Comparing the atmospheric responses to reduced Arctic sea ice, a warmer ocean, and increased CO2 and their contributions to projected change at 2 °C global warming

We consider the combined and individual influences of Arctic sea-ice loss, sea surface temperature (SST) warming, and the direct radiative effect of increased CO 2 on the Northern Hemispheric climate. The surface climate (e.g., temperature, precipitation) and atmospheric circulation responses (e.g., sea level pressure, wind) to these drivers are quantified using simulations from the Polar Amplification Model Intercomparison Project (for sea-ice loss and SST warming) and the Cloud Feedback Model Intercomparison Project (for increased CO 2 ). We verify the linear additivity of the PAMIP-derived winter responses to sea-ice loss and SST change. The responses to SST change are of greater magnitude than that due to sea-ice loss or due to CO 2 direct radiative forcing in most seasons and regions, excluding the Arctic. Notably however, sea-ice loss is at least as important as SST change for the winter atmospheric circulation response over the North Atlantic and Siberia. The dynamical responses to sea-ice loss and SST change oppose each other in many regions in winter, while the responses to SST and CO 2 direct radiative forcing are often opposing in summer. Such opposing responses are less evident for the thermodynamical response. The sum of all three responses reproduces well the spatial patterns of change at 2 °C global warming in winter and autumn in the Coupled Model Intercomparison Project phase 6 projections, but overestimates their magnitude.