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.
What I would love to know, but have not seen explained, is what ice needs to be present to obtain 5m/century sea level rise. Was that the result of huge North American ice sheets disintegrating into the ocean? Or was that working with ice more similar to what we have available now in Greenland and the Antarctic?
ReplyDeleteIt's also worth noticing that if the ice mostly did not melt until it fell into the ocean, that the heat to melt it is there; the heat capacity of the ocean is really difficult to comprehend. I did the math; if you dumped those ice caps in the ocean, they'd melt: http://dr2chase.wordpress.com/2012/02/04/numbers-that-were-larger-than-i-had-imagined/
dr2chase: there's no question there was a lot more ice available to melt coming out of the LGM than there is now. OTOH the rate of change in the forcing is massively higher now. What the result will be is...controversial.
ReplyDeletedr2chase,
ReplyDeleteSorry to be a bit late commenting. The following is the title and abstract of a recent paper, my emboldening:
QUOTE
Northern Hemisphere ice-sheet responses to past climate warming
Anders E. Carlson and Kelsey Winsor
During ice-age glacial maxima of the last ~2.6 million years, ice sheets covered large portions of the Northern Hemisphere. Records from the retreat of these ice sheets during deglaciations provide important insights into how ice sheets behave under a warming climate. During the last two deglaciations, the southernmost margins of land-based Northern Hemisphere ice sheets responded nearly instantaneously to warming caused by increased summertime solar energy reaching the Earth. Land-based ice sheets subsequently retreated at a rate commensurate with deglacial climate warming. By contrast, marine-based ice sheets experienced a delayed onset of retreat relative to warming from increased summertime solar energy, with retreat characterized by periods of rapid collapse. Both observations raise concern over the response of Earth’s remaining ice sheets to carbon-dioxide-induced global warming. The almost immediate reaction of land-based ice margins to past small increases in summertime energy implies that the Greenland Ice Sheet could be poised to respond to continuing climate change. Furthermore, the prehistoric precedent of marine-based ice sheets undergoing abrupt collapses raises the potential for a less predictable response of the marine-based West Antarctic Ice Sheet to future climate change.
END QUOTE
5m/100yr increases probably resulted from coincidental maxima in loss rate from land ice sheets and rapid pulse loss events from marine sheets.
PDF - paywall free.
http://www.see.ed.ac.uk/~shs/Methane/Carlson%20ice%20sheet%20response.pdf