Plants follow a seasonal rhythm of leaf growth in spring and leaf loss in autumn, known as phenology. Over recent decades, warming temperatures have caused leaves to appear earlier in spring and persist longer into autumn, especially across northern regions. These shifts do not only affect ecosystems locally—they also change how land and atmosphere exchange energy, water, and carbon, with consequences for the global climate.This article reviews what is currently known about how phenological changes influence climate through multiple pathways, including changes in surface reflectivity, evaporation, cloud formation,
and carbon uptake, identifies key unknowns, and offers a perspective on how to address these gaps. As an initial step to tackle these gaps, we present new simulations from a fully coupled Earth system model that isolate the climate response to observed phenological trends. We find that a 10-day extension of the growing season leads to a small but measurable global cooling, strongest in northern latitudes. This suggests that recent phenological changes have acted as a negative feedback to global warming.