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Climate: Turning the Temperature Down?

The Economist has had a sneak peak at a draft of the next blockbuster climate report of the Intergovernmental Panel on Climate Change (IPCC), which is due out in September.  These reports come in three large volumes, typically over 3,000 pages in all.  Which means no one—probably literally no one—reads the entire thing (certainly not a single journalist), and hence the importance of the short (typically 25 pages) “Summary for Policy Makers” (which means: “Scary Stuff for Headline Writers and Climateers”), which is a political document reflecting what the head UN bureaucrats want emphasized.  So it is entirely predictable that the headlines in September will be variations on the old standby, “World Still Doomed: Women, Children and Minorities Hardest Hit.”  (We’ll do a contest here in Power Line for the best cliché-ridden headline.)  The former head of the UN climate circus is already promising that the next IPCC report will “scare the wits out of everyone,” though it is hard to see how this is possible, since the climateers and their camp followers are clearly witless to begin with.

So it will be interesting to see whether and how the media handles (or obscures) what is likely to be the real story in the details of the next report, i.e., the lowering of the official estimates of climate sensitivity.  As The Economist explains:

The sign in question is about climate sensitivity. This is the measure used by researchers of how much they expect the world’s average temperature to increase in response to particular increases in levels of carbon dioxide in the atmosphere. According to one table from the unpublished report, which was seen by The Economist (below), at CO2 concentrations of between 425 parts per million and 485 ppm, temperatures in 2100 would be 1.3-1.7°C above their pre-industrial levels. That seems lower than the IPCC’s previous assessment, made in 2007. Then, it thought concentrations of 445-490 ppm were likely to result in a rise in temperature of 2.0-2.4°C. . .

Both the 2007 IPCC report and a previous draft of the new assessment reflected earlier views on the matter by saying that the standard measure of climate sensitivity (the likely rise in equilibrium temperature in response to a doubling of CO2 concentration) was between 2°C and 4.5°C, with 3°C the most probable figure. In the new draft, the lower end of the range has been reduced to 1.5°C and the “most likely” figure has been scrapped. That seems to reflect a growing sense that climate sensitivity may have been overestimated in the past and that the science is too uncertain to justify a single estimate of future rises.

If this does turn out to be the case, it would have significant implications for policy. Many countries’ climate policies are guided by the IPCC’s findings. They are usually based on the idea (deriving in part from the IPCC) that global temperatures must not be allowed to increase by more than 2°C above pre-industrial levels, and that in order to ensure this CO2 concentrations should not rise above 450 ppm. The draft table casts doubt on how solid the link really is between 450 ppm and a 2°C rise.

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