18 January 2018

The Emergent Constraints of Climate Sensitivity are Linked to Shortwave Cloud Feedback

Science

Relationships between equilibrium climate sensitivity (ECS) and observable metrics of the current climate in model ensembles, so-called emergent constraints, are examined. The relationships between the emergent constraint metric and ECS are largely attributable to a statistical connection with shortwave low cloud feedback, the leading cause of intermodel ECS spread. 

Impact

The findings suggest that any proposed or forthcoming ECS constraint based on the current generation of climate models should be viewed as a potential constraint on shortwave cloud feedback, and physical links with that feedback should be investigated to verify that the constraint is real. In addition, any proposed ECS constraint should not be taken at face value since other factors influencing ECS besides shortwave cloud feedback could be systematically biased in the models.

Summary

Differences among climate models in equilibrium climate sensitivity (ECS; the equilibrium surface temperature response to a doubling of atmospheric CO2) remain a significant barrier to the accurate assessment of societally important impacts of climate change. Relationships between ECS and observable metrics of the current climate in model ensembles, so-called emergent constraints, have been used to constrain ECS. However, those relationships are generally not well understood. Here a statistical method (including a backward selection process) is employed to achieve a better statistical understanding of the connections between four recently proposed emergent constraint metrics and individual feedbacks influencing ECS. The relationship between each metric and ECS is largely attributable to a statistical connection with shortwave low cloud feedback, the leading cause of intermodel ECS spread. Additional analysis is conducted with a few thousand artificial metrics that are randomly generated but are well correlated with ECS. The relationships between the contrived metrics and ECS can also be linked statistically to shortwave cloud feedback. Thus, any proposed or forthcoming ECS constraint based on the current generation of climate models should be viewed as a potential constraint on shortwave cloud feedback, and physical links with that feedback should be investigated to verify that the constraint is real. In addition, any proposed ECS constraint should not be taken at face value since other factors influencing ECS besides shortwave cloud feedback could be systematically biased in the models.

Contact
Stephen A. Klein
University of California at Los Angeles
Publications
DeAngelis, A.  2018.  "On the Emergent Constraints of Climate Sensitivity."  Journal of Climate 31(2): 863-875, doi:10.1175/jcli-d-17-0482.1.