"proving" Dangerous Anthropogenic Cimate Change

I entangled myself in "discussion" with Richard Tol over on irisheconomy.ie. I say discussion, but Richard seems quick to jump to use of epithets.

The issue I (quite politely) took Richard up on was a statement he made about the efficacy of the hypothesis of Anthropogenic Climate Change. Note I have remove the "Dangerous", because I don't believe that the issue had been extended to that (additionally hoary) question.

Richard's contention was that because, using the DACC hypothesis, we could mathematically model past temperature changes we had all the "proof" we need.

Here is where I introduce some terminology and two important concepts:

NECESSARY condition:
This is something that must be true (i.e. is necessary) in order to support a particular hypothesis. As in test of the statement "Serena Williams won the Ladies Singles title at Wimbledon in 1999". If I wanted to test the truth of that statement, I could start with a necessary test - is Serena Williams a woman? Answer: yes (cut the giggling up the back there Thompson!). What does that information prove? It proves that Serena William could have won the Ladies Singles title. Another NECESSARY condition would be was she entered that year? But again, this only proves that she could have won, not did.

So what do we need to move from a helpful could to more useful did. For that we need a SUFFICIENT condition. If this condition holds then that is all we need to prove the claim. So, did Serena Williams collect the All England Club Ladies Singles trophy in 1999? If she did, then we can say she did win.

So what use are NECESSARY and SUFFICIENT conditions? In mathematics, statistics or everyday logic a NECESSARY condition is something you can use to falsify an hypothesis. You can rule things out. A very useful tool and vital in science. A SUFFICIENT condition is used to move an hypothesis in the territory of a theory - not only can we not rule it out as an explanation, but we must rule it in. Exclusively.

Back to Richard and the hypothesis of DACC. Richard presented a claim that modelling past climate is SUFFICIENT to test the hypothesis. The problem is that this is only a NECESSARY condition. I have posted on this blog before about modelling hypotheses and the long and short is that only by making correct predictions (correctly modelling future and otherwise unknowable events) with our model can we meet NECESSARY conditions.

Don't believe me? Here is someone who isn't an "anonymous lizard" who agrees with me.

So there you have it and it deserves repeating, and repeating and repeating. DACC will not be "proven" until they start accurately modelling future climatic outcomes. And the track record on that isn't good to date.