Spotlighting a recent study by Stephen Po-Chedley
Key Points & Overview
- Model differences in the total area of Antarctic sea-ice have a large impact on the inter-model spread of the global lapse-rate and water-vapor feedbacks.
Given the same increase in CO2, climate models disagree in the amount of warming. Oftentimes, to understand the sources of this inter-model spread in future warming, scientist’s use simple metrics to understand the behavior of the larger, more complex models. Equilibrium climate sensitivity (ECS), as defined as the steady-state global-mean surface warming response to a doubling of CO2, is one of those metrics. Previous studies have shown that the large inter-model spread in ECS is predominantly due to differences in model-unique climate feedbacks, which act to amplify or dampen a particular forcing. Uncertainty in near-future warming has also been demonstrated to be the result of this spread in climate feedbacks. However, a large question remains unanswered: what are the sources of the inter-model spread in climate feedbacks?
A recent paper by Stephen Po-Chedley, a postdoctoral research fellow at the Lawrence Livermore National Laboratory, sheds light onto some of the possible reasons for such a large spread in climate feedbacks. Po-chedley, along with a group of researchers, uncovered a process that contributes to the inter-model spread of two particular climate feedbacks: the lapse-rate and water-vapor feedbacks. The water-vapor feedback has a net-positive effect. An increase of temperature in the atmosphere allows more water-vapor to be held in the atmosphere, causing more warming. The lapse-rate feedback, which has a net-negative impact (i.e., dampening a forcing), characterizes how the temperature of the atmosphere will change with height under warming.
By tapping into an ensemble of climate models subject to increasing CO2, Po-Chedley and others demonstrated that the way each model represents Antarctic sea-ice affects local surface warming. This local surface warming bias influences the way local lapse-rate and water-vapor feedbacks are calculated. Circuitously, the discrepancies in Antarctic sea-ice area across climate models have a significant impact on the inter-model spread of the global lapse-rate and water-vapor feedbacks, and thus the global and regional temperature response.
By identifying the sources of inter-model spread in these particular feedbacks, uncertainty in future warming can be reduced both at regional and global scales.