Is climate sensitivity really that high?

Climate sensitivity is the shorthand phrase for how much temperatures will rise if atmospheric CO₂ doubles.  The estimates vary but up to now they've been between 2 and 4.5 degrees C.  Pre-industrial CO₂ averaged 280 ppm (parts per million), and it is now about 417 ppm and rising by 2.4 ppm on average each year, and this rate of increase is accelerating.  Global temperatures are rising by 0.2 degrees C since 1970, but this rate of increase also appears to be accelerating.

A post from .... and Then There's Physics:

Can climate sensitivity be really high?  The answer to the question in my post title is – unfortunately – yes. The generally accepted likely range for equilibrium climate sensitivity (ECS) is 2oC – 4.5oC. This doesn’t mean that it has to fall within this range, it means that it probably falls within this range. There is still a chance that it could be below 2oC and a chance that it could be greater than 4.5oC. However, there is a difference between it possibly being higher than 4.5o and this being likely.

There’s been quite a lot of recent coverage of studies suggesting that the ECS may be higher than 5oC. My understanding is that one reason for this increase in the ECS in some climate models is an enhanced short-wavelength cloud feedback in these models. There are also some indications that these high-sensitivity models do a better job of representing some of the cloud processes than was the case for the earlier generation of models.

However, there are also indications that the high-sensitivity models struggle to fit the historical temperature record, and that lower sensitivity models (at least in terms of the transient climate responses) better match some observational constraints. As I understand it, it’s also difficult to reconcile these very high climate sensitivity estimates with paleoclimatological constraints.

So, I think it is interesting, and somewhat concerning, that some of the newest generation of climate models are suggesting that the equilibrium climate sensitivity (ECS) could be higher than 5oC. That these newer models seem to also represent some relevant processes better than the previous generation does provide some indications that it could indeed be that high. However, it’s also possible that these models are poorly representing some other processes that may be unrealistically inflating their ECS values.

Given that there are also other lines of evidence suggesting that the ECS is unlikely to be as high as 5oC makes me think that we should be cautious of accepting these high ECS estimates just yet. I do think it’s worth being aware that it could be this high, but I don’t think it’s yet time to change that the ECS is likely to fall between 2oC and 4.5oC, with it probably lying somewhere near 3oC.

Links:Climate worst-case scenarios may not go far enough, cloud data shows – Guardian article about the new high climate sensitivity studies.

Short-term tests validate long-term estimates of climate change – Nature article about a recent study that tested one of these high climate sensitivity models.

CMIP6 – some of my recent posts about the newest generation of climate models.

Meanwhile, whatever the longer-term response of the system to rising CO₂ levels, the rate of increase in temperature seems to be accelerating:

If the 20-year trend continues, we'll reach 2 degrees above pre-industrial temperatures by 2050.  If the 10-year trend continues, that'll happen by 2035.  The problem is that the steeper path is more likely because of the steady increase in CO₂ emissions. 

We really need to start cutting emissions now.  They'll fall in 2020 over the year as a whole because of the covid crash, but we must do as much as we can to ensure the rebound in 2021 in emissions is as small as possible, while aiming for zero emissions by 2050.