Potential Problems Measuring Climate Sensitivity from the Historical Record

Abstract This study investigates potential biases between equilibrium climate sensitivity inferred from warming over the historical period (ECS hist ) and the climate system’s true ECS (ECS true ). This paper focuses on two factors that could contribute to differences between these quantities. First is the impact of internal variability over the historical period: our historical climate record is just one of an infinity of possible trajectories, and these different trajectories can generate ECS hist values 0.3 K below to 0.5 K above (5%–95% confidence interval) the average ECS hist . Because this spread is due to unforced variability, I refer to this as the unforced pattern effect. This unforced pattern effect in the model analyzed here is traced to unforced variability in loss of sea ice, which affects the albedo feedback, and to unforced variability in warming of the troposphere, which affects the shortwave cloud feedback. There is also a forced pattern effect that causes ECS hist to depart from ECS true due to differences between today’s transient pattern of warming and the pattern of warming at 2×CO 2 equilibrium. Changes in the pattern of warming lead to a strengthening low-cloud feedback as equilibrium is approached in regions where surface warming is delayed: the Southern Ocean, eastern Pacific, and North Atlantic near Greenland. This forced pattern effect causes ECS hist to be on average 0.2 K lower than ECS true (~8%). The net effect of these two pattern effects together can produce an estimate of ECS hist as much as 0.5 K below ECS true .