The surface temperature response to greenhouse gas forcing displays a characteristic pattern of polar-amplified warming1,2,3,4,5, particularly in the Northern Hemisphere. However, the causes of this polar amplification are still debated. Some studies highlight the importance of surface-albedo feedback6,7,8, while others find larger contributions from longwave feedbacks4,9,10, with changes in atmospheric and oceanic heat transport also thought to play a role11,12,13,14,15,16. Here, we determine the causes of polar amplification 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 polar amplification depends strongly on the location of CO2 forcing. In particular, polar amplification is found to be dominated by forcing in the polar regions, specifically through positive local lapse-rate feedback, with ice-albedo and Planck feedbacks playing subsidiary 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 polar amplification. Therefore, understanding polar amplification requires primarily a better insight into local forcing and feedbacks rather than extra-polar processes.
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The model source code to reproduce these experiments can be obtained from http://www.cesm.ucar.edu/models/cesm1.2/ and the modifications to prescribe spatially varying CO2 concentrations can be obtained from the corresponding author.
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M.F.S. was supported by the Institute for Basic Science (project code IBS-R028-D1) and NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Sciences. C.M.B. was supported by NOAA grant CPO NA115OAR4310161. C.P. was supported by a JISAO postdoctoral fellowship. K.C.A. and Y.D. were supported by NSF grants AGS-1752796 and OCE-1523641. S.M.K. and D.K. were supported by the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT and Future Planning (2016R1A1A3A04005520). S.M. was supported by the Australian Research Council (grant numbers FT160100162 and CE170100023). F.-F.J. was supported by NSF grant AGS-1813611 and Department of Energy grant DE-SC0005110. Computing resources were provided by the University of Southern California’s Center for High-Performance Computing.
About this article
Nature Climate Change (2018)