Shifts in the Annual Cycle and Associated Impacts on Northern Hemisphere Summer Onset Under Global Warming

Abstract Global warming has altered the timing of seasonal onset, yet the relative roles of long‐term mean warming and changes in the annual cycle (AC) remain unclear. Using observational data sets and CMIP6 simulations, here we quantify how long‐term warming and the AC contribute to changes in seasonal onset across Northern Hemisphere midlatitudes during 1950–2014. While long‐term warming drives a widespread advance of summer onset, regional, and latitudinal contrasts arise primarily from AC variations. In North America and northern Asia, these cycle changes offset the warming‐induced advance and contribute at magnitudes comparable to long‐term warming, highlighting their critical role in shaping regional climate responses. Based on single‐forcing experiments and a simplified surface energy balance model, we further show that greenhouse gases enhance damping and reduce effective heat capacity, advancing the AC phase, whereas aerosol forcing increases effective heat capacity and weakens damping, thereby delaying summer onset.