the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Oct 2025
Emergent constraints on climate sensitivity and recent record-breaking warm years
Abstract. The Earth’s climate sensitivity remains a significant source of uncertainty in climate projections. A key metric is the Transient Climate Response (TCR), which incorporates aspects of both the Equilibrium Climate Sensitivity (ECS) and ocean heat uptake, and correlates well with historical global warming simulated by Earth System Models (ESMs). CMIP6 ESMs display a wider range of TCR values compared to earlier phases, with many exceeding the IPCC AR6 very likely (90 % confidence) range of 1.2–2.4 K. These high-sensitivity models also predict that warming will exceed the 2 °C Paris climate agreement limit, even under the relatively low emissions SSP1-2.6 scenario. Record global temperatures in 2023 and 2024 highlight how close the world already is to 1.5 °C of warming, raising doubts about whether the 2 °C limit remains within reach. Here, we use the latest observational data to update emergent constraints on TCR and projected warming. We estimate a TCR of 1.81 K with a very likely range of 1.28 K to 2.33 K, which represents a small increase compared to estimates that use observational data through to 2019. Furthermore, we find that warming projections constrained by data through to 2024 fall within the low to mid-range of CMIP6 ESM projections for both the mid- and late-21st century, indicating that limiting global warming to below 2 °C remains feasible.
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Status: open (until 24 Dec 2025)
- RC1: 'Comment on egusphere-2025-4899', Anonymous Referee #1, 12 Nov 2025 reply
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- 1
Review of "Emergent constraints on climate sensitivity and recent record-breaking warm years" by Boardman and co-authors.
In this paper the authors repeat a now well established emergent constraint on TCR and future warming with a few more years of data, notably the record breaking 2023 and 2024. The result is slightly higher medians and slightly more constrained ranges, though nothing Earth shattering, which is the point of the paper.
I have the following two major points, which should not be very hard to address:
1) In numerous places results are compared to IPCC AR6 as if that was some gold standard or independent estimate. But that is nothing but an assessment of literature available at the time of the cut-off date, and this literature includes studies of the kind used here (Jimenez, Nijsse, Tokarska), along with estimates based on longer term instrumental record warming. I would strongly suggest to go through the paper and carefully revise all these instances.
2) The method of smoothing down weighs the warm 2023 and 2024 years. I would like to see results without any smoothing, comparing up to 2019 to an analysis including 2024. This is important since the point of the paper is that these years do not change the results substantially, but the chosen analysis method implicitly down weighs these very years. It would also be good to try using the much simpler delta method, and years of Jimenez-de-la-Cuesta who stopped in 2005 to avoid stitching errors with scenario runs. Ideally, since the inclusion of the recent warm years is the central theme of the study, it would be good to completely take apart how much of the change originates from the inclusion of 2023 and 2024, how much comes from the updated data sources (e.g. HadCRUT5), and if possible how much is due to methods.
After addressing these two major points, and considering the below list of minor points I see no reason to uphold publication of this work.
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3, also pattern effects matter for TCR
16, as per IPCC AR6 the surface air temperature is used
23, "Both metrics can be computed"
32, not often recognised, but 'very likely' means greater than 90 percent confidence.
35-36, this statement misrepresents the cited paper.
68, I would have called emissions and estimated aerosol forcing stable for decades from the 1970s onwards. Perhaps be more specific about the timing of the decline, and also provide references.
74, paleoclimate estimates are also observational.
78, consider deleting "Oceanic Ni.o Index of +2.0 K occurring"
78-79, not sure how this statement is relevant, could be deleted.
80-82, This is a problematic statement. It is unclear who is meant to perceive, who is applying pressure, and who in in 'climate science' is under pressure. Either things should be clarified with documented data and/or scientific references, or I would suggest deleting.
107-108, The sentence seems misplaced as it opens up a new theme, or perhaps starting a new paragraph would be good for readability?
Table 1 caption contains a double negation.
190, please replace 'significant' with something else, e.g. 'excessive' or 'stronger than observed'. The word 'significant' is usually used in statistical tests.
Table 2, The row "IPCC AR6 (Unconstrained)" is misleading. These values taken from Chapter 7 are constrained by observations, including information from the above mentioned studies.
Table 2, perhaps include results from Jimenez-de-la-Cuesta (2019).
201-203, this is unsurprising since these two studies used more or less the same models and methods.
225, the stated difference is incorrect, according to Table 2 the difference is 0.21 K.
233, 'it was not used'
235-236, I would strongly suggest to delete this statement as it is unsubstantiated speculation.
239-240, This is not a standardised initialisation, the initial conditions are simply some state taken from the piControl and it happens to be the first.
246-251, this text could be deleted as it contains nothing concrete.
252-258, the purpose/conclusion of this paragraph is unclear, it would benefit from rewriting and focusing on physics. What you expect here is rather irrelevant.
259-260, rather unsurprising.
Conclusions are pleasantly concise and clear! After revising the manuscript you may wish to include a conclusion regarding how much upward revision is due to the inclusion of 2023 and 2024.