A New Paper Makes Low Sensitivity Models More Implausible

A paper was published this week that argues that low-sensitivity models do a poor job of reproducing CERES-derived EEI trends. In the words of the paper, the authors used CMIP6 models "to illustrate that low climate sensitivity models have an EEI trend behavior that is inconsistent with the satellite-derived EEI trend." Even though models with an ECS near 3°C do a good job of reproducing current warming, CMIP6 models often differ in the in EEI trends. For instance,

The CERES data show a stronger trend in EEI than the multi-model CMIP6 mean and higher EEI in 2023 than any of the CMIP6 models. However, for individual CMIP6 models and ensembles, EEI is comparable to or higher at other periods than the CERES value in 2023.
The difference in trends can be seen by comparing the red CERES line to the black CMIP6 model mean. Even though the interannual variability in the CMIP6 models is consistent CERES observations, the observed trend in EEI, especially since about 2010, is higher than the model average.
The difference is more pronounced when comparing the CERES-observed trends in LW-EEI and SW-EEI with those reproduced by models. Low sensitivity models tend to show weak SW and LW trends in EEI/°C, while higher sensitivity models produce a stronger LW response to GHG-forcings. A few of these also produce a stronger SW contribution to EEI, which can be attributed to reductions in low cloud cover and/or aerosol pollution, not to mention smaller surface areas of snow and ice.
CERES-observed trends are 3.2W/m^2/K for SW-EEI and -1.7 W/m^2/K for LW-EE, indicating that EEI has increased at ~1.5 W/m^2 per 1°C warming since 2001. HadCRUT5 shows 0.5°C warming from 2001 to 2023, indicating an increase in EEI of 0.75°C, which is consistent with observations. But this is more consistent with higher sensitivity models. The paper finds that the higher sensitivity models with a strong SW and LW response to forcings perform better at reproducing CERES-derived observations in EEI trends.
The negative LW EEI trend is driven by surface temperature increase and moderated if GHGs are increasing during the simulations. Cloud changes further amplify the clear-sky relationship between LW EEI and SW EEI trends.
This study appears to be the latest in a series of recent analyses suggesting that the central ECS estimate of 3°C may actually be on the low end of plausible values, and the actual value may be closer to the upper end of the IPCC's likely range of 2.5°C to 4°C. In fact, the models that performed best by the metric discussed in this paper had sensitivities of ~4°C or higher. They conclude,
The trends in net EEl and surface warming trend over the first two decades of this century provide little constraint on climate sensitivity. However, we present robust findings for trends in LW and SW EEI. These trends, and their relationship to climate sensitivity, are more physically based than the net EEI trend. ... All models, given as the 99.999% level of the distribution, with an ECS of 2.93 K or below, are outside the CERES range.
Until recently, I haven't been paid very close attention to arguments that ECS may exceed 4°C. Warming seems to be occurring at rates consistent with ECS between 3°C to 3.5°C. However, this depends on the strength of SW feedbacks (like low cloud cover) and forcings from reflective aerosols being relatively small. If ECS is high and forcings from reflective aerosols also strong, we might see comparable warming rates through the 20th century, but as we continue to reduce aerosol pollution, we would get a stronger warming response, and this is consistent with recent observations in both temperature and EEI. I'm not not fully convinced this is a case yet. One nagging thought I have about this paper is that the CERES-observed EEI trends end in 2023 at a peak, and since we're only looking at the last couple decades, the observed trend in EEI may become smaller in the following months and years. Also, HadCRUT5 shows 1.12°C from 1970 to 2024, so if the same 1.5 W/m^2/°C trend has held since 1970, we'd expect to see a 1.68 W/m^2 increase in EEI, but that looks to be a bit high (we were 1.4 W/m^2 at the end of 2024). I don't know how much to make of that. However, I'm becoming convinced that I need to consider high ECS values more plausible than I used to. ECS values between 3.5°C and 4°C are more plausible to me now than I would have considered them even just a year ago.

References:

[1] Gunnar Myhre et al. ,Observed trend in Earth energy imbalance may provide a constraint for low climate sensitivity models.Science388,1210-1213(2025).DOI:10.1126/science.adt0647



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