Tuesday, December 4, 2012

We're Melting! Melting!

“You cursed brat! Look what you've done! I'm melting! melting! Oh, what a world! What a world! Who would have thought a good little girl like you could destroy my beautiful wickedness? Oooooh, look out! I'm going! Oooooh! Ooooooh!”

                                                                                          From the Wizard of OZ

Dorothy watches the Wicked Witch of the West melt in the Wizard of Oz.  Image Credit: Everett Collection.

How many times have you seen this movie?  I won’t spoil the ending if you haven’t.  However, I couldn’t help remembering this line from the movie Wizard of Oz as I read a new report this week.

The report was published in the November 30 issue of the journal Science.  It was the work of 47 experts in glaciology from 26 institutions worldwide and the best estimate we have of the amount of ice being lost from Greenland and Antarctica.  The team analyzed different sets of data covering the period from 1992 to 2011 and included satellite measurements from NASA.

Areas affected by the melt seasons of 1992 and 2005.  Click on image for a larger view.  Image Credit: University of Colorado.

Scientists found that the two ice sheets are losing more than three times as much ice now than they were in the 1990s.  Two-thirds of that is coming from Greenland with the rest coming from Antarctica.  This is not surprising since the arctic region is warming twice as fast as the rest of the globe.  Earlier studies from the University of Colorado have demonstrated the increasing melt of the Greenland Ice Sheet.

Monthly changes in Antarctic ice mass, in gigatones, as measured by NASA's Gravity Recovery and Climate Experiment (GRACE) satellites from 2003 to 2011. The data illustrate the continuing loss of ice from the continent. The plots here depict results from five different IMBIE team members using different methods. The data have been adjusted to reflect new models of post-glacial rebound.  Image Credit: NASA-JPL/Caltech; NASA GSFC; CU-Boulder; Technical University of Munich; Technical University of Denmark; Delft University of Technology, Aerospace Engineering, Netherlands; Durham University, UK; Leeds University, UK.

Map of Antarctica.  Click on image for a larger view.  Image Credit: Landsat Image Mosaic of Antarctica team.

All of the recent measurements show that Antarctica is losing ice.  Most of that appears to be from the West Antarctic Ice Sheet and the loss is from the more rapid advance of the glaciers to the sea.

By the Numbers:

The team concluded that over the past 19 years the ice mass for the ice sheets changed by the following (Gt/yr – gigatonne per year):

            Greenland                             -142 +/- 49 Gt/yr
            East Antarctica                     +14 +/- 43 Gt/yr
            West Antarctica                    -65 +/- 26 Gt/yr
            Antarctic Peninsula             -20 +/- 14 Gt/yr

Only the East Antarctic Ice Sheet saw an increase in ice mass, however this was overwhelmed by the ice loss from West Antarctica and the Antarctic Peninsula.  Furthermore, Greenland was found to be losing ice mass at an accelerating rate.  The loss was – 51 +/- 65 Gt/yr in the period 1992-2000, however the rate had increased to -263 +/- 30 Gt/yr in the most recent period of 2005-2010.  This is a significant increase in the loss of ice.

One of the lead authors, Erik Ivins of NASA's Jet Propulsion Laboratory in Pasadena, California said “Both ice sheets appear to be losing more ice now than 20 years ago, but the pace of ice loss from Greenland is extraordinary, with nearly a five-fold increase since the mid-1990s.  In contrast, the overall loss of ice in Antarctica has remained fairly constant, with the data suggesting a 50-percent increase in Antarctic ice loss during the last decade."

Bottom Line:

The researchers estimated that the ice loss contributed 11.2 +/- 3.8 mm to global mean sea level between 1992 and 2011.  This represents about 20% of the rise in sea level during this time and amounts to a contribution of 0.59 +/- 0.20 mm/yr.  The remaining contribution came from the melting of mountain glaciers and the warming of the oceans.

"What is unique about this effort is that it brought together the key scientists and all of the different methods to estimate ice loss," said Tom Wagner, NASA's cryosphere program manager in Washington. "It's a major challenge they undertook, involving cutting-edge, difficult research to produce the most rigorous and detailed estimates of ice loss from Greenland and Antarctica to date. The results of this study will be invaluable in informing the IPCC as it completes the writing of its Fifth Assessment Report over the next year."

Current estimates of global mean sea level rise are 3.11 mm/yr since 1993.  The rate of change from 1870 to 2000 was about 1.7 mm/yr.  Everything is telling us that global mean sea level is rising and it is due to global warming.  Furthermore, the rate of change is accelerating.  Current projections call for a rise of about a foot between now and 2050.  Then sea level is expected to rise at least another two feet between 2050 and 2100.