Tuesday, March 23, 2010

Sea level rise prediction cage fight

RC spends much of its time defending the IPCC (to the hilt). And broadly speaking, this is fair enough, especially in the context of the public discourse where the alternative is not a slightly nuanced and updated version, but rather a torrent of wingnut denialist nonsense.

So it was a bit of a surprise to see Stefan Rahmstorf's broadside about "Sealevelgate":

In its latest report, the IPCC has predicted up to 59 cm of sea level rise by the end of this century. But realclimate soon revealed a few problems.

[...]

Some scientists within IPCC warned early that all this could lead to a credibility problem, but the IPCC decided to go ahead anyway.

Nobody cared about this.

Reading between the lines a little, I somehow don't think this is really just about the AR4. Scientifically speaking, that is old news, indeed it was old news by the time it was published. Rather, this looks like more like an opening salvo over the forthcoming AR5. Alert readers may have noted that the current Chair of WG1, Thomas Stocker, was a co-author on the Siddall et al sea level rise paper that Rahmstorf (with Vermeer) forced the retraction of, and was also one of the Coordinating Lead Authors on the relevant chapter (10) of the AR4. It may not be too great a stretch to imagine that there may be some strained relations there.

I don't have any particular stake in the sea level rise predictions. I do think it was pretty spineless and inappropriate of the IPCC to duck the issue in the way that they did (by basically ignoring dynamical ice sheet response - if they really thought this was not going to happen, they should have said so clearly). OTOH the conceptual time series models presented by Rahmstorf and others may be a bit too simplistic to have much credibility for predictive purposes. I suppose I won't know what I really think until I look into it myself...which may be some time away. In the meantime, pass the popcorn, as Eli would say.

6 comments:

Martin said...

> the Siddall et al sea level rise paper
> that Rahmstorf (with Vermeer) forced the
> retraction of

Yes, an AK47 is a thing of beauty.

Hank Roberts said...

No particular stake?

If there's a change in the modeled expectation of sea level rate of change, does that suggest local changes in salinity or basin deep water production or circulation that would affect Arctic sea ice rate of change?

I hear "it's all connected ...."

Jesús said...

Von Storch & Zorita seem pretty convinced that Rahmstorf is wrong based on two published comments (see here or here). In case you finally decide to dig into it, please, let us know your opinion. I left here the relevant links:

- Stefan Rahmstorf, 2007: A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 19 January 2007: Vol. 315. no. 5810, pp. 368 – 370 | DOI: 10.1126/science.1135456

- Simon Holgate, Svetlana Jevrejeva, Philip Woodworth and Simon Brewer2 (2007): Comment on “A Semi-Empirical Approach to Projecting Future Sea-Level Rise”. Science 28 September 2007, Vol. 317. no. 5846, p. 1866 | DOI: 10.1126/science.1140942

- Torben Schmith, Søren Johansen and Peter Thejll (2007): Comment on “A Semi-Empirical Approach to Projecting Future Sea-Level Rise”. Science 28 September 2007, Vol. 317. no. 5846, p. 1866 | DOI: 10.1126/science.1143286

- Stefan Rahmstorf (2007): Response to Comments on “A Semi-Empirical Approach to Projecting Future Sea-Level Rise”. Science 28 September 2007, Vol. 317. no. 5846, p. 1866 | DOI: 10.1126/science.1141283

*Update of Rahmstorf 2007:
- Martin Vermeer and Stefan Rahmstorf (2009): Global sea level linked to global temperature. PNAS December 22, 2009 vol. 106 no. 51 21527-21532

James Annan said...

Hi Jésus,

Yes I was aware of some of that - had forgotten the HvS thing though I did read it at one time. I tend to agree that the stats are a bit shaky, but that doesn't make the result completely unreasonable (one advantage that climate scientists have over more theoretical time series analysts is the physical reasoning underpinning the models). At one point I had a plan to spend some time on this, but I ended up waiting for input from colleagues...and have other things to do too. But it might come around again...

Jesús said...

Thanks for your comments, James.

I cannot assess the stats, but, as I see it, observations show that physical models are underestimating see level rise. Confronting this shortcoming, Rahmstorf has proposed a reasonable way to assess by how much: getting the statistical relationship between T and SLR from the observational record. If his stats were wrong, I guess it means that we could improve the "formula", but the method would still be a good first-order approximation. The method might be wrong if physical processes (behind SLR) change significantly during this century. I don't expect this, and, with temperatures rising (as expected), the rather constant rate of rise proposed by the IPCC seems rather implausible. The 20th Century has seen accelerating SLR with rising temperatures; therefore I think that, under future rising temperatures, SLR should continue to accelerate rather than to keep its pace (because of the contribution from the ice sheets). I've just recalled that there is an interesting presentation by Rahmstorf (in the conference '4 degrees and beyond') commenting on all this and explaining the semi-empirical approach (including the last improvement from Vermeer): PPT and audio.

Cheers!

James Annan said...

Thanks for the further links. I agree that the model is physically plausible, but there is a question of whether the data are sufficient to estimate the parameters - and if not, it will depend heavily on prior assumptions which don't seem to be clearly stated. (I'm not saying it is wrong, only that I'm not sure it is right.)