Climate Feedback Analysis of the GFDL IPCC AR4 Global Warming Simulation

Both observed and modeled global warming pattern shows a large surface polar warming and a large upper atmospheric warming in the tropics. This pattern leads to an amplification (reduction) of the temperature gradient at upper levels (surface). Physical processes behind this temperature change are the external radiative forcing, and subsequent feedback processes that may amplify or dampen the climate response. This unique warming pattern suggests that high latitudes are very sensitive to climate change and also the area where the largest warming projection uncertainties occur. The objective of this study is to apply a new coupled atmosphere-surface climate feedback-response analysis method to quantify the contributions of the external forcing alone (doubling of carbon dioxide), and subsequent feedback processes to the 3-D global warming pattern in the GFDL_CM2.0 model. The feedbacks under consideration include the water vapor feedback, surface albedo feedback, surface turbulent heat flux feedback, and the sum of the change in cloud radiative forcing (CRF), vertical convective, and large-scale scale dynamical feedback. The partial temperature changes due to the external forcing and due to individual feedbacks are additive and their sum converges toward the temperature change produced by the original GFDL_CM2.0 global warming simulations. Therefore, our attributions of the global warming patter to individual thermodynamic and dynamical processes are mathematically robust and physically meaningful. The partial temperature change due to the water vapor feedback is found to be the largest contributor to the globally averaged surface warming. It is twice as large as the warming due to the external radiative forcing alone. The surface albedo feedback and change in surface cloud radiative forcing increase the surface temperature by a smaller amount. In addition, the changes in atmospheric cloud forcing and large-scale dynamics, as well as the surface turbulent heat flux feedback, contribute to an overall damping the surface warming. In terms of spatial pattern of global warming, the external forcing alone would cause a large surface warming in the extratropics. The water vapor feedback strengthens the tropical warming substantially and the ice/snow albedo feedback contributes to polar warming amplification. The atmospheric dynamical feedbacks associated with the enhancement of vertical convection in the tropics acts to amplify the warming in the upper troposphere at the expense of reducing the warming in the lower troposphere and at the surface in the tropics. The dynamical feedbacks due to the strengthening of the poleward energy transport contribute to a warming in the entire troposphere and the surface in high latitudes. At the surface and in the lower troposphere, the additional warming brought by the change in circulations strengthens the warming due to thermodynamical forcings (e.g., external forcing, water vapor feedback, and ice albedo feedback). In the upper troposphere, the warming brought by the change in circulations dominates the cooling due to thermodynamical forcings. As a result, the entire troposphere becomes warmer. The stratospheric cooling is entirely due to the external radiative forcing.