So far, I have not said very much about the “attribution question.” That question is basically the following:
How much, if any, of the recent increase in global surface temperatures is attributable to increased levels of CO2 in the atmosphere?
Although this is an interesting and important question, I do not think it is a critical question. Even if all of the warming of the last 50-60 years is attributable to CO2, it does not necessarily follow that the CAGW Hypothesis is correct, since such warming could have resulted from moderate climatic sensitivity to CO2.
On the other hand, if little or none of the recent warming is attributable to CO2, it doesn’t necessarily mean that the CAGW Hypothesis is wrong, but it does put a rather big dent in the hypothesis, since it suggests that the climate is significantly less sensitive to CO2 than the warmist position claims.
In any event, it’s worth noting that the warmist case for attribution is surprisingly weak. In large part it rests on untested computer simulations. I touched on why this is a problem in an earlier post:
But this morning, I saw essentially the same problem expressed in words. Phil Jones, a leading warmist, has apparently responded to the following question with the answer that follows:
H – If you agree that there were similar periods of warming since 1850 to the current period, and that the MWP is under debate, what factors convince you that recent warming has been largely man-made?
The fact that we can’t explain the warming from the 1950s by solar and volcanic forcing . . . .
* * *
So essentially, the warmists’ argument is that (1) they do not know what could have caused recent warming besides CO2; so (2) it must have been because of CO2.
When you think about it, this is an incredibly weak argument. A lot of things aren’t known about the climate. For example, what caused the Little Ice Age? And if we don’t know what caused the Little Ice Age, how do know that the same thing (working in reverse) didn’t cause recent warming?
It’s even possible that recent warming (as well as the Little Ice Age) were the unpredictable result of fluctuations in a chaotic system. Nobody knows if it will be snowing in Boston on March 1 because the weather is too chaotic and unpredictible. There seems to be an unspoken assumption that weather evens out over time like spins of a roulette wheel making climate predictable over periods of 50 or 100 years. But why should we believe this? To me, it’s at least as likely that climate too is unpredictable on 50 or 100 year time scales.
So what does all this say about the CAGW Hypothesis? It seems to me it weakens it a bit. If we cannot even nail down the attribution question, it’s that much harder to have confidence in future effects of CO2 on the climate. But then again, the CAGW Hypothesis was never very strong to begin with.