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TAR vs SAR: The Triumph of Storyline over Science
Wigley, T.M.L. and Raper, S.C.B. 2002. Reasons for larger warming projections in the IPCC Third Assessment Report. Journal of Climate 15: 2945-2952.

In an attempt to quantify changes in the world's climate due to anthropogenic activities expected to occur between 1990 and 2100, the Second Assessment Report (SAR) of the Intergovernmental Panel on Climate Change (IPCC) projected a mean global temperature increase of 0.9 to 3.5C under a "no climate policy" scenario that assumes humanity does nothing to intentionally influence earth's climate in the interim; while the IPCC's Third Assessment Report (TAR) projected a mean global temperature increase of 1.4 to 5.8C for the same scenario. Wigley and Raper thus set out to answer the obvious question raised by these dramatically different results: "Why are the more recent projections so much larger?"

What was done
The authors begin by stating that the different results of the IPCC's second and third assessment reports arise from differences in the two major components of the reports' climate model calculations: (1) differences in emissions scenarios, which are derived from what Wigley and Raper refer to as "storylines," and (2) differences in science, which one would hope would be due to advancements in science. The authors then describe these differences and how they altered the conclusions of the two reports.

What was learned
With respect to the high-end warming of the two assessment reports -- which is what climate alarmists always cite in support of their call for immediate implementation of draconian regulations to reduce anthropogenic CO2 emissions -- Wigley and Raper report that the projected 1990-2100 warming rose by 2.3C (from a value of 3.5C in the SAR to a value of 5.8C in the TAR), but that advancements in science were responsible for only 4% of that increased warming, which translates to an additional temperature increase of a mere 0.1C between the time of the two reports. All the rest of the extra warming projected in the TAR report (2.2 out of 2.3C) was thus the result of nothing more than a scarier worst-case "storyline."

What it means
Either there were no advancements in climate science between the second and third assessment reports of the IPCC (which we know to be false) or the various advancements that were made resulted in some climatic factors becoming more important and other climatic factors becoming less important, so that the total scientific effort that occurred between the publication of the SAR and the TAR left the final high-end result of the SAR essentially unaltered. In other words, it was not science that gave the climate alarmists something to crow about when the catastrophic high-end warming result of the TAR was released to the public, it was storytelling.

Reviewed 2 April 2003