Kate Marvel and colleagues have just published an interesting paper on how [i]nternal variability and disequilibrium confound estimates of climate sensitivity from observations. Essentially they compare three different ways of estimating Equilibrium Climate Sensitivity (ECS):
- atmosphere-only simulations with observed sea surface temperatures, sea ice concentrations, and natural and anthropogenic forcings,
- historical simulations using the same forcings, but in which sea surface temperatures and sea ice are predicted, not prescribed,
- and abrupt 4xCO2 simulations that are run to equilibrium.
recent decades appear to have experienced a pattern of sea surface temperatures that excited unusually negative feedbacks in tropical marine low clouds, leading to an even lower estimate of climate sensitivity than would have been expected under more usual historical conditions.
I don’t know that I need to say much more. This is another paper suggesting that observationally-based ECS estimates tend to be biased low, partly because feedbacks are likely to become stronger in the future, and partly because the pattern of sea surface warming in recent decades has lead to less warming than might have been typically expected. It doesn’t mean that this is necessarily right, but does suggest that
ECS estimates inferred from recent observations are not only biased, but do not necessarily provide any simple constraint on future climate sensitivity.