Some IPCC SPM comments

I'm not going to give a broad overview, as there is is already a plethora of rather boring bloggorhea (sorry guys, but it is :-) ) on the subject. Go and read RC if you want the "consensus" view. Or just read the document itself, it's simple enough. I'm just going to pick out a few bits that are particularly interesting to me.

1. First, they predict continued warming for the next 20 years of "about 0.2C per decade", up from a "likely" range of 0.1-0.2C in the TAR. That change is not really surprising - it had been clear for some time that the actual warming rate was closer to the upper than the lower end of the TAR range. I don't know the history of the TAR but I guess that their selection of endpoints for their range owed as much to rounding as a deliberate selection of 0.15C as a central estimate. Even back then the recent trend was above 0.15C and forecast to increase over time, especially under the implicit assumption of no volcanic eruptions (a big one could knock as much as 0.1C off a decadal average temperature). This new estimate is still a long way short of the probabilistic predictions that have been published though (at least, 2 such papers that I recently re-read).

Apparently there is a new Science paper which talks of a recent trend of >0.2C per decade. Every time I've looked at GISTEMP (eg here) it shows just a bit under 0.2C to me, so I'll have to check exactly what this new paper did. Anyway, "about 0.2C per decade" is fine by me.

2. On climate sensitivity, there is the much-leaked change from 1.5-4.5C (TAR) to 2-4.5C (AR4) at the same "likely" level. I think the change at the lower end may be as much due to increasing recognition that 1.5 is a firm limit, as stronger confidence that the value of S is actually greater than 2C. Forster and Gregory's recent estimate was 1.7C and there are several others with a strong likelihood close to the lower end of the range. Anyway, depending on how "likely" is interpreted, this phrase still acknowledges perhaps as much as 15% probability of S sneaking below the 2C threshold, but it cannot do so by much.

What they have said about the upper end of the range is more...interesting. They have added the phrase:

"Values substantially higher than 4.5°C cannot be excluded, ...".

A literal interpretation of this is completely vacuous (we can never assign a probability of precisely zero), so I'm not at all sure what they mean by including it. Note how it carefully avoids using the calibrated probabilistic language that has been adopted (likely, very likely etc). I can't help but be amused by RC's comment:

"the governments (for whom the report is being written) are perfectly entitled to insist that the language be modified so that the conclusions are correctly understood by them and the scientists. [...] The advantage of this process is that everyone involved is absolutely clear what is meant by each sentence. Recall after the National Academies report on surface temperature reconstructions there was much discussion about the definition of 'plausible'. That kind of thing shouldn't happen with AR4."

I predict discussion about the definition of "cannot be excluded". I will be discussing it, at least! I complained about this ambiguous phrasing (which appeared in similar form in a couple of chapters) in my review of the last draft, and explicitly asked the authors to explain more clearly what they meant by it. I've also asked a couple of authors who used similar phrasing in their papers but have not got a reply out of them. I find it hard to avoid the conclusion that this "cannot be excluded" phrase was deliberately chosen specifically for its meaninglessness, in order to to be able to present a "consensus" rather than a strong disagreement about the credibility of such high values. I'm sure that those who assign a probability of 5% or even more to S greater than 6C will consider that this phrase supports them, even though Stoat parses it as "they do go on to diss > 4.5 oC a bit" due presumably to the "but agreement of models with observations is not as good for those values" which completes the sentence I partially quoted above. "Not as good" also has no probabilistic interpretation of course.

[Update

Based on this first-hand report, the phrase was indeed chosen specifically for its ambiguity.]

3. There is more probabilistic confusion in the discussion of attribution of past climate changes (I've written about this before). This is perhaps most clearly demonstrated in

"It is very unlikely that climate changes of at least the seven centuries prior to 1950 were due to variability generated within the climate system alone."

One thing they might have said (and perhaps thought they were) is that an unforced system is very unlikely to exhibit the observed level of climate changes. That is an essentially frequentist statement about ensembles of model runs. But what they have actually said appears to be the Bayesian statement that they believe that there was external forcing in the real world. No shit Sherlock! The cavalier way in which the detection and attribution community freely switches between frequentist and Bayesian approaches to probability, without any clear explanation, gives every impression that they do not understand the difference (or even perhaps realise that there might be a difference). Their writing about the recent warming is similarly clumsy:

"it is extremely unlikely that global climate change of the past fifty years can be explained without external forcing, and very likely that it is not due to known natural causes alone."

The first statement again is essentially frequentist, the second Bayesian. The existence of anthropogenic forcing as a contribution to the recent climate changes is not merely "very likely", it is at least "virtually certain"! (I don't believe there is a single working climate scientist who would argue that the anthropogenic forcing has been precisely zero. There is of course debate over its magnitude - some legitimate, some specious.)

I should point out that this criticism doesn't invalidate (or even weaken) the broad thrust of the report. I'm grumbling about the D&A stuff primarily because it forms the basis of the confusion in the climate sensitivity debate, rather than actually mattering in itself. They could have written things in a clear and correct manner without substantively affecting the overall message.