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In the Tropical Rain belts with an Annual cycle and Continent Model Intercomparison Project (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.

Plain Language Summary In both observations and computer model simulations, the polar regions (especially the Arctic) warm more than the rest of the world in response to increased greenhouse gas concentrations. Scientists disagree on the reasons for this polar amplification of warming. The melting of ice floating in the ocean, which lets more sunlight be absorbed, is often given as an explanation, but climate models with no sea ice also display polar amplification. We ran hundreds of experiments with a simple climate model in order to understand the reasons for polar amplification in more complex models that lack sea ice. We found that the main reason is that the atmosphere transports energy from the tropics to the poles, so much so that even processes that initially add energy mostly to the tropics cause polar amplification. Our methods produce different explanations from past studies because they did not fully account for this movement of energy.