Den stora tråden om global uppvärmning/växthuseffekten

Jag har ingen aning om resten av det där brevet, men gissar att de talade enbart om PC1, vilken mycket riktigt lätt får hockeyklubbsform med den metod MBH använde, men det påverkar inte slutresultatet. Om du inte förstår vad PC1 är så får du börja med att läsa lite om den PCA som användes.

First let’s dispense with the last claim, that non-centered PCA isn’t right. This point was hammered by Wegman, who was recently quoted in reader comments thus:

“The controversy of Mann’s methods lies in that the proxies are centered on the mean of the period 1902-1995, rather than on the whole time period. This mean is, thus, actually decentered low, which will cause it to exhibit a larger variance, giving it preference for being selected as the first principal component. The net effect of this decentering using the proxy data in MBH and MBH99 is to produce a “hockey stick” shape. Centering the mean is a critical factor in using the principal component methodology properly. It is not clear that Mann and associates realized the error in their methodology at the time of publication.”

Just plain wrong. Centering is the usual custom, but other choices are still valid; we can perfectly well define PCs based on variation from any “origin” rather than from the average. It fact it has distinct advantages IF the origin has particular relevance to the issue at hand. You shouldn’t just take my word for it, but you *should* take the word of Ian Jolliffe, one of the world’s foremost experts on PCA, author of a seminal book on the subject. He takes an interesting look at the centering issue in this presentation.

It has recently come to my notice that on the following website, related to this one, my views have been misrepresented, and I would therefore like to correct any wrong impression that has been given.

http://tamino.wordpress.com/2008/03/06/pca-part-4-non-centered-hockey-sticks/

An apology from the person who wrote the page would be nice.

In reacting to Wegman’s criticism of ‘decentred’ PCA, the author says that Wegman is ‘just plain wrong’ and goes on to say ‘You shouldn’t just take my word for it, but you *should* take the word of Ian Jolliffe, one of the world’s foremost experts on PCA, author of a seminal book on the subject. He takes an interesting look at the centering issue in this presentation.’ It is flattering to be recognised as a world expert, and I’d like to think that the final sentence is true, though only ‘toy’ examples were given. However there is a strong implication that I have endorsed ‘decentred PCA’. This is ‘just plain wrong’.

The link to the presentation fails, as I changed my affiliation 18 months ago, and the website where the talk lived was closed down. The talk, although no longer very recent – it was given at 9IMSC in 2004 - is still accessible as talk 6 at http://www.secamlocal.ex.ac.uk/people/staff/itj201/RecentTalks.html

It certainly does not endorse decentred PCA. Indeed I had not understood what MBH had done until a few months ago. Furthermore, the talk is distinctly cool about anything other than the usual column-centred version of PCA. It gives situations where uncentred or doubly-centred versions might conceivably be of use, but especially for uncentred analyses, these are fairly restricted special cases. It is said that for all these different centrings ‘it’s less clear what we are optimising and how to interpret the results’.

I can’t claim to have read more than a tiny fraction of the vast amount written on the controversy surrounding decentred PCA (life is too short), but from what I’ve seen, this quote is entirely appropriate for that technique. There are an awful lot of red herrings, and a fair amount of bluster, out there in the discussion I’ve seen, but my main concern is that I don’t know how to interpret the results when such a strange centring is used? Does anyone? What are you optimising? A peculiar mixture of means and variances? An argument I’ve seen is that the standard PCA and decentred PCA are simply different ways of describing/decomposing the data, so decentring is OK. But equally, if both are OK, why be perverse and choose the technique whose results are hard to interpret? Of course, given that the data appear to be non-stationary, it’s arguable whether you should be using any type of PCA.

I am by no means a climate change denier. My strong impressive is that the evidence rests on much much more than the hockey stick. It therefore seems crazy that the MBH hockey stick has been given such prominence and that a group of influential climate scientists have doggedly defended a piece of dubious statistics. Misrepresenting the views of an independent scientist does little for their case either. It gives ammunition to those who wish to discredit climate change research more generally. It is possible that there are good reasons for decentred PCA to be the technique of choice for some types of analyses and that it has some virtues that I have so far failed to grasp, but I remain sceptical.

Ian Jolliffe

2: den måttliga uppgång av medeltempen (knappt 1 grad på 70år) tror jag beror på att befolkningen har ökat, med vad det nu för med sig, naturliga variationer mellan istider samt att omloppsbanan runt solen förändrats en aning.

3: jag köper inte Co2 argumentet, såvitt jag förstår är mängden vattenånga en faktor som har långt större påverkan än den lilla mängden Co2 i atmosfären, varav utav i sin tur ungefär 5% tillskrivs "mänsklig påverkan"

Sedan blir den här typen av diskussioner alltid enkelriktad, eftersom alla som följer massan vet ju att de har rätt och att andra som ifrågasätter, per definition är foliehattar, förnekare och dåligt pålästa - själv vet jag av erfarenhet att grupptillhörighet ofta är det största självbedrägeriet.

Däremot är jag mycket orolig för miljön, med allt som dumpas i haven och naturen!

Vattenånga är den absoluta merparten av CO2 därefter kommer planetens vegetation.

Men det är ju bra att vi är överens om att våra åsikter är lika relevanta, även om dina är lite mer relevanta än mina.

Apropå att jorden är platt och förmodligen varit det väldigt länge, om det inte ifrågasatts - platthetsförnekarna hade det också jobbigt i början!