Transient and Quasi-Equilibrium Climate States at 1.5°C and 2°C Global Warming

Recent climate change is characterized by rapid global warming, but the goal of the Paris Agreement is to achieve a stable climate where global temperatures remain well below 2°C above pre-industrial levels. Inferences about conditions at or below 2°C are usually made based on transient climate projections. To better understand climate change impacts on natural and human systems under the Paris Agreement, we must understand how a stable climate may differ from transient conditions at the same warming level. Here we examine differences between transient and quasi-equilibrium climates using a statistical framework applied to greenhouse gas-only model simulations. This allows us to infer climate change patterns at 1.5°C and 2°C global warming in both transient and quasi-equilibrium climate states. We find substantial local differences between seasonal-average temperatures dependent on the rate of global warming, with mid-latitude land regions in boreal summer considerably warmer in a transient climate than a quasi-equilibrium state at both 1.5°C and 2°C global warming. In a rapidly warming world, such locations may experience a temporary emergence of a local climate change signal that weakens if the global climate stabilizes and the Paris Agreement goals are met. Our research demonstrates that the rate of global warming must be considered in regional projections.