After the last IPCC (AR4) I spent some time on the available literature as well as conversing with several qualified people (scientists) and concluded (purely on the basis of how it all seemed to add up), that the AR4 estimates of sea level rise (SLR) probably were conservative and that a rise of around a metre by 2100 was a reasonable estimate, that 1.5 metres was plausible, but that much more seemed unlikely in the timescale, given the range of evidence.
Having gone through chapter 13 of the new AR5, I am steadily moving towards the view that a metre by 2100 is possibly the lower bound of SLR, that 1.5 metres is now the reasonable estimate, and that a larger rise is not so much unlikely as uncertain, which is a very different thing.
Of course I'm not allowed to print the stuff (well..) in the draft itself, but I extracted some parts, particularly in relation to the contribution to Sea Level Rise of the Antarctic Ice Sheet and the Greenland Ice Sheet, thinking to comment as a whole. But there is too much - even reduced to the bits which seemed most relevant to the dialogues here and on other websites.
In addition, there are plenty of better qualified commentators already working on this in the public domain. Thanks to Steve Bloom, who provided these sample links (you will need to expand the comments). This is also thematically connected to the discussion of the cryosphere (Chapter 4) here.
What I wanted to draw out of the AR5 were the potential reasons why others might think that their assessment is conservative. I have come across 9 slight oddities which will be dealt with a bit at a time (otherwise it's just too much). The first one concerns the basis for estimating the SLR contribution from the Antarctic.
It looks to me as if the core statement of estimated likely SLR from this source is based on Little et. al. (2013). [NOTE - the Little paper in question is behind a paywall at Nature Climate Change, but available as a pdf via Google Scholar] Actually, that's what the report says. But then it goes on to say, apparently, that the reason for choosing this as the reference point is that it's somewhere in the middle of the available ( process model-based) assessments, not including outliers (!) , and not taking into considering semi-empirical model estimates. But this is undermined in the selfsame explanatory paragraph by stating that it falls at the lower end of the estimates from the other process model-based assessments which were available. [NOTE - the Little paper in question is behind a paywall at Nature Climate Change, but available as a pdf via Google Scholar]
It's important to remember that the people who put this stuff together for the IPCC are experienced professional scientists and they work very hard to present what is the representative view in a balanced way. Yet, throughout the chapter, there are side-comments and minor details which, put together, strongly suggest that their chosen rubric is a consciously conservative one.
The other point which is frequently being made is that this is an example of over-dependence on the process models. There is enough evidence in the chapter to support this view, some of it overt. As things stand, I am inclined to think that not enough weight has been given to either the semi-empirical models, which consistently produce SLR rates higher than the process models, or to the people on the ground, who also seem consistently to question the AR5 estimates and suggest they are understating things.
As was pointed out to me, the most important source of difference between estimates of sea level rise is the Antarctic. Why? In broad terms, the contribution from other sources, such as the Greenland Ice Sheet, can be imagined as adding to SLR in a way comparable to the way a tap adds water to a bath, steadily. With Antarctica, this is also the case, but there also is an identifiable (but uncertain) risk that someone is planning to jump into the bath - via a destabilisation of the West Antarctic Ice Sheet (definitely more on this in a later post). The analogy also works when we think about the proposed timescales for change. There's a huge difference between adapting to a slightly overfilled bath by being careful, and clearing up after the idiot went and jumped in.
There is a lot more to be said on this subject, suffice it to say for know that I am now more concerned, after reading the report, than I was before, about the ability to adapt to some of the possible changes in the next forty-odd years, and even more worried about the following half century.