Sources of inter-model spread in climate feedbacks

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.
Stephen Po-Chedley received his PhD in 2017 from the Department of Atmospheric Sciences.

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.

About the Article

Co-Authors: Kyle C. Armour, Cecilia M. Bitz, Mark D. Zelinka, Benjamin D. Santer, Qiang Fu
Published: |
Journal of Climate