The observed and projected Arctic warming pattern is characterized by an early winter maximum and a summer minimum. While a robust feature of Arctic climate change, the seasonal expression of surface warming remains incompletely understood. Previous explanations attribute the seasonality to surface energy budget changes induced by climate feedbacks. However, these hypotheses cannot explain key features of the simulated seasonal structure: seasonal heating rate changes and the early winter warming maximum. We find that the increase in the thermal inertia of the Arctic system due to the transition from a lower thermal inertia surface (sea ice cover) to a higher thermal inertia surface (ice free ocean) captures these key seasonal features. Our analysis shows that the early winter Arctic warming maximum results from a slowing of the background surface cooling rate from summer to winter, not from an additional net energy input into the Arctic surface during that time.