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Politics : The Environmentalist Thread -- Ignore unavailable to you. Want to Upgrade?

To: Alastair McIntosh who wrote (7051)	7/24/2006 10:00:28 AM
From: Wharf Rat	Respond to of 36917

There was no significance; didn't change a thing. Wegman had been tasked solely to evaluate whether the McIntyre and McKitrick (2005) (MM05) criticism of Mann, Bradley and Hughes (1998) (MBH) had statistical merit. That is, was their narrow point on the impacts of centering on the first principal component (PC) correct? He was pointedly not asked whether it made any difference to the final MBH reconstruction and so he did not attempt to evaluate that. Since no one has ever disputed MM05's arithmetic (only their inferences), he along with the everyone else found that, yes, centering conventions make a difference to the first PC. This was acknowledged way back when and so should not come as a surprise... ...But, and this is where the missing piece comes in, no-one (with sole and impressive exception of Hans von Storch during the Q&A) went on to mention what the effect of the PC centering changes would have had on the final reconstruction - that is, after all the N. American PCs had been put in with the other data and used to make the hemispheric mean temperature estimate. Beacuse, let's face it, it was the final reconstruction that got everyone's attention.Von Storch got it absolutely right - it would make no practical difference at all. This is what MBH would have looked like using centered PC analysis: Red is the original MBH emulation and green is the calculation using centered PC analysis (and additionally removing one of the less well replicated tree ring series). (Calculations are from Wahl and Amman (2006), after their fig. 5d). Pretty much the same variability and the same 'hockey stick'. We'd be very surprised if anyone thought that this would have made any difference to either the conclusions or the subsequent use of the MBH results. In fact, it's even more simple, Throw out that PC analysis step completely, what do you get? Again, red is the original MBH98 multiproxy+PC analysis, green is if the raw data are used directly (with no PC analysis at all). (This comes from Rutherford et al (2005) and uses a different methodology - RegEM - to calibrate paleoclimate proxy data against the modern instrumental record, but that doesn't make any difference for this point). Why doesn't it make any difference? It's because the PC analysis was used to encapsulate all of the statistically relevant information in the N. American tree ring network and so whatever patterns are in there they will always influence the final reconstruction. So what would have happened to the MBH results if Wegman and his colleagues had been consulted on PC centering conventions at the time? Absolutely nothing. Can we all get on with something more interesting now? realclimate.org