Arguments
In 2020, a large group of scientists published a paper in which they used multiple lines of evidence to assess Earth’s climate sensitivity. The lines of evidence they used were the physical processes that determine climate sensitivity, the historical climate record, and the paleoclimate record. The key results were:
Climate Sensitivity
Posted on 1 September 2025 by Ken Rice
This is a re-post from And Then There's Physics
- The 66% range is 2.6–3.9 K for the Baseline calculation and remains within 2.3–4.5 K under robustness tests.
- the corresponding 5–95% ranges are 2.3–4.7 K, bounded by 2.0–5.7 K.
- all three lines of evidence are difficult to reconcile with an equilibrium climate sensitivity, characterised by an effective sensitivity S, below 2K.
- the paleoclimate evidence provides the strongest evidence against S > 4.5 K.
All of this seems quite reasonable. A likely range from just above 2K to about 4.5K, little evidence to support an equilibrium climate sensitivity below 2K, and evidence against it being above 4.5K.
Unsurprisingly, however, Nic Lewis has views. He has a published a response in which he objectively combines climate sensitivity evidence and finds that
[t]he estimates of long-term climate sensitivity are much lower and better constrained (median 2.16 °C, 17–83% range 1.75–2.7 °C, 5–95% range 1.55–3.2 °C)
and that
[t]his sensitivity to the assumptions employed implies that climate sensitivity remains difficult to ascertain, and that values between 1.5 °C and 2 °C are quite plausible.
As far as I can tell, the differences are mostly due to different choices about the various parameters. Given that different choices of values can give such large variations in the results, does seem to suggest that climate sensitivity remains difficult to ascertain. However, it’s less clear that values between 1.5 °C and 2 °C are quite plausible, although it does depend on what one means by plausible.
I realise that one can select a set of potentially plausible parameters that will give values between 1.5°C and 2°C, but given that we’ve already warmed by ~1.5oC, that the planetary energy imbalance has recently been above 1 Wm-2, and that we haven’t yet reached in change in anthropogenic forcing equivalent to a doubling of atmospheric CO2, values in this range don’t seem particularly plausible.
I do like Nic Lewis’s work and I have learned quite a lot by working through some of it. However, I do think a weakness is a reluctance to properly interrogate why his work seems to suggest values for climate sensitivity that are lower than many other experts would regard as plausible.
I think there’s a tendency to think that if you’ve justified all your assumptions, carefully chosen your parameters, and ensured that the methodology is robust, that the results should then stand. In my view, it’s always worth sanity checking the results. I realise that you have to be careful of not introducing additional biases, but you also have to be careful of trusting a result simply because the analysis is supposedly objective.
"values between 1.5°C and 2°C are quite plausible." So are values between 3.5°C and 4.5°C. In either case, the issue isn't their plausibility, it's their permanence. You don't just throw away this petri dish once it reveals it's secrets to you. Not unless you've terraformed yourself another planet.
It's amazing how contrarians often focus on one tail of the distribution and argue in favour of it, while pretending the other taill does not exist.
Ken Rice:
How many articles containg "Clime Sensitiyity" in their titles have you posted on your And There'e Phusics website?
John @ 3:
Ken may or may not respond himself here, but whenever SkS reposts his articles it is worth going to his web site to see the comments on the originals.
At the end of the original post over at Ken's, you can see that his articles are grouped into classes and tagged with various markers. Under this post ("Climate Sensitivity"), you can see that it is under the group "Climate Sensitivity", and that is also has tags for "ECS" and "Equilibrium Climate Sensitivity" (which are the same thing in terms of climate science, but different tags).
Links to those three groupings of Ken's posts are:
Climate Sensitivity
ECS
Equilibrium Climate Sensitivity
There are dozens hundreds of posts listed under these categories (which overlap)...
....but to answer your question ("how many?") directly, it's either:
(I guess that's not really an either/or question, though. It's probably both too many and not enough.)
Bob: "It's amazing how contrarians often focus on one tail of the distribution and argue in favour of it, while pretending the other taill does not exist."
Indeed. Judith Curry'smuncertainty monster is a two-tailed beast. For every potential outsized gain in the left tail, there is a mirror-image risk of an outsized loss in the right tail. The contrarian pretends this right tail doesn't exist or is negligible. That's also related to the gambler's fallacy, where a blind eye is attached to losing.
Yet, we're screwed when it comes to removing the CO2 already in the atmosphere. It's 100% certain that it will go up in the future. No way it will go down, physically impossible due to the properties of CO2 and the fat right tail of time to sequestration.
I would like to go back to an older post here on the tropospheric hot spot. I know the discussion, especially like Santer et al 2005. I do not really care if the hot spot is there or not. The interesting fact is that it should be there, and what jumps into my eyes when I see this graph..
Roughly speaking the graph suggests a 1.5K increase in Ts in the tropics, and about a 3K increase ot Tz, if you assume the average emission altitude to be in 450-400mb range in the tropics. I know, some emissions will occur from below, some from above, but the higher the more warming, so it should largely cancel out anyway.
This graph indirectly implies a very low climate sensitivity. If you assume Tz ~261K in the tropics and a Planck Feedback of 3.6W/m2 there, you can do some math. For Tz +3K, going from 261 to 264, we can calculate..
(264^4-261^4)*5.67e-8 = 12.3W/m2 delta OLR
Planck Feedback would only amount to 1.5 * 3.6 = 5.4W
So you would get a negative lapse rate feedback of 12.3 -5.4 = 6.9W/m2
Or normalized per K of warming of 6.9/1.5 = 4.6W/m2.
This figure is insanely large, way larger than all positive feedbacks combined. Eventhough I only used ballpark estimates, the fundamental problem is simply the huge increase in Tz. Of course one could say it is mainly a thing of the tropics, but the "hot spot" expands well beyond 30° latitude, and the tropics between 30° S and N account for 1/2 of the planet. Even if you just halve it, you are still left with 4.6/2 = 2.3W/m2 of negative lapse rate feedback.
Although the "hot spot" does not seem to materialize, the fact that it should in theory, and the logical consequences to it, is kind of a non-negligible detail when considering climate sensitivity.
[BL] If you want to "go back to an older post", then the proper thing to do is to go back to that post. And read it in its entirety, along with the comments. Here is a direct link to the Advanced tab on the tropospheric hot spot:
https://skepticalscience.com/tropospheric-hot-spot-advanced.htm
The figure that you present is included in that post. And its significance is discussed in detail. It does not imply what you think it implies.
Please go to the correct thread, read the post, and place any further comments there.