There has been some discussion of Cowtan and Way 2013 take on HadCRUT4 at Lucia’s, Judy Curry’s, Nick Stokes and elsewhere. HadCRUt4 has run cooler than other datasets (including UAH satellite) in recent years. Cowtan and Way observe that HadCRU does not estimate temperature in many Arctic gridcells. Because Arctic temperatures have risen more than low-latitude temperatures, they state that recent HadCRU temperatures are biased low. (Since GISS extrapolates into the Arctic, it is less affected by this bias.)

In the context of IPCC SOD FIgure 1.5 (or similar comparison of models and observations), CW13 is slightly warmer than HadCRUT4 but the difference is small relative to the discrepancy between models and observations; the CW13 variation is also outside the Figure 1.5 envelope.



Figure 1. Cowtan and Way 2013 hybrid plotted onto IPCC AR5SOD Figure 1.5

Next, here is a simple plot showing the difference between the CW13 hybrid and HadCRUT 4. Up to the end of 2005, there was a zero trend between the two; the difference has arisen entirely since 2005.



Figure 2. Delta between CW Hybrid (basis 1961-1990) and HadCRUT4.

In their online commentary, Cowtan and Way praise Hansen for being the first person to report the effect of missing Arctic data on global temperature. However, no material discrepancy had arisen between their index and the HadCRUT4 index as of 2005 so that Hansen was, according to Cowtan and Way’s own data, observing a discrepancy that had not yet arisen, making their following praise to Hansen seem somewhat premature:

Probably the first mention of an underestimation of recent warming due to poor Arctic coverage comes from Hansen in 2006, who sought to explain why the NASA temperature data showed 2005 as being a record breaking warm year, in contrast to the Met Office temperature record.

That there are continuing defects in HadCRU methodology should hardly come as a surprise to CA readers. Attempts to reconcile and/or explain discrepancies between HadCRU and GISS also seem worthwhile to me.

Nor do efforts to apply kriging seem misplaced to me in principle. On the contrary, for someone with experience in ore reserves, it seems entirely natural e.g. see for example, some of Jeff Id’s discussion of Antarctica. I notice that their methodology results in changes to the Central England gridcell. While I don’t object to the use of kriging or similar methods to estimate values in missing gridcells, I don’t see any benefit to altering values in known gridcells, if that’s what’s happening here. (I haven’t parsed their methods and don’t plan to do so at this time.)

Co-author Way was an active participant at the secret SKS forum, where he actively fomented conspiracy theory allegations. Uniquely among participants in the secret SKS forum, he conceded that Climate Audit was frequently correct in its observations (“The fact of the matter is that a lot of the points he [McIntyre] brings up are valid”) and urged care in contradicting Climate Audit (“I wouldn’t want to go up against that group, between them there is a lot of statistical power to manipulate and make the data say what it needs to say.”) [Update Nov 21: While Way did wrongly associate me with conspiracy theory on a couple of occasions, including a tasteless accusation of being a “conspiracy wackjob”, the vast majority of his language is temperate and reasonable and shows remarkable appreciation of the statistical points of our critique, with the slurs being a sort of incidental sideswipe. See the next post.]

Update: Here is annotation of IPCC AR5 SOD Figure 11.12 comparing observations to CMIP5 4.5, with both HadCRU4 outside envelope.



Bart Verheggen compared CMIP5 RCP8.5 to observations, saying that “recent observations are at the low side of the CMIP5 model range”.



However, my own calculations using RCP8.5 show that observations are outside the envelope. Verheggen’s calculations are not consistent with similar calculations by others (including IPCC) and I presume that he’s made an error somewhere.



