New paper in Nature Climate Change: Energy budget constraints on climate sensitivity in light of inconstant climate feedbacks [pdf]

Equilibrium climate sensitivity — defined as the long-term near-surface air temperature change that would result from a doubling of atmospheric CO2 — is a key metric of climate change. Global energy budget constraints suggest a climate sensitivity around 2°C, which is lower than that suggested by other estimates, such as from palaeoclimate reconstructions, process-based observational analyses (e.g., emergent constraints), and global climate model simulations. A key, but often unstated, assumption of these energy budget constraints is that the same climate sensitivity inferred today also applies to the distant future. Yet, global climate models robustly show that feedbacks vary over time, with a strong tendency for climate sensitivity to increase as equilibrium is approached. This paper considers the implications of inconstant climate feedbacks for energy budget constraints on climate sensitivity.

The analysis shows that the long-term value of climate sensitivity is, on average, 26% above that inferred during transient warming within global climate models, with a larger discrepancy when climate sensitivity is high. Moreover, model values of climate sensitivity inferred during transient warming are found to be consistent with energy budget observations, indicating that the models are not overly sensitive. Using model-based estimates of how climate feedbacks will change in the future, in conjunction with recent energy budget constraints, produces a current best estimate of equilibrium climate sensitivity of 2.9°C (1.7–7.1°C, 90% confidence). The findings suggest that climate sensitivity estimated from global energy budget constraints is in agreement with values derived from other methods and simulated by global climate models.