The story so far... IPCC sez sea-level rise (SLR) by 2100 (0.18 to 0.6 m), but this excluded dynamic effects on the grounds that present understanding of the relevant processes is too limited for reliable model estimates. It even said so fairly explicitly in the SPM table heading (Model-based range excluding future rapid dynamical changes in ice flow) but naturally enough this caveat gets ignored as not providing any useful numbers. RC noticed, of course. It does become obvious, however, that you can't really get any very exciting numbers out without some rapid ice sheet response.
But saying something sensible about the ice sheet response is very tricky. Pfeffer et al. have now had a go, and RC have a sensible scientific report on it. The "problem" however is stuff like the U Colorado press release titled "Global Sea-Rise Levels By 2100 May Be Lower Than Some Predict, Says New CU-Boulder Study". This is quite odd, given that the "headline" from the papers abstract is that 0.8-2.0m is "most likely" starting point for refinements, which is numerically higher than the IPCC.
The PR says, Despite projections by some scientists of global seas rising by 20 feet or more by the end of this century as a result of warming.... RC replies We stress that no-one (and we mean no-one) has published an informed estimate of more than 2 meters of sea level rise by 2100. Tellingly, the statement in the paper that suggests otherwise has no reference.. Well, the obvious assumption is that Pfeffer et al. are having a go at Hansen. And since the paper says Also argued is that feedbacks unaccounted for in the IPCC estimate could quickly cause several meters of very rapid SLR (11, 12)., with 11, 12 both being Hansen refs, this seems like a very good assumption. Indeed Hansen has said: "If we follow business as usual I can't see how west Antarctica could survive a century. We are talking about a sea-level rise of at least a couple of metres this century." (comment 15 at RC provides another example). I recall disbelieving Hansen a few times but can't find myself explicitly criticising his SLR estimates... this is close. I think Hansen has done his best to imply meters of SLR; and Gore in AIT was deliberately ambiguous about his timescales or lack thereof. So I'm not too surprised to see people getting back at him; Hansen has got off very lightly with a number of (what look to me like) quite outrageous papers. RP lays into the RC take, if you're looking for blood.
Just to finish off... what of the science in Pfeffer et al.? RC call it "back of the envelope" without really saying a lot about it. I'm not really convinced that they've done anything that useful, other than provide some numbers for people to argue over. Their range of numbers mostly seem to come from making arbitrary assumptions about how much various icestreams etc might speed up. They have no new physics or interpretation to constrain it; all they've done is a big integration on a what-if basis. So their upper bound isn't an upper bound, and their lower bound isn't one either. If you don't like their big numbers, you're quite free to call their assumptions unrealistic: indeed they pretty well do so themselves: dynamic discharge was calculated by accelerating outlet glacier velocities by an order of magnitude in the first decade. In Antarctica, PIG/Thwaites was accelerated from present-day net discharge in the first decade and held thereafter to the highest outlet glacier velocity observed anywhere.... OTOH, if you really want larger numbers, you could get those by assuming something different happens, like a nice collapse of the WAIS, which still can't be ruled out on physical grounds.
I think you really missed the point of the Pfeffer et al paper. Which as I understand it is that to achieve SLR of 2m ice *all* of the glaciers in the GIS that discharge to the sea would have to accelerate to rates >10x faster than *ever* observed starting *right now.* It's not just "what if," as you describe it. It's simple physics: discharge of mass M through an area of X requires what velocity V? And the V they calculate is much faster than anything observed, which is not "arbitrary assumptions," so 2m SLR seems unlikely. They're not assuming that the ice streams *will* speed up to those or any rates, just asking "What would it take?" Yes, it's not proof. It's a creative approach to solving the problem of how to get a realistic estimate when you can't model the chaotic physical processes of ice dynamics.
["a creative approach to solving the problem of how to get a realistic estimate" - but it *isn't* a realistic estimate. As you've just pointed out, its an unrealistic estimate! -W]
You are once again missing the point....
The paper gives a realistic physical constraint on SLR from the GIS. The paper responds to a number of previous papers that document acceleration of outlet glaciers from the the GIS and suggest that acceleration is a harbinger of destabilization that will lead to massive SLR. Pardon me for using the word "estimate" when I should have used constraint. Now go reread the paper.
[I'll forgive you your last impoliteness. Your words are ambiguous, but I think you are now saying: it provides an upper bound, which is "realistic" in the sense of not likely to be exceeded. Thats true, but not really terribly useful. Its easy to construct a not-to-be-exceeded upper bound. What is hard is to construct something that actually approximates the truth. I don't think they've managed the latter; and the former is in doubt too -W]