Polar amplification dominated by local forcing and feedbacks

Abstract

​The surface temperature response to greenhouse gas forcing displays a characteristic pattern of polar-amplified warming, particularly in the Northern Hemisphere. However, the causes of this polar amplification (PA) are still debated. Some studies highlight the importance of surface-albedo feedback, while others find larger contributions from longwave feedbacks, with changes in atmospheric and oceanic heat transport also thought to play a role. Here, we determine the causes of PA using climate model simulations in which CO2 forcing is prescribed in distinct geographical regions, with the linear sum of climate responses to regional forcings replicating the response to global forcing. The degree of PA depends strongly on the location of CO2 forcing. In particular, PA is found to be dominated by forcing in the polar regions, specifically through positive local lapse-rate feedbacks, with ice-albedo and Planck feedbacks playing lesser roles. Extra-polar forcing is further shown to be conducive to polar warming, but given that it induces a largely uniform warming pattern through enhanced poleward heat transport, it contributes little to PA. Therefore, understanding PA requires primarily a better insight of local forcing and feedbacks rather than extra-polar processes. © The Author(s), under exclusive licence to Springer Nature Limited 2018