EAPS

Title: Why does climate sensitivity go up as ocean heat uptake declines? A linear systems perspective.

Transient climate sensitivity tends to increase on multiple timescales in climate models subject to an abrupt CO2 increase, which suggests that:
(1) we may be systematically underestimating equilibrium sensitivity
in the models, and
(2) constraining equilibrium sensitivity from observations is a
tricky business at best.
I will argue that the radiative feedback and ocean heat uptake processes governing transient sensitivity are intimately connected. Idealized GCM experiments show that the global climatic impact of spatially localized ocean heat uptake patterns are very sensitive to their geographical distribution. I will discuss these results in terms of the efficacy concept and a linear systems perspective in which responses to individual climate forcing agents are additive. Heat uptake can be treated as a slowly varying forcing on the atmosphere and surface, whose efficacy is strongly determined by its spatial pattern. An illustrative linear model demonstrates the emergence of increasing climate sensitivity as a simple consequence of the slow decay of high-efficacy sub-polar heat uptake. Shortwave cloud feedbacks are a key player in this increasing climate sensitivity, both in idealized and CMIP5-type models. I propose a causal physical mechanism linking sub-polar heat uptake to a global-scale increase in lower-tropospheric stability.