Transient and quasi-equilibrium climate states at 1.5°C and 2°C global warming

Inferences about conditions at or below 2 degrees 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 degrees C and 2 degrees 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 degrees C and 2 degrees C global warming.