Comment on "Can uncertainty in climate sensitivity be narrowed further?" by Sherwood and Forest (2024)

Abstract. This comment addresses assertions made by Sherwood and Forest (2024) [SF24] regarding the narrowing of the range of equilibrium climate sensitivity (ECS), particularly at the low end. SF24 challenged a previous study by Lewis (2022) [L22] that found a narrower and substantially lower ECS level. This comment clarifies that, contrary to SF24's claims, L22 did not rule out a high ECS level based on historical evidence, and did identify and correct errors in Sherwood et al. (2020). Those errors included use of an invalid likelihood estimation method that, ironically, substantially underestimated likelihood at high ECS levels for their historical evidence. This comment also discusses the role of priors in Bayesian ECS estimation and explains why the subjective Bayesian approach favoured by SF24 risks producing unreliable inference for uncertain parameters such as ECS. Finally, the importance of considering structural uncertainties in climate models, particularly concerning tropical warming patterns, is extended beyond the points raised by SF24. Such uncertainties could affect ECS estimation not only from historical period evidence but also from climate process understanding, paleoclimate data and emergent constraints, but seem more likely to suggest existing ECS estimates are too high than too low.