Antarctica melting?

Melting ice in Antarctica produced global sea level rises of 0.4 mm/year between 2002 and 2005, according to a new study published March 2, 2006 in the on-line journal Science Express. The study, titled "Measurements of Time-Variable Gravity Show Mass Loss in Antarctica", by University of Colorado researchers Isabella Velicogna and John Wahr, used satellite data from two NASA satellites called the Gravity Recovery and Climate Experiment (GRACE). The satellites measured the changing pull of gravity from the two large ice sheets covering Antarctica to determine how much ice was on the continent, and how fast the ice was changing. Most of the melting discovered was from the West Antarctic Ice Sheet. This is the smaller of the two ice sheets covering Antarctica, and holds enough ice to raise global sea levels 20 feet should it completely melt. The rock on which the West Antarctic ice rests is below sea level, and the sheet could be melting on its underside due to warming ocean waters penetrating there and melting it from both below and along the edges. The study found little melting of the huge East Antarctic Ice Sheet (which would raise global sea levels 200 feet if it were to melt). This ice sheet is on rock high above sea level, so warmer ocean waters cannot affect it. Additionally, the East Antarctic Ice Sheet has average temperatures so cold that even a 5-10C increase in temperatures is not expected to seriously threaten it.

The net Antarctic melting reported comes as a surprise, since the "official" prediction from the latest 2001 report from the Intergovernmental Panel on Climate Change is that global warming should cause increased precipitation over Antarctica this century. This increased precipitation is expected to exceed Antarctica's melting enough to decrease global sea level. This decrease in sea level by 2100 is predicted to be about 3 inches (8 cm), � 4 inches (10 cm), but would be offset by increases in sea level due to thermal expansion of the seas due to warmer water temperatures, plus melting of Greenland and glaciers on other continents.

As I reported in my blog on Greenland's greenhouse, total global sea level rise in recent years has been between 1.5 and 2.9 mm/year. Thus, the .4 mm/year contribution from Antarctica found by the new study represents a significant portion of this rise. However, another study published in December 2005 in the Journal of Glaciology titled, "Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002" found a rate of melting for Antarctica five times smaller, for the earlier period 1992-2002. This research, performed by a team led by NASA scientist H. Jay Zwally, used satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2, and found a net melting of only .08 mm/year from Antarctica. Did Antarctica's melting really increase 5-fold in past three years? If so, is this a short term fluctuation, or indication of a long term trend? I'm of the opinion that's it's too soon to tell. It is extremely difficult to do mass balance studies of these huge ice sheets, since it requires finding a small change in a very large number. The same problem affects the recent estimates of Greenland's mass balance. The new study from the Journal of Glaciology also reported that between 1992 and 2002, the total mass of the Greenland Ice Sheet increased and thus Greenland caused a .03 mm/year decrease in sea level. This result is in contradiction to the two studies I quoted in my Greenland blog, by Box et al. (2004), who found that Greenland contributed to a net increase in global sea level of 1.5 mm/year, and Rignot et al. (2006), who found a .23 mm/year rise for the year 1996, increasing to .57 mm/year by 2005. I'd like to see at least three to five more years of satellite measurements before concluding that Antarctica or Greenland are undergoing significant melting. The European Space Agency is launching a satellite called CryoSat in March 2009 that should help answer these questions. If you want a more technical discussion of the issues, realclimate.org published a nice analysis last week.

Coverage in the press
It was interesting to watch the reaction of the press to the release of the new study. The New York Times titled their article, "Loss of Antarctic Ice Increases", and did a reasonable job covering some of the uncertainties. The USA Today was a bit more alarmist, headlining their article, "Study: Antarctic ice sheet in 'significant decline'". The Washington Post had a very alarmist title to their article, "Antarctic Ice Sheet Is Melting Rapidly". The facts and uncertainties involved in the making ice sheet balance measurements do not support this claim, as of now. Although any news of an increase in melting from Antarctica or Greenland is worthy of concern, I thought that in general, the media's headlines on the matter were too alarmist, given the uncertainties involved.

My next blog will be Thursday, when perhaps I'll be able to talk about Phoenix's first rain in 142 days. They've got a 20% chance of rain on Wednesday!

Jeff Masters

References
Box, J.E., D.H. Bromwich, and L-S Bai, 2004. Greenland ice sheet surface mass balance 1991-2000: Application of Polar MM5 mesoscale model and in situ data. J. Geophys. Res., 109, D16105, doi:10.1029/2003JD004451.

Rignot, E., and P. Kanagaratnam, "Changes in the Velocity Structure of the Greenland Ice Sheet" Science 311, 986-990, 17 February 2006, DOI: 10.1126/science.1121381