Revisiting the Role of the Water Vapor and Lapse Rate Feedbacks in the Arctic Amplification of Climate Change

Abstract:

The Arctic tends to warm faster than the rest of the globe in observations and climate model simulations of global warming. The factors causing this Arctic amplification are often described in the context of climate feedbacks, using a standard feedback analysis that partitions the total surface warming change into contributions from each feedback. Such analyses have identified the lapse rate feedback as the largest contributor to Arctic amplification, with the water vapor feedback being the main opposing factor by warming the tropics more than the Arctic region. However, the interpretation of such analyses is complicated by the nonlinear interaction of the individual climate feedback processes with each other and with the meridional heat transport in the climate system. Here we focus instead on the physically intuitive approach to how much each feedback process contributes to warming by suppressing the process during forced warming and evaluating the resulting change in temperature, i.e., we examine the effect of locking each feedback process. We investigate this using a moist energy balance model (MEBM) with spatially-varying feedbacks that are specified according to comprehensive climate model simulations, which has been shown previously to simulate surface temperature changes that closely match comprehensive climate model results. Comparing the simulated warming with simulations in which the radiative effects of each feedback are held fixed in the MEBM, we find that the effect of water vapor heats the Arctic region more than the tropics, which can be explained in terms of the interactions between water vapor and other positive feedbacks in the Arctic. We furthermore find that the lapse rate feedback has a neutral effect by cooling the Arctic and the tropics by approximately equivalent amounts. The results highlight that identifying the contributions of each physical process to polar amplification depends crucially on the details of how the question is asked. When warming is attributed to processes by comparing how the climate would change if the process were not active, the water vapor feedback is found to be the primary physical reason that the Arctic region warms more than the tropics.