Is the ECS very high?

Since I’ve written about climate sensitivity quite a lot, I though I would briefly comment on Sabine Hossenfelder’s recent video, I wasn’t worried about climate change. Now I am. In this context, climate sensitivity refers to Equilibrium Climate Sensitivity (ECS), which is an estimate of how much our climate will warm if atmospheric CO₂ doubles. Technically, it is determined by doubling atmospheric CO₂ in a climate model and determining how much it warms before returning to equlibrium. 

In the IPCC’s AR6 WG1 report, the likely ECS range was 2.5^oC to 4.5^oC with a best estimate of 3^oC. A recent paper assessing Earth’s climate sensitivity using multiple lines of evidence suggested that the likely ECS range is 2.6^oC to 3.9^oC and is bounded by 2.0^oC and 5.7^oC. 

However, the most recent group of climate models have a subset with climate sensitivities higher than the top of the IPCC’s likely range (i.e., above 5^oC). This has been referred to as the “hot model” problem and it’s been suggested that these models should be given less weight.

On the other hand, Sabine’s video highlights a paper that tested one “hot” model to assess some of the processes relevant for climate sensitivity, in particular the cloud physics. These tests use short-term trends, but can still be useful for assessing the physics in climate models, as long as the model used to do the short-term forecast is the same as the one used to do the climate projection. There’s also been a recent paper by James Hansen and collaborators, suggesting that the ECS is 4.8^oC ± 1.2^oC.

So, there are some indications that the ECS may be higher than the standard IPCC range suggests. However, it is just one model that has been tested using short-term trends, and the Hansen et al. work is just one paper. There are plenty of other lines of evidence suggesting that the ECS is probably between about 2^oC and 4.5^oC with a best estimate around 3^oC. 

One should also bear in the mind that even the standard ECS range doesn’t preclude these high ECS values, they’re just regarded as not very likely. When it comes to climate change, the ECS doesn’t have to be this high to be worried, and one could still include the possibility of the ECS being this high when thinking about what to do about climate change. 

As usual, feel free to express a different view in the comments.