Real-world efficacies
Jan 21, 2016
Bishop Hill in Climate: Models, Climate: sensitivity

BH readers have long been aware that low estimates of climate sensitivity based on observations are little affected by the pause and will therefore be little affected by this year's El Nino either. The discussion in this area will continue to focus on subjects like efficacies and aerosols. Marvel and colleagues are apparently formulating a response to Nic Lewis's critique, which is sure to be interesting. While we're waiting, I have been passed a preprint of a forthcoming Piers Forster paper in Annual Review of Earth and Planetary Sciences, which makes some interesting observations on the efficacy debate.

Shindell (2014) suggested that a higher transient efficacy of aerosol forcing could cause an underestimate of ECS in the historic record. Depending on the efficacy assumed for the aerosol forcing, the underestimate of ECS could be as much as 1.1 K (Kummer & Dessler 2014). The Shindell (2014) and Shindell et al. (2015) studies could only draw speculative conclusions due to a lack of forcing information within the historic CMIP5 model integrations analyzed. Marvel et al. (2015) explored this properly by computing efficacies and ERFs for individual forcing terms. They showed that within the GISS model, accounting for efficacies in the historical response could raise the implied best estimate of ECS from the historical period from around 2.0 K to around 3.0 K. However, my preliminary investigations (not shown) could not find any evidence of this effect in dedicated aerosol and carbon dioxide perturbation experiments within two climate models [Community Climate System Model version 4 (CCSM4) and Hadley Centre Global Environment Model version 2 (HadGEM2)]. Nevertheless, it could still be important, as the efficacies of different ERF mechanisms have not been properly assessed across models. The groundbreaking efficacy study by Hansen et al. (2005) needs updating: Efficacies need evaluating across other models and for transient change.

One has to ask though: if efficacies similar to those in the GISS model are eventually unearthed elsewhere, what does this tell us about the real climate system?

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