Mike’s AGU Trick

By Steve McIntyre

There has been considerable recent discussion of the fact that observations have been running cooler than models – see, for example, Lucia’s discussion of IPCC AR5 SOD Figure 9.8 (see here). However, Michael Mann at AGU took an entirely different line. Mann asserted that observations were running as hot or hotter than models. Mann’s assertion was taken even further by Naomi Oreskes, who asserted that climate models were under-estimating relative to observations. Oreskes squarely placed the blame for the supposed underestimates on climate skeptics.

In today’s post, I’ll look closely at the illustration in Mann’s AGU presentation, an illustration that gave an entirely different impression than the figure in the IPCC draft report. The reason for the difference can be traced to what I’ve termed here as “Mike’s AGU Trick”.

The IPCC AR5 SOD Graphic

An excerpt from IPCC AR5 SOD Figure 9.8 is shown below, clearly showing that the multimodel ensemble (red) is running noticeably hotter than observations (black). In my opinion, the difference is not merely “noticeable” but “statistically significant”, but that’s a story for a different day.

Figure 1. Excerpt from IPCC AR5 SOD Figure 9.8, comparing model ensemble to observations.

Mann’s AGU Presentation

However, Mann at AGU asserted that observations were running as hot or hotter than models. Mann’s model comparandum was Hansen’s Scenario B, which is widely regarded as the most reasonable scenario to use to interpret Hansen’s “forecast” – see past CA points on this issue.

Figure 2. Mann’s AGU slide comparing observations to Hansen’s 1988 Scenario B projection.

I took the photo with a new phone, with which I was then unfamiliar and unfortunately can only provide a muddy zoom on the graphic. Despite the muddiness, you can see that observations (red) appear to cohere with Hansen’s 1988 forecast (blue). In the loop below, I’ve overplotted data for models and observations to show more clearly what was shown to the AGU audience. (There was a bit of detective work in figuring this out – see below.) Click on the figure below for a loop illustrating the components of the zoomed figure). (Note: see below for Mann’s use of his AGU Trick to hide the divergence in a presentation a few months earlier at Rutgers).

Figure 3. Blowup of Mann’s slide comparing Hansen’s Scenario B to observations. Blue – Hansen’s Scenario B; red – “observations”.

Mann’s AGU slide obviously has a completely different rhetorical impression than the IPCC graphic. Whereas the discrepancy between observations and models was immediately noticeable in the IPCC graphic, Mann’s AGU graphic showed no such discrepancy. There were two reasons for the difference, the combination of which I’ll call “Mike’s AGU Trick” and will analyse below.

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Read the full post here: http://climateaudit.org/2013/03/02/mikes-agu-trick/

Reading the comments is also entertaining, particularly in watching Nick Stokes trying to defend the Mann, while McIntyre wipes the floor with his argument.

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