Can the ECS be less than 2 degrees?

I thought I would post this video of a talk by A.E. Dessler, that’s being doing the rounds. You may all already have seen it. I saw it first at Rabett Run. The video basically discusses whether or not the Equilibrium Climate Sensitivity (ECS) could be less than 2^oC. For those who don’t know, the ECS is the eventual change in global surface temperature if the atmospheric CO₂ concentrations were to double. The video explains everything quite nicely, so I don’t really have to say more, but I thought I would make a brief comment.

The video, in a sense, is a very good illustration of how one might undertake a serious discussion about global warming. One could argue that the global quantity we would most like to understand is the ECS. It tells us how much hotter the planet will eventually get given a certain rise in atmospheric CO₂ concentration. How would one step through such a discussion about the ECS? Well, I suspect that if you were to question a large number of climate scientists, almost all (if not all) would agree that – in the absence of any other changes – a doubling of atmospheric CO₂ would increase the equilibrium surface temperature by about 1^oC. This has been tested in laboratories, there are satellite measurements of changes to the outgoing spectrum, and you can play yourself with the MODTRAN code. I know there are no scientific facts, but this is probably pretty close to being one. There is very strong agreement about this.

Now CO₂ concentrations can’t simply change without other things changing too. The net change to the equilibrium temperature if CO₂ concentrations double is not going to be 1^oC. It could be higher, or it could be lower. However, past climate history tells us that there is no evidence that the net feedbacks can be negative. In other words, there is no evidence that a doubling of atmospheric CO₂ can lead to an eventual change in surface temperature of less than 1^oC (there are probably also quite good heuristic arguments as to why this isn’t likely). This, I suspect, would also be almost completely accepted by active climate scientists today. In fact, paleoclimatology evidence suggests that the ECS is probably quite high, closer to 3^oC (or higher) than to 1^oC. So, in a sense one could establish – at this point – what most (if not all) current climate scientists agree. The ECS is certainly bigger than 1^oC and almost certainly bigger than 1.5^oC.

Now we come to what the video discusses in more detail and probably does a better job of than I can do. Current evidence (and past climate history) tells us that as atmospheric temperatures rise (due to increased CO₂ concentrations) we will see increases in other greenhouse gas concentrations and in atmospheric water vapour. The influence of these are also understood well and so most climate scientists would agree that if you add the effect of increased CO₂ to the effect of increased water vapour and other greenhouse gases, the ECS rises to about 2^oC. Maybe there isn’t as much agreement about this than about the effect of CO₂ alone, but I suspect the agreement amongst climate scientists is still strong.

Now we get to areas that are not as certain. The effect of clouds, for example. Clouds can both have a positive and/or a negative feedback (increase albedo but also absorb long-wavelength radiation). However, again the evidence does not support that the net effect of clouds is to provide a negative feedback that would reduce the ECS below 2^oC. Increased surface temperatures will also reduce polar ice and reduce albedo, increasing the absorption of solar energy. Effectively, most evidence suggests that the feedbacks are positive and that the ECS has to be bigger than 2^oC. So, there is “complete” agreement about the ECS being greater than 1^oC, very strong agreement that it is bigger than 1.5^oC, and most climate scientists would probably agree that it is likely to be greater than 2^oC.

However, this isn’t really what I’m trying to get at here. If people were genuinely interested in understanding the science, they would be willing to listen to the type of argument presented in this video, which quite nicely shows what scientists think they understand well and are confident about and what they understand less well and are less confident about. Doing this would give a better sense of what we can be confident will happen, what might happen but we’re less sure about, and what is unlikely to happen. This doesn’t mean you have to ultimately agree with the scientists. They could be wrong. But if they are, they’re wrong about some fairly fundamental things and one would need to show that these fundamental things were wrong if one was to significantly contradict our current scientific understanding of global warming.

So, in my opinion, the debate isn’t possible if people aren’t even willing to actually listen to and understand the evidence. I’ve been writing about this for about 5 months now and I’ve been trying to remain civil and have been trying to engage with anyone who’s willing to discuss this. I don’t even care if we never actually reach an agreement. However, the more I engage, the more I encounter people who completely dismiss the evidence without even attempting to show that they actually understand it, or have tried to understand it. My intention is certainly to remain civil, but this will partly be done by simply ignoring those who’ve illustrated that they’re unwilling to consider that the evidence might have any merit. An ever increasing list of people, unfortunately.