Energy Budget Constraints on Climate Sensitivity in Light of Inconstant Climate Feedbacks

Global energy budget observations suggest an equilibrium climate sensitivity around 2C (e.g., Otto et al., 2013) -- lower than estimates from paleoclimate reconstructions, process-based observational analyses, and global climate model simulations. A key assumption of these energy budget constraints is that the climate sensitivity inferred today also applies to the distant future; that is, global climate feedbacks are assumed to be constant. Yet, global climate models robustly show that climate feedbacks vary over time, with a strong tendency for climate sensitivity to increase as equilibrium is approached. Here I consider the implications of inconstant climate feedbacks for energy budget constraints on equilibrium climate sensitivity. I find that the climate sensitivity inferred during transient warming is, on average, 31% below equilibrium climate sensitivity within global climate models. Moreover, model values of climate sensitivity inferred during transient warming are found to be consistent with the range suggested by the 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 current energy budget constraints produces a best estimate of equilibrium climate sensitivity of 2.4C (1.3-5.6C, 5-95% confidence interval). These findings largely reconcile global energy budget estimates of equilibrium climate sensitivity with those derived from other methods and simulated by global climate models.