This is something I've been meaning to blog about for some time. It comes up a lot in the context of the hurricane wars, over at RPJnr's blog. A recent comment of his provides a nice opening:
[Quotes RPjr lecturing on the null hypothesis tested via detection of a climate signal]
There is, however, an entirely different but equally valid approach that could also be used from the outset, which is: what is our estimate of the magnitude of the effect? The critical distinction is that the null hypothesis has no particularly priviledged position in this approach.(Emphasis added.)
This distinction between detection and estimation is related to that between a frequentist and Bayesian approach to probability....The answers that these two approaches provide may be very different in any given situation, and neither is necessarily right or wrong a priori, but it is surely self-evident that the Bayesian approach is more relevant to decision-making. If we have any reasonable expectation that certain policies would have particular bad effects, it would be ridiculous to wait until such effects could be shown to have occurred at some arbitrary level of statistical significance (that's not a point specific to climate change, of course).
....It is trivial to create situations in which a currently undetectable effect can be reasonably estimated to be large, and the converse is equally possible - an easily detectable (statistically significant) influence may be wholly irrelevant in practical terms. I suspect that this forms a large part of the difference in presentation between various parties in the hurricane debate - the evidence may not yet rule out the null hypothesis of no effect, but some people estimate that AGW is likely to have a substantial effect (even if the ill-defined error bars on their estimate do not exclude zero). In principle, exactly the same evidence could support both of these conclusions, although I don't personally know enough about hurricanes to make a definitive statement in that particular case.
It is amusing to see Roger, very much at the sharp end of policy-relevant work, promoting the scientifically "pure" but practically less useful detection/frequentist approach rather than the more appropriate estimation/Bayesian angle. It's not surprising, although perhaps a little disappointing, that the IPCC explicitly endorses that view. But by placing the null hypothesis in a priviledged position from which it can only be dislodged by a mountain of observational evidence, this approach provides a strong inbuilt bias for the status quo which cannot be justified on any rational decision-theoretic grounds.
IMO this needs to be repeated every time RPjr repeats the same tired argument in a new format and a new paper. Or maybe in a shortened format - "who cares about detection, it's estimation that counts." Certainly when Roger says:
When you next hear someone tell you that worthy and useful efforts to mitigate climate change will lead to fewer natural disasters, remember these numbers and instead focus on what we can control.You know he's being disingenuous and that everything he said before that about detection is irrelevant to whether disasters are reasons to do something to control climate change.
So William says "[RPjr's work] addresses the question 'is climate change going to cause disasters so expensive that we'd be better off not changing the climate *because of that*'?" Well, it depends. In his academic work, it doesn't address that question, at all, it's about detection. When he turns to a public venue, then he uses the same stuff to make very questionable policy claims.