Accelerated north-east shift of the global green wave trajectory

Viewed from space, a "green wave" seasonally traverses Earth's surface, from the north in boreal summer to the south in austral summer. This wave represents vegetation phenology, driven primarily by solar irradiation and modulated by climate variability and ecosystem dynamics. Despite its significance for multiple Earth system processes, we lack a unified metric to characterize and understand its dynamics. Here, we propose a concept to quantify global phenology by tracking the green wave's centroid using satellite and Earth system model data. The resulting trajectory summarizes global phenological dynamics and directional trends. Earlier reports on global greening led us to hypothesize a rapidly northward shifting trajectory during boreal summer and a moderate southward shift during austral summer. Contrary to this expectation, we find that the centroid moves northward during both summer periods, with the austral summer shift consistently exceeding the boreal shift across datasets. As a consequence, the amplitude of the green wave trajectory is decreasing, a trend projected to intensify throughout this century. We also detect an accelerating eastward shift, a phenomenon not previously reported. Tracking the green wave's centroid reveals how regionally changing land dynamics affect the global functioning of Earth's terrestrial biosphere.