Thread by @ClimateBen

Last year I was surprised the IPCC decided climate sensitivity of 4.1-4.5°C is no longer considered likely. The likely range is now 2.5-4°C. I'd understood key recent papers showed this range was likely, not to mention Dr James Hansen's scepticism of models & suggestion of 6°C.🧵

1. A key IPCC paper:

If CO₂ hits 560 ppm & stays there 'there is up to an 18% chance that temperatures will rise to 4.5°C above pre-industrial levels'.

'this “likely range” has narrowed to, at most, 2.3°C to 4.5°C – or possibly an even narrower range*.'theconversation.com/just-how-sensi…

2. Sherwood et al. (2020): 'used multiple lines of evidence' to argue that ECS above 4.5C is unlikely.
frontiersin.org/articles/10.33…

A climate scientist did offer me an explanation of the narrowed 2.5-4C range but I rather struggled with the rationale.

3. The new IPCC range of 2.5C-4C is 'fully in line' with a study suggesting a range of 2.6 to 4.2C:

carbonbrief.org/in-depth-qa-th…

Ok, but wouldn't it perhaps have been prudent to stick with 4.5C as an upper limit? Well, I'm not a scientist.

4. "Cloud feedbacks are still too uncertain for comfort one way or another. We need climate models that simulate clouds much better than those we have.”

'The authors estimate a central ECS value of 3.2C, with a 17% to 83% range of 2.6 to 4.2C.'

July 2021
carbonbrief.org/clouds-study-f…

5. ECS is likely to be between 2.6C and 4.1C.
July 2020
carbonbrief.org/guest-post-why…

6. 'For decades, evidence from observed warming, Earth’s distant past and climate models have suggested that ECS falls in a likely range of 2.0-4.5C. But the latest generation of climate models suggest the ECS might be even higher.'

Oct 2020
carbonbrief.org/guest-post-how…

6b. 'As for whether climate sensitivities above 5C are plausible, a complete answer to that question requires further testing
(we evaluated cloud feedbacks from 150-year simulations using version 2 of the Community Earth System Model (CESM2), which estimates a high ECS of 5.3C.)'

7. "Our study implies that CESM2's climate sensitivity of 5.3 C is likely too high.'
April 2020
sciencedaily.com/releases/2020/…

8. 'The predecessor to CESM2, the CESM1.2 model, did a remarkably good job of simulating temperatures during the Early Eocene, according to the researchers. It has an equilibrium climate sensitivity of 4.2 C.'

news.umich.edu/some-of-the-la…

9. June 2021

'the paper “adds to a long line of research showing that clouds are complicated and climate models simulate them poorly”.'

Many scientists expect at least 560 ppm of atmospheric CO2 this century, so ECS of 2.5-4C is horrific anyway.

carbonbrief.org/cooling-effect…

10. See replies to this tweet:

https://twitter.com/ClimateBen/status/1428044676490305536?t=5S_Wwke0isv6bAJtnu66Ig&s=19

11.

According to Dr James Hansen, staying at or above 400 ppm for long enough will take us to 3.5°C (climate sensitivity of 6C).

It looks like we'll be above 400 ppm for centuries.

Thread:

https://twitter.com/ClimateBen/status/1437144308562472968?t=Ov2np8VOMUeJ7XOB-CDFQA&s=19

12. We're heading for environnemental ruin at 2C by the 2040s according to both the IPCC and James Hansen, so debating climate sensitivity may seem somewhat academic to many.