Posted on 18 September 2024 by Ken Rice

This is a re-post from And Then There's Physics

I wrote a post a little while ago commenting on a Sabine Hossenfelder video suggesting that she was now worried about climate change because the Equilibrium Climate Sensitivity (ECS) could be much higher than most estimates have suggested. I wasn’t too taken with Sabine’s arguments, and there were others who were also somewhat critical.

Sabine has since posted a response to the various reactions. I think this response is rather unfortunate and doesn’t really engage with the criticisms of her earlier video. She suggests that Andrew Dessler and Zeke Hausfather have lost touch with reality because they say:

Arguments over ECS are distractions. Whether it’s 3C or 5C is a bit like whether a firing squad has 6 rifleman or 10.

It might be a bit flippant, but I think they’re probably just being realistic. Whatever the ECS, the goal will be to rapidly decarbonise our societies and the rate at which we do so will probably be determined more by societal and political factors than by whether the ECS is 3°C or 5°C.

Sabine then goes on to criticise those who highlight that there are many lines of evidence and that we shouldn’t focus too much on individual studies. Sabine argues that she is making a different point and suggests that climate scientists are suffering from confirmation bias. The high-ECS ‘hot’ models have already been used in IPCC reports and arguing now that they should use climate sensitivity to screen out models implies an unjustified bias against the possibility that the ECS could actually be as high as these models suggest.

Essentially, once we’ve started collecting data and doing some analysis, we shouldn’t then change how the data is used, or modify the analysis, simply because the results aren’t consistent with previous expectations. However, this isn’t quite that simple. This is an ensemble of models that are developed to try and understand the physical climate system.

We can look at how well these models compare with observations. The ‘hot’ models tend to have poor agreement with historical temperatures and struggle to reproduce the last glacial maximum. If we select models based on their transient climate response (TCR) they do a better job of matching observations. So, the argument that we should screen models isn’t simply because they have a higher ECS than might be expected.

Of course, Sabine is correct that we can’t actually rule out high ECS values. The latest IPCC report says that the “best estimate of ECS is 3°C, the likely range is 2.5°C to 4°C, and the very likely range is 2°C to 5°C”. This certainly doesn’t rule out an ECS between 4oC and 5oC and doesn’t even entirely rule out values of 5oC and above, even if it suggests these are very unlikely.

Given that the highest risk is from the low-probability high-impact events, it seems entirely reasonable to be particularly concerned about the possibility that the ECS is something like 5oC, or higher. None of the information presented by climate scientists has ever really suggested that people shouldn’t do so. However, in general, the broader societal response has not been focussed on this possibility. I doubt that this is going to change anytime soon, and it’s certainly not because climate scientists have failed to highlight the potential risks associated with global warming and climate change.