Earlier leaf-out in response to climate warming has been recorded in northern temperate and boreal regions. In turn, this shift modifies climate by altering seasonal cycles of surface energy, water, and carbon budgets. Here, we use the Community Earth System Model 1.2 to investigate climate feedbacks from advanced leaf-out in northern temperate and boreal vegetation. An imposed 12-day earlier leaf-out in this region, consistent with recent observations, enhances annual surface warming in the Northern Hemisphere. We identify warming hotspots in the Canadian Arctic Archipelago (~0.7 °C), east and west edges of Siberia (~0.4 °C) and southeastern Tibetan Plateau (~0.3 °C). We attribute this enhanced warming to combined effects of indirect water vapor, cloud, and snow-albedo radiative feedbacks through intensified poleward water vapor transport rather than direct vegetation albedo and latent heat biophysical feedbacks. With continued warming, positive feedbacks between climate and leaf phenology are likely to amplify warming in the northern high latitudes.