The Rahmstorf et al 2007 comparison of observations to outcome is invalid for the 1990-2000 decade, because the projected emissions, concentrations, forcings and temperatures for each of the scenarios was originally different. The original model outputs for these variables has been replaced, for all six of the illustrative scenarios, with a simple average of four scenarios.

The example I gave of the differences between scenarios in the trends in energy output from coal shows that the original estimates/projections of fossil CO2 emissions would also have differed between scenarios. And the translation of any given level of cumulative emissions into concentration levels would also have differed initially, because five different models were used to produce the six scenarios. And the translation of atmospheric concentration levels into forcings, and from forcings into global mean temperatures, would also have differed between the scenarios produced by the different modelling teams. .

As the IPCC removed all of these differences BY ASSUMPTION for the 1990-2000 decade, Rahmstorf et al 2007 did not in fact evaluate model projections against observations for this period. As for the subsequent decade, all six scenarios projected INCREASES in global mean temperature. This doesn’t square with the observations.

Hi Ian,

You write, “Unfortunately, the authors of Rahmstorf et al misunderstood the basis of the ‘projections’ for 1990-2000 in the TAR. As explained in the Special Report on Emissions Scenarios (2000), ‘the 1990 and 2000 emissions scenarios were standardized in all the SRES scenarios, with emissions diverging after the year 2000′ (Box 5.1, p. 243).”

But if one goes to the Rahmstorf et al. paper in Science, one sees (if one squints hard enough!) that the temperature projections for the IPCC scenarios only diverge after the year 2000 (i.e., they are all the same from 1990 to 2000). This can be better seen from the (apparent) reprint/blowup on the “Skeptical Science” website.

So the temperature projections attributed to the IPCC scenarios in the Science paper only diverge after the year 2000, which seems completely consistent with the fact that the forcings in the IPCC scenarios only diverged after the year 2000.

Or am I missing something?

David Benson, Thanks for your suggestion that I read the recent ‘Skeptical Science’ posting on ‘Comparing IPCC Scenarios to Observations’, which draws on Rahmstoorf [sic] et al (2007) to argue, inter alia, that the IPCC’s 2001 projections ‘underestimated temperature rise with observations warmer than all projections.’

Unfortunately, the authors of Rahmstorf et al misunderstood the basis of the ‘projections’ for 1990-2000 in the TAR. As explained in the Special Report on Emissions Scenarios (2000), ‘the 1990 and 2000 emissions scenarios were standardized in all the SRES scenarios, with emissions diverging after the year 2000′ (Box 5.1, p. 243).

The SRES went on to explain that ‘The standardized scenarios share the same values for emissions in both 1990 and 2000′, and that ‘The 1990 and 2000 emissions estimates for all gases, except SO2, were set to be equal to the initial values in the unadjusted four marker scenarios.’ So, for example, the projections for emissions AND FOR TEMPERATURE for the A1FI scenario, which was not a marker, were set BY ASSUMPTION as equal to those of the average of the four markers.

As an indication of the possible scale of the effect of this procedure, the A1FI scenario assumed that the global energy supply from coal would increase by 30% between 1990 and 2000, compared with an increase of about 5% for the A1B and B1 scenarios.
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So when Rahmstorf et al conclude that ALL OF the IPCC projections underestimated the temperature rise, they are not in fact evaluating the performance of models against observations – at least as far as the 1990-2000 period is concerned (the greater part of the comparison). In fact, the comparison is invalid in the context in which it was used in the article published in ‘Science’ on 4 May 2007.

I’m not confident that there is a strong correlation.

http://rankexploits.com/musings/2008/ipcc-falsification-can-we-identify-trends-that-are-not-consistent-with-noisy-data/

Compare that to the responses of Romm and Connolley. There is an incredible difference in tone and content.

David B. we updated the Rahmstorf et al. analysis here in January. Lucia’s analysis is completely consistent with that.

http://tamino.wordpress.com/2007/12/16/wiggles/

http://tamino.wordpress.com/2007/08/31/garbage-is-forever/

http://tamino.wordpress.com/2008/01/09/dead-heat/

http://tamino.wordpress.com/2008/01/24/giss-ncdc-hadcru/

http://www.skepticalscience.com/ Comparing-IPCC-projections-to-observations.htm