Mid-Pliocene Climate Forcing, Sea Surface Temperature Patterns, and Implications for Modern-Day Climate Sensitivity

Abstract Characterized by similar-to-today CO 2 levels (∼400 ppm) and surface temperatures approximately 3°–4°C warmer than the preindustrial, the mid-Pliocene warm period (mPWP) has often been used as an analog for modern CO 2 -driven climate change and as a constraint on the equilibrium climate sensitivity (ECS). However, model intercomparison studies suggest that non-CO 2 boundary conditions—such as changes in ice sheets—contribute substantially to the higher global mean temperatures and strongly shape the pattern of sea surface warming during the mPWP. Here, we employ a set of CESM2 simulations to quantify mPWP effective radiative forcings, study the role of ocean circulation changes in shaping the patterns of sea surface temperatures, and calculate radiative feedbacks during the mPWP. We find that the non-CO 2 boundary conditions of the mPWP, enhanced by changes in ocean circulation, contributed to larger high-latitude warming and less-stabilizing feedbacks relative to those induced by CO 2 alone. Accounting for differences in feedbacks between the mPWP and the modern (greenhouse gas–driven) climate provides stronger constraints on the high-end of modern-day ECS. However, a quantification of the forcing of non-CO 2 boundary condition changes combined with the distinct radiative feedbacks that they induce suggests that Earth system sensitivity may be higher than previously estimated. Significance Statement Climate records from past warm intervals in Earth’s geologic history are frequently used to constrain modern-day equilibrium climate sensitivity. Yet, climate warmth during these periods is not solely driven by the radiative effect of CO 2 , but also by environmental and geographic features, such as large-scale ice sheet and vegetation changes, that are not expected to occur in the near future. Using the mid-Pliocene warm period as an example, we find that the surface patterns of warming induced by non-CO 2 paleoenvironmental boundary conditions lead to a more-sensitive climate state than the modern. This implies that near-term warming under CO 2 forcing may be smaller than expected but that on geological time scales, future warming may be significantly larger.