Externally forced symmetric warming in the Arctic and Antarctic during the second half of the twentieth century

In recent decades, the two polar regions have exhibited strikingly different changes, with much greater warming in the Arctic than the Antarctic. However, the warming asymmetry between the two polar regions is quite small during the second half of the twentieth century. By using a multi-member ensemble of simulations with the Community Earth System Model, this study investigates the relative contributions of greenhouse gases, aerosol, and ozone forcings to the responses of Arctic and Antarctic surface temperature during 1955–2000. Results show that both the greenhouse gases- and aerosols-induced changes are greater in the Arctic than in the Antarctic, yet they are opposite and act to balance each other, leaving a limited warming in the Arctic and hence a small bipolar asymmetry. Using a radiative kernel, feedback analysis reveals that both greenhouse gases and aerosol forcings influence the polar surface temperature through albedo feedback related to sea ice changes and lapse rate feedback related to strong surface temperature inversion. The ozone forcing can hardly excite any surface temperature changes over the polar regions even in the Antarctic with the strongest ozone depletion, which is due to a cancellation between the cooling effect from radiative forcing and cloud radiative feedback, and the warming effect from lapse rate feedback and enhanced atmospheric heat transport from lower latitudes.