Just a quick-fire post on climate sensitivity, because that is, after all is said and done, what all this business is about.
We keep hearing from alarmists on here and elsewhere that ‘uncertainty’ in estimates of climate sensitivity means that we cannot disregard the high end estimates generated from the GCMs, meaning, effectively, that current urgent CO2 emissions reductions are justified. This is despite the fact that empirically derived observationally based estimates are generally lower than those estimates emergent from the GCMs. Climate scientists have attempted to justify the higher estimates and downplay the lower estimates, most notably a recent attempt from Marvel, Schmidt et al—which fell flat on its face here and here.
Estimates of 2xCO2 equilibrium climate sensitivity (EqCS) derive from running global climate models (GCMs) to equilibrium. Estimates of effective climate sensitivity (EfCS) are the corresponding quantities obtained using transient GCM output or observations. The EfCS approach uses an accompanying energy balance model (EBM), the zero-dimensional model (ZDM) being standard. GCM values of EqCS and EfCS vary widely [IPCC range: (1.5, 4.5)°C] and have failed to converge over the past 35 years. Recently, attempts have been made to refine the EfCS approach by using two-zone (tropical/extratropical) EBMs. When applied using satellite radiation data, these give low and tightly-constrained EfCS values, in the neighbourhood of 1°C. These low observational EfCS/two-zone EBM values have been questioned because (a) they disagree with higher observational EfCS/ZDM values, and (b) the EfCS/two-zone EBM values given by GCMs are poorly correlated with the standard GCM sensitivity estimates. The validity of the low observational EfCS/two-zone EBM values is here explored, with focus on the limitations of the observational EfCS/ZDM approach, the disagreement between the GCM and observational radiative responses to surface temperature perturbations in the tropics, and on the modified EfCS values provided by an extended two zone EBM that includes an explicit parameterization of dynamical heat transport. The results support the low observational EfCS/two-zone EBM values, indicating that objections (a) and (b) to these values both need to be reconsidered. It is shown that in the EBM with explicit dynamical heat transport the traditional formulism of climate feedbacks can break down because of lack of additivity.
Take home message:
The central conclusion of this study is that to disregard the low values of effective climate sensitivity (≈1°C) given by observations on the grounds that they do not agree with the larger values of equilibrium, or effective, climate sensitivity given by GCMs, while the GCMs themselves do not properly represent the observed value of the tropical radiative response coefficient, is a standpoint that needs to be reconsidered.
More inconvenient peer-reviewed science for those who wish to promote the idea of a global ‘climate emergency’ supposedly based upon sound science and a >90% consensus of experts. Mind you, with the way Cook’s 97% ‘consensus’ was confected, Prof. Bates would probably be included as one of those experts—even after publishing this paper!