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Steve McIntyre On Cowtan and Way (2013)

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Steve McIntyre, Climate Audit

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.

cowtanway2013 vs ipcc ar5sod figure 1_5
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.

cowtanway2013_difference between hadcru4 and cw2013
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.

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