Did the IPCC Pull Its Punches In the Science Group Press Release?

The global climate models have been under criticism because of the recent plateau in the global surface temperature record, despite the smooth increase in climate model predictions. In the press release, the likelihoods are given for reaching 2 degrees C by the end of this century for the top two of the four projections for the excess radiative forcings by the end of the century. The 2 C limit over pre-industrial times is the European goal for limiting warmth, period.

However, in the press release, the IPCC did not state what the means of predictions were for the two largest radiative forcing cases, of 6.0 and 8.5 Watts per meter squared. In the largest case of 8.5, the mean temperature increase over pre-industrial times at the end of the century is 3.7 C, almost twice the 2.0 C case that was discussed in the press release. This case is the most important, since it may correspond to business as usual in taking no effective steps to lower CO2 emissions. In the 6.0 W/m^2 case, the mean was 2.2 C, only slightly more than the 2.0 C discussed. These values are given in the 36 page Summary for Policy Makers, but the full report for policy makers may not come out for almost a year.

In that same table, they give the 5% and 95% limits of the predictions, within which 90% of the predictions lie. I would have preferred the one sigma width range, within which 2/3 of the predictions fall, as being the more common and more probably useful limits. Noting that the given limits are almost symmetric, and that the 5% and 95% limits are at 1.64 sigmas from the mean, I estimated the one sigma limits as follows: for RCP 8.5, the temperature rise is 3.7 pm 0.67 C; for RCP 6.0, 2.2 pm 0.52 C; for RCP 4.5, 1.8 pm 0.46 C; and for RCP 2.6, 1.0 pm 0.43. The key difference in results is of course not the accuracy of the calculations, but our capability of achieving low greenhouse gas sources of energy and transportation.

I have the highest respect for the expertise and dedication of climate scientists, and those that have labored on the IPCC reports. But in the press release of the science working group, since there is no specification of what temperature increases greater than 2 degree C may be, the real urgency of climate change responses has been blunted. Hopefully this can be corrected in the near future by the IPCC, but also by individual climate scientists, and by the press.

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For two decades I have been active in learning about energy and the environment, and in reporting on those topics for a decade. For the last four years I have added science policy. Lately, I have been reporting on the Covid-19 pandemic of our times.
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