In the TRACMIP ensemble of aquaplanet climate model experiments,
CO2-induced warming is amplified in the poles in 10 out of 12 models,
despite the lack of sea ice. We attribute causes of this amplification
by perturbing individual radiative forcing and feedback components in a
moist energy balance model. We find a strikingly linear pattern of
tropical versus polar warming contributions across models and processes,
implying that polar amplification is an inherent consequence of
diffusion of moist static energy by the atmosphere. The largest
contributor to polar amplification is the instantaneous CO2 forcing,
followed by the water vapor feedback and, for some models, cloud
feedbacks. Extratropical feedbacks affect polar amplification more
strongly, but even feedbacks confined to the tropics can cause polar
amplification. Our results contradict studies inferring warming
contributions directly from the meridional gradient of radiative
perturbations, highlighting the importance of interactions between
feedbacks and moisture transport for polar amplification.