Wednesday, 16 October 2013

Sea Oddity - it's a godawful small affair

Well, RC went and stole a march by posting about chapter 13, and deservedly their material will get more attention because the contributors are specialists in the area. But whilst they discuss some technical matters, they don't seem to have covered some of the 'oddities' which, to my eye, point towards an innate conservatism in the Sea level rise assessment. Since RC has done it I will also, after all, use some of the draft, but will try to do so sparingly. Interested readers can read it for themselves.

As usual this is the 'outside' view, so I expect (and hope) for feedback.

First up is the paragraph about the two model intercomparison papers, Bindschadler et. al. (2013) and Nowicki et, al (2013), which concludes:

"The resultant projection included contributions from lubrication, marine melt and SMB-coupling and generated a mean SLR at 2100 of 162 mm over five models, or 53 mm if an outlier with anomalously high response is removed (including SMB results in SLR at 2100 of 223 and 114 mm for five- and four-model means, respectively). This comparison lends further weight to our confidence."

I don't have a problem with the concept of removing outliers as such, and it is clear from the very large differences in results that the outlier must be well out, but if they are included in the model intercomparisons, should they not be included in the assessment of the range? My beef with this is that they seem to be saying that once they have fiddled a bit with the original papers (and I'll admit, I might also be inclined to discount an outrageous outlier if there was no sensible reason to consider its' plausibility), they end up with a number which adds confidence to their 'chosen' assessment range. This doesn't seem, on the surface, to be entirely transparent.

Next up is the 'introduction' of the Marine Ice Sheet Instability material:

"There is an underlying concern that observations presage the onset of large-scale grounding line retreat in what is termed the Marine Ice Sheet Instability (MISI; Box 13.2), and much of the research assessed here attempts to understand the applicability of this theoretical concept to projected SLR from Antarctica."

Now call me a pedant (it's a reasonable charge), but the language chosen: 'this theoretical concept' at this point, whilst it may be technically correct, might predispose a reader to imagine that MISI is an implausible scenario. Later on, the Chapter includes the memorable:

"In summary, based on ice-dynamics theory, numerical simulations, and paleo records, it is likely that abrupt and irreversible ice loss from WAIS is possible. However, theoretical considerations, current observations, numerical models, and paleo records currently do not allow a quantification of the perturbation that is necessary to destabilize the ice sheet."

Can someone tell me what that first sentence means? It's a cracker.

Last up for this post is what appears to my eyes to be mildly remarkable:

"More detailed regional modelling using scenario A1B illustrates the potential for warm water to invade the ocean cavity underlying the Filchner-Ronne ice shelf in the second half of the 21st century, with an associated 20-fold increase in melt (Hellmer et al., 2012). Based on the limited literature, there is medium confidence that oceanic processes may potentially trigger further dynamical change particularly in the latter part of the 21st century, while there is also medium confidence that atmospheric change will not
affect dynamics outside of the Antarctic Peninsula."

Wow. So there is another hypothetical process that indicates a possibility of a larger potential sea level rise, which includes local changes in orders of magnitude. Does this not indicate that the reasonable range of SLR might be upped a bit? After all, we are talking about a range, aren't we?

I'm going to finish this off in another post because I am tired.

Comments, please.