26.07.2022 RSES Seminar Series: Callum Shakespeare (RSES, ANU)

Longwave radiation and surface temperature: a climatic chicken and egg conundrum
Callum Shakespeare (RSES, ANU)
ABSTRACT: Increases in atmospheric carbon dioxide (CO2) manifest at the Earth’s surface as an increase in the downwelling longwave radiation (DLR) from the sky above. Climate models predict large increases in DLR under even moderate emissions scenarios. It is often argued that this enhanced heat flux from the atmosphere drives an increase in land and ocean surface temperatures... but does it? Climate models do not determine how much of the DLR increase is a direct forcing due to the increased CO2, versus how much is a response (or feedback) to surface temperature changes. This distinction is important to consider when using coupled climate model output (e.g., reanalyses) to drive other physical models such as for the ice sheets, ocean and land surface, where the surface temperatures may diverge from those represented in the climate model. I will describe a novel theoretical framework for predicting DLR and thereby disentangling longwave forcing and feedbacks in climate models. The analysis shows that over 90% of the modelled change in DLR is a feedback to surface temperature changes. In other words, surface temperature drives changes to DLR and not the other way around. Therefore, significant caution is called for in interpreting the results of studies that treat the DLR as a forcing independent of the surface temperature (e.g., those imposing DLR from climate model output). I consider how the forcing of such models could be improved to correctly capture the DLR-temperature feedback.