Calculating ECS from the Indicators or Global Climate Change

A couple months ago, I saw that the "Indicators of Global Climate Change 2024" (IGCC24) was published, which is an annual update on the state of global climate and how it is changing in response to human and natural forcings. The data provides the most up to date information on human emissions and associated changes in radiative forcings as well as an evaluation of the change in temperature. A graphical summary of this is below.

From IGCC24

I thought it would be helpful to show how updated values for warming, EEI, and radiative forcings would impact calculations for Equilibrium Climate Sensitivity (ECS). Here's their accounting of the various natural and anthropogenic forcings. Note that solar forcing in the diagram above is only for 2024 only and so disagrees with the accounting in the table below.

Change in Radiative Forcing
Forcer1750-2024
CO22.33 ± 0.28W/m²
CH40.57 ± 0.11W/m²
N2O0.23 ± 0.03W/m²
Halogenated GHGs0.41 ± 0.08W/m²
Ozone0.5 ± 0.25W/m²
Stratospheric water vapor0.05 ± 0.06W/m²
Aerosol–radiation interactions-0.22 ± 0.23W/m²
Aerosol–cloud interactions-0.85 ± 0.6W/m²
Land use change-0.19 ± 0.14W/m²
Light-absorbing particles on snow and ice0.08 ± 0.11W/m²
Contrails and contrail-induced cirrus0.06 ± 0.04W/m²
Total anthropogenic2.97 ± 0.8W/m²
Solar irradiance0.01 ± 0.07W/m²

IGCC24 estimated the 2024 GMST anomaly above the 1850-1900 mean to be 1.52 ± 0.13°C, but it would be unfair to use this value for ECS calculations, since 2024 was significantly warmer than the overall trendline. The IGCC24 estimated anthropogenic warming above the 1850-1900 mean to be 1.36 ± 0.34°C with warming currently occurring at 0.27°C/decade. IGCC24 only provided a 13 year mean for EEI, which I think is too long a time frame to compare to the forcing and GMST values included in the report, so I'm using the 4-year running mean for EEI at the end of 2024 from Leon Simons, which was 1.4 W/m². This shows that EEI has increased by a full 1 W/m² in just 21 years.

from Leon Simons on X

The most recent estimate for the ERF for 2xCO2 is 3.93 ± 0.47 W/m². With this information we can do a quick and simple calculation for ECS using the following energy balance equation:

λ = ΔT/(ΔF - EEI)
λ = 1.36/(2.98-1.4) = 0.86°C/W/m²

ECS = λ*F2xCO2
ECS = 0.86*3.93 = 3.4°C

The uncertainty in this calculation comes from the uncertainties for the forcings, temperature and EEI estimates above. This calculated result obviously doesn't supplant the more rigorous calculations in the peer-reviewed literature, such as those by Sherwood et al 2020. Nor is it an argument against recent observations from satellite measurements that Myhre 2025 showed are more consistent with higher ECS values. Rather, this just shows that observational evidence continues to support the fact that warming is occurring at rates that are consistent with ECS being ~3°C or higher.



References:

[1] Forster, P. M., Smith, C., Walsh, T., Lamb, W. F., Lamboll, R., Cassou, C., Hauser, M., Hausfather, Z., Lee, J.-Y., Palmer, M. D., von Schuckmann, K., Slangen, A. B. A., Szopa, S., Trewin, B., Yun, J., Gillett, N. P., Jenkins, S., Matthews, H. D., Raghavan, K., Ribes, A., Rogelj, J., Rosen, D., Zhang, X., Allen, M., Aleluia Reis, L., Andrew, R. M., Betts, R. A., Borger, A., Broersma, J. A., Burgess, S. N., Cheng, L., Friedlingstein, P., Domingues, C. M., Gambarini, M., Gasser, T., GĂ¼tschow, J., Ishii, M., Kadow, C., Kennedy, J., Killick, R. E., Krummel, P. B., LinĂ©, A., Monselesan, D. P., Morice, C., MĂ¼hle, J., Naik, V., Peters, G. P., Pirani, A., Pongratz, J., Minx, J. C., Rigby, M., Rohde, R., Savita, A., Seneviratne, S. I., Thorne, P., Wells, C., Western, L. M., van der Werf, G. R., Wijffels, S. E., Masson-Delmotte, V., and Zhai, P.: Indicators of Global Climate Change 2024: annual update of key indicators of the state of the climate system and human influence, Earth Syst. Sci. Data, 17, 2641–2680, https://doi.org/10.5194/essd-17-2641-2025, 2025.

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