the maximum extent of the sea ice about doubles the area of ice (land and sea) at the bottom of our planet. The story is that a glitch has been discovered in the estimate of the rate of change (change year over year) in the area of sea ice around Antarctica1,2.


Underlying the news of the glitch is the prior observation that the amount of sea ice has been increasing year over year. This is opposite the observation for the Arctic Ocean where famously, sea ice area has been shrinking year over year3.

First about the glitch; this is due to a difference in the methods used in computing Antarctic sea ice cover from satellite imagery. The method was changed in 1991.
The result just discovered, is that the change revealed itself in a sudden increase, a jump, in sea ice area. The jump is an artifact of the data processing. The debate now underway is which method was correct; the method used before the jump or the method used after? It makes a difference in the rate of increase. If the original method was correct then the rate of increase is very small1, otherwise it is large. Regardless, the area is increasing at some soon to be agreed upon rate.

What’s up with more Antarctic sea ice if Global Warming is raging? Is it over? Is it not happening? The Arctic Ocean evidence alone says it’s not over and other Antarctic data say so too, let alone what other manifestations on Earth are evidence for Global Warming. In fact the ocean temperatures around Antarctica have been increasing dramatically over the last few decades4! That was mentioned in my earlier posts that dealt with loss of Antarctic ice mass on the continent itself not the sea ice (March 25, 2014, June 9, 2014). The warmer ocean is eroding from below the ice shelf extremities of the ice sheet.

Explanations for sea ice increase consistent with Global Warming point to other secondary effects in the Antarctic climate system. The idea that appears the most robust to me at this time is a reported increase in the intensity of the winds that circle the Southern Ocean around Antarctica – as result of ozone depletion5 and Global Warming. The idea is that the increased storminess breaks up ice floes as they form and disperses them. This leaves more areas of open water ready to freeze6. Another idea looks at the imbalance of melting versus freezing rates due to warmer air and ocean temperatures. The melting rate decreases due to upper ocean stratification resulting in greater freezing rates and an increase sea ice extent7.

Are these supported hypotheses? I’m not qualified to judge, but no doubt others with the needed skills will be letting us know very soon.

What is the end game? Will sea ice around Antarctica continue to expand? No. If the Southern Ocean continues to warm under Global Warming then soon it will be too warm to freeze! Climate models project that by year 2100 Antarctic sea ice will have decreased in extent by 25 percent compared to the end of the twentieth century8. But never mind that, the Arctic Ocean will be ice-free in summer by the end of this century8!


1
Eisenman, I., Meier, W. N., and Norris, J. R.: A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated?, The Cryosphere, 8, 1289-1296, doi:10.5194/tc-8-1289-2014, 2014.

2 New York Times: Opinion

3
Vaughan, D.G., J.C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen and T. Zhang, 2013: Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

4
Rhein, M., S.R. Rintoul, S. Aoki, E. Campos, D. Chambers, R.A. Feely, S. Gulev, G.C. Johnson, S.A. Josey, A. Kostianoy, C. Mauritzen, D. Roemmich, L.D. Talley and F. Wang, 2013: Observations: Ocean. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

5
Turner, J., J. C. Comiso, G. J. Marshall, T. A. Lachlan-Cope, T. Bracegirdle, T. Maksym, M. P. Meredith, Z. Wang, and A. Orr (2009), Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent, Geophys. Res. Lett., 36, L08502, doi:10.1029/2009GL037524.

6
Comiso, J. C. (2000), Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements, Journal of Climate, 13, 1674 – 1696.

7
Zhang, Jinlun, Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions. Journal of Climate, 2007, DOI: 10.1175/JCLI4136.1, v. 20 #11-2515-2529.

8
Arzel, O., T. Fichefet, and H. Goosse (2006), Sea ice evolution over the 20th and 21st centuries as simulated by current AOGCMs, Ocean Modelling, 12, 401 – 415.


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