Monday, December 3, 2012
The above is the key figure from a new paper in Science by Shepherd et al, A Reconciled Estimate of Ice-Sheet Mass Balance. It's probably more meaningful to look at the right hand scale, which shows the total amount of sea level rise contribution, to date, from the melting of Greenland (blue), Antarctica (pink), and the two together. You can see that both are melting on balance, and the melt rate is accelerating.
The scientists involved in making these estimates have been doing it for a decade or so now, and are increasingly collaborating and reconciling all the different methods of doing the measurements/calculations: for example this last paper involves 47 authors and completes the reconciliation of four different methods. So the results are presumably getting pretty solid.
The big worry now has to be Greenland, which is surrounded by an Arctic Ocean that increasingly isn't frozen in the summer - the whole Arctic is warming very rapidly. Can Greenland continue to increase it's melt rate further and further (as Hansen for instance has worried about: an exponentially growing breakdown of the ice sheets leading to 5m of sea level rise in the 21st century). I extracted the Greenland line above and plotted it on a log plot:
This is roughly a straight line; ie exponential increase in sea-level contribution with a doubling time of about four years. There may be some indication of some slowing in the exponential, but given the fluctuations and the fairly short timeframe it's hard to say for sure. Also hard to know how far one can safely extrapolate this rough exponential.
At any rate, I don't think Hansen's fear of an exponential break-down of the ice-sheets can be clearly ruled out, and thus it remains a risk to global civilization worth tracking.