Monday, June 02, 2008

More on that SST change

As expected, there have been increasing volumes of hot air expended on the blogosphere about this. RC's post seems pretty reasonable as ever and so is this comment they point to. There is probably little point in speculating in too great detail about the implications, but then again, if one is not going to speculate pointlessly on a blog, there seems little point in having one.

Steve McIntyre was quick to present a hypothetical new surface temperature record, based on slowly phasing out buckets over the 2nd half of the century.

This gives a greatly reduced warming trend over the last 50 and even 30 years, which got Roger Pielke very excited. However, even though Steve may be right about the buckets themselves, his analysis (which, to be fair, he did not present as anything authoritative) ignores the fact that SST observations have increasingly come from other sources such as satellites and buoys. (Furthermore, that graph above seems to show a change of 0.3C for the global mean temperature, which is hard to justify since the adjustment under discussion is only 0.3C for the SST measurements made by some ships, albeit most of them.) I happen to have used some satellite SST data in previous research, and on checking the details I see that the Pathfinder AVHRR were flying from late 1981 (there may even have been older missions for all I know, but I would guess not). This fleet of satellites has provided hi-res global coverage on a regular basis from 1985 (and probably something from the few years prior), so although I do not know the details of the global SST analysis it seems inconceivable that bucket measurements from ships played much of a part subsequent to that date. Turning to buoys, Wikipedia tells me that the US National Data Buoy Development Program started in 1967 and the National Data Buoy Center was formed in 1970, so it seems that at least some data was coming from this source back then too (and note that even if the volume of data was quite small, its relative accuracy may make it outweigh a lot of ship measurements).

So I don't know exactly what the charges will be, but I suspect that the graphic here attributed to CRU is a reasonable first guess.


As you should be able to see at a glance (although Roger apparently cannot), the maximum change in the smoothed record shown there is a little short of 0.2C globally, which is just as expected given a maximum change of no more than 0.3C for the ocean (70% of the earth's surface).

So how does this affect the IPCC's latest report? Well, in the Technical Summary, there is a nice figure (Figure TS.6) which gives 25, 50, 100 and 150 year trends. So let's give this a little update shall we?

Actually, I cannot reproduce either that graph or the Independent's one precisely, as I do not know exactly how they did the smoothing, or (in the case of the IPCC) which data set they used. But this seems acceptably close to both (click for larger version):


The original data are the crosses and the blue line (5 year boxcar smooth), with the new smoothed data forming the dotted blue line which trends down more slowly from 1945-1960. The adjustment I used was a linear term which falls from 70%*0.3C in 1946, to 0 in 1960. Obviously I don't claim this is authoritative but I do believe it is more plausible than a number of other graphs I have seen on the intertubes...

The solid straight lines are the trends plotted by the IPCC. The dotted ones are equivalent trends of the adjusted data. With the help of a bit of rounding, the 50 year trend (1956-2005) changes from 0.13 to 0.12C/decade. By cherry-picking the start date to be 1946 I can get a change as large as 20%, from 0.11C/decade to 0.09C/decade. Woohoo. Let the blogorrhea continue...

Update. It certainly looks like the Independent graphic attributed to CRU is smoothed over rather more than 5 years. Roger says it is a 21y binomial filter, and if that is correct then the adjustment presented in that graph there must be just a hand-drawn guess rather than the results of any realistic calculation, since a 21y smooth would smudge out any adjustment over a rather longer interval than indicated. It's even possible that the attribution to CRU is just for the original data, and the adjustment was drawn on by a journalist. So don't take any of this too seriously for now. I would guess that the data analysts may take this opportunity to have another look at the various assumptions underlying the splicing together of different data sources, and by the time they are done there may be a whole bunch of minor adjustments throughout the time series.

11 comments:

Roger said...

James-

Good stuff, you are catching up;-)

I can replicate The Independent graph, and it is not as good a first guess as you think:

http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/001447visually_pleasing_te.html

If you start your trend analysis in 1950 rather than 1956 (to jibe with what the IPCC did) and carry it declining-linearly through the mid-1960s (to match the adjustment dates specifically as suggested by Thompson et al.) you'll get a reduction in trend of about 20%. Now if you said that we'd be just about saying the same thing.

