Dr. Tom Wigley apparently made some comments about my comments on the Gristmill blog:

Dr. Tom Wigley's comments, and my responses

These were my responses to his comments. (I want to have my responses on my blog, in addition to the Gristmill blog.)

Dr. Wigley apparently writes, "markbahner said that the probability of warming less than 1.4C was 50% and the probability of warming greater than 5C was zero."

That's close, but my exact words were, "The simple fact is that there is approximately a 50/50 chance that the warming will be less than 1.4 deg C...and there is virtually no chance (far less than 1 percent) that the warming will be over 5 deg C."

Dr. Wigley continues, regarding the "projections" in the IPCC Third Assessment Report (TAR), "IPCC did not assign a probability to the 1.4-5.8C range. It was recognized that to so may have been useful, but it was beyond the state of the science (insofar as IPCC can only review the science, not do new science)."

That extraordinary comment provides compelling evidence of the deep scientific pathology of the current state of climate "science." I've personally developed two sets of my own probabilistic predictions for methane atmospheric concentrations, CO2 emissions and atmospheric concentrations, and resultant lower tropospheric temperature increases. The second set is here:

My own probabilistic predictions were developed in my spare time, almost certainly with less than 100 hours of research, thought, and calculations, and only a pocket calculator. By 2001, when the IPCC published the TAR, there had obviously been two previous assessment reports, with more than a full decade having passed since the first assessment report was published in 1990. In that time, hundreds, if not thousands, of people were working full time on climate research. Dr. Wigley is stating that the entire climate change community was unable to come up with probabilistic predictions in over a decade of research, including countless conferences (in some very nice locales) and a great many powerful computers running climate simulations. I'm sorry, but if a simple set of probabilistic predictions for methane atmspheric concentrations, CO2 emissions and atmospheric concentrations, and resultant lower tropospheric temperature increases was beyond the IPCC and the climate change community after more than a full decade of research by hundreds or thousands of people, it's simply because the IPCC and climate change community aren't very interested in doing science.

Dr. Wigley continues, "So Sarah Raper and I did the appropriate probabilistic calculations...We found that there was a non-zero probability of warming less than 1.4C and a non-zero (but smaller) probability of warming above 5.8C. You can estimate the probabilities from the above-cited paper. We found the 90% C.I. to be 1.68C to 4.87C."

I commend Drs. Wigley and Raper (WR) for injecting a modest amount of science into the IPCC process for developing "projections" based on "scenarios." As he reports, the WR paper calculated a 5% probability of warming less than 1.68C, a 50% probability of warming less than 3.06C, and a 95% probability of warming less than 4.87C. Therefore, WR and I agree that the probabililty of warming of 5 deg C or more is very remote (with WR estimating the odds at more than 20 to 1, and me at more than 100 to 1).

The differences between their predictions and mine regard the probability of warming near 1.4 deg C and 3.1 deg C. They estimate less than 5 percent chance of warming less than 1.4 deg C, whereas I estimate approximately a 50 percent chance of warming less than 1.4 deg C. Also, they estimate approximately a 50 percent chance of warming more than 3.1 deg C, whereas I estimate the probability of warming more than 3.1 deg C at less than 5 percent.

How do these differences in predictions arise? Well, the main reason for the differences is that Wigley and Raper assumed that all IPCC TAR scenarios had equal probability of occurrence, whereas I estimated that the most probable climate forcing for the 21st century would be somewhat less than the B1 scenario:

IPCC TAR scenarios and resultant temperature increases

So WR assume equal probability for all IPCC TAR scenarios. Is this a scientifically valid assumption? No, it is not. This is not a close call. Even a layperson can see that the assumption of equal probability is scientifically invalid. James Hansen's Keeling Lecture compared various IPCC scenarios with actual increases in CO2 and methane:

James Hansen's Keeling Lecture, see pages 42-44

Increases since 1990 of both methane and CO2 have been at the very bottom of all IPCC TAR scenarios. The WR assumption of equal probability for all scenarios is like assuming a straight-A student has equal probability of getting an A, B, C, D, or F in the next class he or she takes.

To summarize: The IPCC TAR scenarios are completely invalid, as a matter of science. The fact that the TAR had no probabilistic estimates is clear evidence of the pathology of the current state of climate "science." The Wigley and Raper paper is a significant improvement on the IPCC TAR. However, the WR assumption of equal probabilities for all scenarios is scientifically invalid. If WR had used a "50 percent probability" forcing near or less than the B1 scenario, as would be appropriate, their calculated probability of warming of more than 3.1 deg C would be well below 50 percent, and their calculated probability of warming less than 1.4 deg C would be much higher than 5 percent. Their calculations would produce results near mine, in fact.

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