Miscellaneous Debris, or the AR5 Estimates of SLR

Ever since this came up in a twitter exchange between Aslak Grinsted and Tamsin Edwards somewhere about Halloween,

@flimsin and AR5 SLR projections are incompatible with the views of almost half of ice sheet expert opinions: http://t.co/Hjbb088fBM
— Aslak Grinsted (@AGrinsted) October 22, 2013

Eli has been peeking at a couple of pages on the formers web pages fittingly enough for an ice sheet expert called miscellaneous debris.  As the bunnies will see, it really is scary stuff.

Grinsted, as many ice sheet folk, is, shall the Rabett say, not very impressed with the AR anything's estimate of sea level rise from the wastage of ice sheets, which he describes as optimistic and overconfident, and ok, the AR5 did put a caveat in in fine print, that well, if there is a collapse in Antarctica all bets are off.  Grinsted shows the various estimates of sea level equivalent rise due to the ice sheets wasting away.

 

Most of the labels are self explanatory, but you can get the details amongst the debris.  The ranges are the 5-95% range. the whte line the best guess.

AR5 process based model projections are much more conservative/optimistic and has much more narrow uncertainties than the ice sheet experts (Fig.1). There can be no good reason for why the AR5 authors have much greater confidence in their ability to project ice sheet loss than ice sheet experts themselves. Notably the best guess view of ice sheet experts nearly falls outside the AR5 process based range. The worst case scenario from ice sheet experts is more than 60 cm higher than the worst case from the AR5 process models.

Clearly the process based SLR projections from AR5 are over-confident and too conservative by themselves.

The ice sheet experts estimates come from Bamber and Aspinall (2013). 

A major gap in predictive capability concerning the future evolution of the ice sheets was identified in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change. As a consequence, it has been suggested that the AR4 estimates of future sea-level rise from this source may have been underestimated. Various approaches for addressing this problem have been tried, including semi-empirical models and conceptual studies. Here, we report a formalized pooling of expert views on uncertainties in future ice-sheet contributions using a structured elicitation approach. We find that the median estimate of such contributions is 29 cm—substantially larger than in the AR4—while the upper 95th percentile value is 84 cm, implying a conceivable risk of a sea-level rise of greater than a metre by 2100. On the critical question of whether recent ice-sheet behaviour is due to variability in the ice sheet–climate system or reflects a long-term trend, expert opinion is shown to be both very uncertain and undecided.

Grinsted points out that in the current situation, although the uncertainty monster may not be perfectly represented in models, the experts have laid hands on the beast and they

. . . know the models and they have an idea of what processes are poorly represented. They can have an informed opinion on what that means for projections.

and, oh yes, if you want the total sea level rise, you have to include expansion of the oceans due to warming (22 cm), glacial melt (15 cm) and water mining as in pulling water out of deep reservoirs (5 cm).  Add it all up as Grinsted does and one is well over a meter for worst case and damn close for best guess.