August 28, 2013 | 7
The warming, melting and potential contributions to sea level rise from glaciers in Greenland and West Antarctica in the face of climate change has long since been a serious concern. The behavior of the much larger East Antarctic ice sheet has been much more uncertain and until now has been thought to be relatively insensitive to climate change.
Now an important paper in Nature from Durham University and the University of Zurich has examined a large number of satellite observations of the East Antarctic ice sheet over the last forty years. The authors find that although there is considerable variability in individual glacier advance and retreat, there is a clear overall trend of advance and retreat that tracks well with warming and cooling periods between 1974 and 2010. Glacier movement is thus much more sensitive to climate trends than previously thought. The work casts serious doubt on reassurances about the stability of the East Antarctic ice sheet.
Three epochal patterns emerged from the analysis: 63 per cent of glaciers retreated from 1974 to 1990, 72 per cent advanced from 1990 to 2000, and 58 per cent advanced from 2000 to 2010. These results were consistent with warming and cooling. The effect was most significant along the western South Pacific Coast and was least along the Ross Sea Coast.
The study looked at a large data set of approximately 300 satellite images of glaciers from 1963-2010 to map the positions of 175 glaciers along 5400 km. The data set was thus quite comprehensive. The images showed variation in individual glacier movement that was linked to their width; large glaciers undergoing much more change than smaller ones. However the overall switch from advance to retreat was very highly significant and swamped this individual variability; for instance, between 1974-1990 the glaciers retreated at a median rate of -12.5 m/yr while they advanced at a median rate of 19.7 m/yr during 1990-2000, a change of about 375%.
These changes were consistent with a relatively warm period in the 70s and 80s and cooling in the 90s. The advance and retreat also seem to track well with more subtle changes; for instance while the planet warmed until 2005, there has been a cooling trend since 2005 which is reflected in the slowing down of the retreat. However the paper is careful not to directly connect the glacier change to air temperature, instead recognizing air temperatures as part of a more complex system – the Southern Annular Mode (SAM) which comprises not just the air but also other factors like wind-speed, the interplay between the ocean and the atmosphere and sea-ice concentration. It’s worth noting though that positive trends in the SAM have been linked to ozone depletion and increasing greenhouse gas concentrations over the last two decades, so the correlation with air temperatures might possibly be a good proxy.
In spite of this complexity, the three trends in glacier advance and retreat since the 70s explicitly stand out and are consistent with measures of warming and cooling since then. The study does make it clear that the East Antarctic glaciers, which were thought to be relatively impervious to climate change, are likely as or more sensitive to global warming and other climate change effects as the better understood West Antarctic and Greenland ice sheets. In one sense this finding is very significant since the East Antarctic ice sheet is much larger than its western counterpart; thus, whatever humans are doing to the climate is likely to have potentially huge impacts on the melting of this ice sheet and a corresponding change in sea levels. We are mucking around with these massive wonders of ice at our own peril.
Reference: “Rapid, climate-driven changes in outlet glaciers on the Pacific coast of East Antarctica”, Miles, Stokes, Vieli and Cox, Nature, 2013, 500, p. 563-567