Monday, October 24, 2016

Recent Estimates of the Climate Sensitivity

Another excerpt from our literature review:

Estimates of the climate sensitivity have been the focus of ongoing debate with widely differing estimates (Armour, 2016) and notable differences between observation and model based sensitivity estimates. The consensus in the IPCC 5th Assessment Report (Bindoff et al., 2013) is that the equilibrium climate sensitivity (ECS) falls in the range of 1.5-4.5 K with more than 66% probability. The transient climate response (TCR) falls in the range 1-2.5 K with more than 66% probability. Armour (2016) notes that the range of ECS supported by recent observations is 1-4 K with a best estimate of around 2 K and the TCR is estimated at 0.9-2.0 K. This suggests that climate model based estimates are too sensitive.

Richardson et al. (2016) note that sea surface temperature measurements measure water rather than air temperature, which has warmed faster. Additionally, the most poorly measured regions on Earth, such as the Arctic, have also warmed the most. Richardson et al. (2016) process the CMIP5 model output in the same way as the HADCRUT4 temperature series is constructed – using seawater temperatures and under-sampling some regions. They infer an observation-based best estimate for TCR of 1.66 K, with a 5–95% range of 1.0–3.3 K, consistent with the climate models considered in the IPCC 5th Assessment Report.

Marvel et al. (2016) argue that the efficacy of other forcings is typically less than that of greenhouse gases so that total radiative forcing is less than standard calculations estimate. When single-forcing experiment results are reported to estimate these efficacies, and TCR and ECS are estimated from observed twentieth-century warming, both TCR and ECS estimates are revised upward to 1.7K and to 2.6-3.0 K, depending on the feedbacks included. Armour (2016) highlights the joint (multiplicative) importance of the Richardson et al. (2016) and the Marvel et al. (2016) studies, which together should raise observational ECS by 60%, which reconciles the discrepancy between observation and model based estimates.


Armour, K. C., 2016. Projection and prediction: Climate sensitivity on the rise. Nature Climate Change 6, 896–897.

Bindoff, N. L., Stott, P. A., K. AchutaRao, M., Allen, M. R., Gillett, N., Gutzler, D., Hansingo, K., Hegerl, G., Hu, Y., Jain, S., Mokhov, I. I., Overland, J., Perlwitz, J., Sebbari, R., Zhang, X., 2013: Detection and attribution of climate change: from global to regional. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Marvel, K., Schmidt, G. A., Miller, R. L., Nazarenko, L., 2016. Implications for climate sensitivity from the response to individual forcings. Nature Climate Change 6(4), 386-389.

Richardson, M., Cowtan, K., Hawkins, E., Stolpe, M. B., 2016. Reconciled climate response estimates from climate models and the energy budget of Earth, Nature Climate Change 6, 931-935.

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