Like you I'm not wed to any specific adjustment, and think we should look at all of the uncertainties.

Of course the easiest way to deal with this would be to start a trend analysis after 1960;-)

James Annan said...

Actually Roger, you started by saying the trend would be almost 50% lower. Then you said 30%, this latter calculation being based on an adjustment which was obviously implausible for reasons which I think are now clear enough to you. So from where I'm sitting, your patronising "you are catching up" comment seems a little misplaced, but I'm relieved to see you are not going to continue to claim black is white for another week or two this time.

If you would look at the figure I referenced from the Technical Summary, it is abundantly clear that the intervals used in that graphic all extend well past the year 2000 (I estimated the last year is 2005, although it is possible I'm a year out as I can't match their trends precisely). So other than the choice of smoothing (and minor differences in data source) it is clear that what I have done does exactly "jibe with what the IPCC did".

Roger said...

James-

The smiley face is usually use to denote an attempt at humor. ;-)

I started by posting up the implications of adjustment proposed by Steve McIntyre. Why not? I said it was not the last word (so did he) and I said I was open to any other suggestions.

I then followed up with an estimate based on the graph proposed by Real Climate identified as a "good guess". Now I've replicated that graph, and it turns out to be quite misleading with a major error. Interesting. no?

There is nothing magical about 1950 or 50 years or anything else. Nor is there anything sacrosanct about the IPCC 2007 -- understandings evolve. Frankly, I don't have any idea what the future consensus will be on this subject, do you?

In the long run science will work its course, meantime, people will speculate, discuss, and try things out. So what? I am sure that this discussion has only begun, so get used to it.

Steve Bloom said...

t this point I'm completely mystified as to why Roger thinks he has any useful role to play. Will he now follow McIntyre off into solar-crazy land?

James, looking at the CRU documentation it appears that their SST data set continues to make use of ship-based measurements only (i.e. no sat data) right up to the present.

James Annan said...

No satellite data seems odd. Mind you, it has problems of its own and needs calibrated to in-situ data (ie buoys), but I would think it could still be useful to fill in the gaps.

As for Roger...I can't help but think of the navigational technique known as "aiming off". At least you know which side of reality he is!

Simon Donner said...

The 1 deg x 1 deg HadISST1 dataset uses COADS, the source of the 1945 problem, and also AVHRR (satellite) observations. The 5 deg x 5 deg dataset doesn't use AVHRR observations. From what I recall, the 1985- AVHRR pathfinder SST dataset compiled by NOAA tracks reasonably well with the Hadley data. Adding that 0.3 C adjustment to the entire post 1945 record is unfounded.

Steve Bloom said...

Thanks for the clarification, Simon. It makes sense for them to still collect the ship data, but the lack of any reference to sat data seemed odd.

I have to say that the look of the CRU site is more or less sub-Web 1.0. OK, looks aren't everything, but then after a further five minutes I still couldn't find any obvious reference to the existence of the 1x1 set, to say nothing of anything relating to the Thompson et al results.

OTOH, up at the top of the home page was a link to the latest news: "Climate scientists at UEA are celebrating after winning a Silver Medal for their debut garden at the Royal Chelsea Flower Show"(!) I clicked to see the exciting details... but the link didn't work. Figures. :(

James Annan said...

Thanks for the further info. On checking what I wrote when I looked into this 10 years ago, it seems like the satellite data are actually a bit worse than I remembered, with persistent regional biases of perhaps several tenths of a degree. So further in-situ calibration would be required for a truly high-precision reconstruction. (If the biases are really persistent over long time scales, then this would not affect anomalies, but that is probably too optimistic an assumption).

AdamW said...

You could try the Hadley Centre site instead?

http://www.metoffice.gov.uk/research/hadleycentre/obsdata/index.html

(see also link at the bottom).

Steve Bloom said...

Thanks, Adam, that's much better! I've always been a bit murky on the whole Had vs. CRU business.

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