I recently wrote a post about the Brown & Caldeira paper which suggests that climate sensitivity may be on the high side of the range. Rather predictably, Nic Lewis has a guest post on Climate Etc in which he looks at the Brown & Caldeira analysis and claims that global warming will not be greater than we thought.

I think this claim is simply wrong. Even if he has found some issue with the analysis in Brown & Caldeira, that still would not justify a claim that global warming will not be greater than we thought. At a basic level, all that the Brown & Caldeira paper is suggesting is that the models that most closely match some observed properties of the climate system tend to have climate sensitivities on the high side of the range. This also isn’t the first time this has been suggested.

One could argue that there are also studies that suggest that climate sensitivity could be on the low side of the range. Also, as Ray Pierrehumbert says in this comment, it’s not necessarily the case that models that do best at representing short-term fluctuations will also do best when it comes to the feedbacks that determine ECS. One might then argue that we should really just stick with something like the IPCC range for climate sensitivity; it is almost certainly a reasonable representation of what is likely.

The debate, in my view, therefore should not really be about whether climate sensitivity could be high, or low, but what we should do in case it is high (which, to be clear, is not to say that people shouldn’t study this). The impact of climate change almost certainly increases with warming and, if we can quantify this impact, probably increases non-linearly. In other words, the impact of 3oC of warming is likely to be more than twice as great as the impact of 1.5oC. If addressing climate change carried more risks than the risks associated with climate sensitivity being high, then maybe we would do nothing and hope it was low. This, however, seems very unlikely.

One of the key things is simply to emit less CO 2 into the atmosphere than we could, and this seems quite possible, if maybe not easy. There are many possible ways to do so and if one is concerned about big government and too much regulation, then just promote something like a carbon tax. Even if you’re not particularly concerned, one could still consider that an economically optimal pathway suggests peaking emissions by about 2040 and not increasing them much between now and then (see Figure 2).

Unless one assumes that climate sensitivity is very low and ocean acidification will have no adverse impacts (and there’s a word for those who think this), I can’t see a scenario under which we shouldn’t be thinking about how reduce our emission of CO 2 into the atmosphere. The irony, I think, is that what probably leads people to avoid considering ways to reduce our emissions (big government, too much regulation, interfering in the market,….) is precisely what we will get if climate sensitivity does turn out to be high and we decide, in the future, that we need to rapidly reduce our emissions. This doesn’t seem very sensible.