James Annan, more or less a supporter of catastrophic man-made global warming theory, explains how typical climate sensitivities (of the order of magnitude of 3 or more) used by catastrophists are derived (in an email to Steve McIntyre) As a reminder, climate sensitivity is the amount of temperature rise we would expect on earth from a doubling of CO2 from pre-industrial 280ppm to 560ppm.
If you want to look at things in the framework of feedback analysis, there’s a pretty clear explanation in the supplementary information to Roe and Baker’s recent Science paper. Briefly, if we have a blackbody sensitivity S0 (~1C) when everything else apart from CO2 is held fixed, then we can write the true sensitivity S as
where the f_i are the individual feedback factors arising from the other processes. If f_1 for water vapour is 0.5, then it only takes a further factor of 0.17 for clouds (f_2, say) to reach the canonical S=3C value. Of course to some extent this may look like an artefact of the way the equation is written, but it’s also a rather natural way for scientists to think about things and explains how even a modest uncertainty in individual feedbacks can cause a large uncertainty in the overall climate sensitivity.
This is the same classic feedback formula I discussed in this prior article on feedback. And Dr. Annan basically explains the origins of the 3C sensitivity the same way I have explained it to readers in the past: Sensitivity from CO2 alone is about 1C (that is S0) and feedback effects from things like water vapour and clouds triples this to three. The assumption is that the climate has very strong positive feedback.
Note the implications. Without any feedback, or feedback that was negative, we would not expect the world to heat up much more than a degree with a doubling of CO2, of which we have already seen perhaps half. This means we would only experience another half degree of warming in the next century or so. But with feedbacks, this half degree of future warming is increased to 2.5 or 3.0 or more degrees. Essentially assumptions about feedback are what separates trivial nuisance levels of warming from forecasts that are catastrophic.
Given this, it is instructive to see what Mr. Annan has to say in the same email about our knowledge of these feedbacks:
The real wild card is in the behaviour of clouds, which have a number of strong effects (both on albedo and LW trapping) and could in theory cause a large further amplification or suppression of AGW-induced warming. High thin clouds trap a lot of LW (especially at night when their albedo has no effect) and low clouds increase albedo. We really don’t know from first principles which effect is likely to dominate, we do know from first principles that these effects could be large, given our current state of knowledge. GCMs don’t do clouds very well but they do mostly (all?) suggest some further amplification from these effects. That’s really all that can be done from first principles.
In other words, scientists don’t even know the SIGN of the most important feedback, ie clouds. Of course, in a rush to build the most alarming model, they all seem to rush to the assumption that it is positive. So, yes, if the feedback is a really high positive number (something that is very unlikely in natural, long-term stable physical processes) then we get a climate catastrophe. Of course if it is small or negative, we don’t get one at all.
My Annan points to studies he claims shows climate sensitivity net of feedbacks in the past to be in the 2-3C range. Note that these are studies of climate changes tens or hundreds of thousands of years ago, as recorded imperfectly in ice and other proxies. The best data we have is of course for the last 120 years when we have measured temperature with thermometers rather than ice crystals, and the evidence of this data points to a sensitivity of at most about 1C net of feedbacks.
So to summarize:
Climate sensitivity is the temperature increase we might expect with a doubling of CO2 to 560 ppm from a pre-industrial 280ppm
Nearly every forecast you have ever seen assumes the effect of CO2 alone is about a 1C warming from this doubling. Clearly, though, you have seen higher forecasts. All of the "extra" warming in these forecasts come from positive feedback. So a sensitivity of 3C would be made up of 1C from CO2 directly that is tripled by positive feedbacks. A sensitivity of 6 or 8 still starts with the same 1C but has even higher feedbacks
Most thoughtful climate scientists will admit that we don’t know what these feedbacks are — in so many words, modelers are essentially guessing. Climate scientists don’t even know the sign (positive or negative) much less the magnitude. In most physical sciences, upon meeting such an unknown system that has been long-term stable, scientists will assume neutral to negative feedback. Climate scientists are the exception — almost all their models assume strong positive feedback.
Climate scientists point to studies of ice cores and such that serve as proxies for climate hundreds of thousands of years ago to justify positive feedbacks. But for the period of history we have the best data, ie the last 120 years, actual CO2 and measured temperature changes imply a sensitivity net of feedbacks closer to 1C, about what a reasonable person would assume from a stable process not dominated by positive feedbacks.