The Catastrophe Stems Completely From Feedback
Over at Climate Skeptic, I dissect climate models to show that the future warming in the models from CO2 alone is not much more than 0.5C. All the catastrophe comes from positive feedbacks that modelers assume dominate the climate, an odd assumption for such a long-term stable system. I summarize as follows:
- 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.
Posted on January 2, 2008 at 10:54 PM | Permalink
By the way, Arnold Kling is trying to track down exactly what these positive feedbacks are that are in the models. Keep an eye on his blog over the next few weeks.
Some of the feedbacks do have evidence for them, and so are not just guesses. Of course, if you posit such a feedback, you also have to explain how the earth is not already much warmer.
Posted by: Daublin | Jan 4, 2008 10:24:51 AM
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