Back to the AR5 on the subject of the Antarctic, and in particular the Marine Ice Sheet Instability Hypothesis. The AR5 confidently asserts that 'it is likely that abrupt and irreversible ice loss from WAIS is possible'. Why does this matter? Because if it occurred, that additional sea level rise which the AR5 does not expect, of 'several tens of centimetres', would become a reality.
For non-scientists, (usual warnings apply), a 'sudden and irreversible ice loss' refers to the possible 'collapse' of the mechanism which currently constrains the export of ice into the oceans and therefore the contribution to sea level rise. This would be signalled by rapid, non-linear changes to the Pine Island Glacier (PIG) and Thwaites Glacier (TG). These two represent a substantial proportion of the potential sea-level contribution of the WAIS, which is why they are important. One of the keys to recognising that a sudden process is under way is the retreat of the Grounding Line (GL) of the glaciers. Recent research suggests that Pine Island is more risky than Thwaites because of the relative basal geography of the two.
The process follows a theoretical sequence: The glaciers at the sea end thin, meltwater seeps down from above, the glacier fractures deep into the outlet section, then collapses; the glaciers behind speed up, pushing more ice seawards, and the grounding line (where the sea can no longer get 'under' the glacier, retreats inland.
In Chapter 13, The AR5 discusses the condition under which such an event might occur and the consequences:
"Two processes that could trigger GL retreat are particularly relevant to contemporary polar climate change. The first is the presence of warmer ocean water under ice shelves, which leads to enhanced submarine iceshelf melt (Jacobs et al., 2011). The second is the presence of melt water ponds on the surface of the ice shelf, which can cause stress concentrations allowing fractures to penetrate the full ice-shelf thickness. This process appears to have been a primary factor in the collapse of the Larsen B Ice Shelf (LBIS) over the course of two months in 2002 (MacAyeal et al., 2003). The collapse of the LBIS provided a natural demonstration of the linkage between the structural integrity of an ice shelf and the flow of grounded ice draining into it. Following the breakup of LBIS, the speeds of the glaciers feeding the collapsed portion of the shelf increased two-to-eightfold, while the flow of glaciers draining into a surviving sector was unaltered (Rignot et al., 2004; Scambos et al., 2004; Rott et al., 2011). This indicates that a mechanical link does indeed exist between shelf and sheet, and has important implications for the future evolution of the far more significant PIG and TG systems of the WAIS."
Dutrieux, P., Vaughan, D. G., Corr, H. F. J., Jenkins, A., Holland, P. R., Joughin, I., and Fleming, A. H.: Pine Island glacier ice shelf melt distributed at kilometre scales, The Cryosphere, 7, 1543-1555, doi:10.5194/tc-7-1543-2013, 2013. There are other observations and calculations substantiating that there is water under the PIG, that it is relatively warm, that it is progressively undermining the glacier. There seems little reason to doubt that this condition is currently being met.
Condition two requires the presence of meltwater at the top of the glacier. I can't find the source (maybe someone can help), but I am certain that I have very recently seen a record of observation of this phenomenon - I believe it was at the Thwaites Glacier, but there is no reason to suppose that the phenomenon would exist at one glacier and not the other.Forthcoming papers (not yet published) look at the 'condition' of PIG and seem to offer further evidence that the conditions exist for a full-depth fracture of the PIG.
So; it would appear that the required conditions for the fracture and subsequent collapse of the Pine Island Glacier are being met. If this is correct, the next 'critical' stage would be the appearance of a glacier-wide fracture, probably during the Austral Summer. On the surface, it very much feels like this is no longer a matter of 'if', but of 'when'.
Prediction a non-linear shift is, of course impossible. It has to be a sophisticated form of educated guesswork as to when this could occur. To me, given that other indicators suggest that the transformational process in the cryosphere are systematically understated in the 'official' lines, it could as easily be this Summer, or ten years, or twenty, thirty years away.
If any of my readers can give guidance as to the plausible timeline in which we are working here, it would be much appreciated. Likewise, if someone thinks that I am misrepresenting or overstating the case. But from the viewpoint of this blogger, that 'likely that it is possible' statement of the AR5 goes nowhere near capturing the reality of the situation or the risk. As a consequence, the decision to eliminate sea level rise estimates which incorporate a collapse in the 21st century look at best misguided, at worst, much too conservative.
I'll look out for other connections on the subject: for example, Aslak Grinsted's excellent summary of the estimates, and RealClimate's ongoing discussions.
I think this is a real, substantive risk which has a significant timeline for the current generation, and that serious and urgent investment in Antarctic research is now essential.