What is the actual strength of the water vapor feedback? There has been some confusion about this over the years. The original Dessler report of 2008 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2008GL035333#d37306572 came up with a very specific number, 2.04 W/m2 per degree (C) of warming. This was the result of research that has never been seriously challenged, yet Dessler himself was a source of confusion at the time. Using the 75% rule, which has likewise stood for years without being seriously challenged, two W/m2 should result in about 1.5C of temperature increase, with only a small margin of error. From the very beginning, Dessler has often been quoted as saying things like “We now think the water vapor feedback is extraordinarily strong, capable of doubling the warming due to carbon dioxide alone.” Those are his own words, taken from an early news release (https://www.eurekalert.org/news-releases/551246). They have been commonly accepted and repeated by others, probably more so than the research data itself.
As we all know by now, a difference of one-half degree in global temperature averages is quite significant in terms of damaging effects. For that reason any confusion over the size of the feedback effect should be resolved as fully as possible, with an understanding that the 2.04 watts number is not necessarily conclusive. Yesterday, when comparing the two radiative forcing charts, I mentioned the unusual increase in CO2 forcing—which, unlike all the others, includes the water vapor feedback effect—over a period of just 8 years. The forcing rise amounts to about 15%. The actual CO2 ppm numbers show a rise of only 5% over these years, which suggests that the IPCC has quietly made an upward adjustment in the strength of the feedback component.. How big an adjustment? In the absence of better data, here is how I make the calculation, based on use of the well-accepted figure of very close to 1.0C for how much a doubling of CO2 concentration will affect global temperatures. Using the logarithmic scale, we would have accomplished half of a double, or 0.5C, over pre-industrial levels at about 395ppm. This means we have yet to reach +0.6C, and thus, again using the 75% rule, CO2 by itself—no feedbacks allowed—cannot be credited with a post-1750 forcing increase as high as a full 0.8 W/m2. The new forcing chart appears to have a total reading somewhere above 2.0 W/m2 for the CO2 combination, maybe not quite 2.1. The difference in forcing between CO2 alone and the feedback added on to it is must be greater than 1.2 watts in the IPCC’s calculation. That makes the addition at least 150% more than starting point below 0.8 watts. Switching to degrees, the spread between over 0.9C and under 0.6C also leaves us with an increase of more than 150%. Dessler’s original data of 2.04 W/m2 has thus been resolved in a way that makes it look conservative. He may well be surprised to hear about it.
Have I handled this properly? If you think not, please send me an explanation, via email to the address near the top, so I can post a retraction. This is a pretty scary number, if everything works out to more than 150%, as appears to be the case. I’m sure the people who do climate modelling have already been informed. They may already be applying the new number to all future projections of CO2 increases. That would help to explain the recent warnings of anxiety that there is no time at all left to be wasted in carbon removal efforts. The so-called carbon budget must itself be shrinking as a result of the expanded feedback.
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Carl