There have been a couple of recent papers presenting analyses that claim to have narrowed the likely range for equilibrium climate sensitivity (ECS). One is Dessler et al. (currently a discussion paper under review) which suggests that the 500hPa tropical temperature better describes the planet’s energy balance and infers an ECS of 2.1K to 3.9K. The other is Cox et al. who use variability of temperature about long-term historical warming to constrain the ECS to 2.2K to 3.4K. Both suggest a narrower range than that suggested by the most recent IPCC report (1.5K to 4.5K).
James Annan has a post which suggests that these new papers are interesting but that there may be unaccounted for uncertainties. I largely agree and won’t say any more myself. I was, however, going to mention a few aspects of this that I think are relevant.
I thought how this was framed in the media was somewhat unfortunate. For example, Yes, global warming will be bad. But these scientists say it won’t reach the worst-case scenario. It does indeed seem that these studies are suggesting that the worst case scenarios might be less likely than we had previously thought. However, the public debate seems to be dominated by those who think everything will be fine (Lukewarmers) and those who are mostly in the middle of the mainstream. In fact, there is often quite a lot of pushback against any who present worst case scenarios.
The significance of these new studies to the public climate debate therefore seems to be that they largely rule out the Lukewarmer position. Yet, this is not really how they’ve been presented. One prominent Lukewarmer has even claimed that these studies are a vindication for Lukewarmers. Presenting these studies as having ruled out the worst case scenarios, rather than the best case scenarios, probably hasn’t shifted the public climate debate very much.
Another issue is that there is often an apparent confusion between climate sensitivity and how much we will warm. Yes, climate sensitivity is relevant, but so is how much we emit. These new studies have potentially narrowed the range, but they don’t really change the best estimate (about 3K). The more extreme scenarios (both low and high) may be less likely, but we can still potentially emit enough to warm substantially. In a sense, how much we will probably warm is largely unchanged.
Additionally, a common way to quantify how we could achieve some temperature target is to present a carbon budget; the amount of CO2 we can emit if we want some chance of staying below the target. Given that these new analsyses don’t really change the ECS best estimate, the carbon budget that would give us a 50:50 chance of staying below some target should be unchanged. Often, however, the carbon budget is presented as giving us a 66% chance of staying below some temperature target.
Given that the range might now be narrower, this might suggest that the carbon budget for a 66% chance might be slightly bigger. However, it’s not only the uncertainty in climate sensivity that constrains this; there are also carbon cycle uncertainties (i.e., what fraction of our emissions will be taken up by the natural sinks). Hence, I suspect that the impact on the carbon budget framework might be small (I might be wrong about this).
Also, although I’m mostly in favour of working within a carbon budget framework (it’s a pretty straightforward metric) I do sometimes think that there might be a better way to present it (to be fair, I don’t have any good suggestions as to what this should be). A carbon budget that gives us a 66% chance of staying below some temperature target doesn’t mean that we will do so 2/3 of the time, and fail 1/3 of the time. There is only one outcome; we will either stay below the temperature target, or we will not.
If we think that there is now a bigger chance of staying below some temperature target, it’s not clear to me that we should then adjust the carbon budget. Maybe it would be better to present it as there now being a bigger chance of succeeding, than suggesting that we can now emit more while still having the same chance.
Okay, I think that’s long enough, so I’ll stop there. The latter part of this post is not as clear as I would have liked, so if anyone has other suggestions as to what we should do given these new results, feel free to make them in the comments.
Andrew Dessler’s comment is worth reading. I had forgotten that their paper was more presenting what might be a better way to constrain the energy balance, rather presenting a firm estimate for an ECS range.