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.
  • 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.

    http://econlog.econlib.org/