<p>Terrestrial ecosystems have become greener over the years through two different processes: increasing maximum greenness or extending growing season. The integrated signal of these two processes has resulted in a persistent increase in Earth’s greenness, which serves as key evidence of contemporary global change. However, it remains unclear which process has predominantly driven Earth’s greening, as the mechanisms underlying increased maximum greenness and growing season extension are distinct. Here, we decompose long-term satellite greenness observations and quantify the respective contributions of maximum greenness and growing season length to Earth’s greening. We find that greening in non-forest areas is dominated by increased maximum greenness induced by elevated carbon dioxide and land use change, while greening in extra-tropical forests is primarily driven by growing season extension, particularly a lengthened autumn growing season associated with climate change and elevated carbon dioxide. This substantial contribution of autumn phenology to global greening is not captured by current dynamic global vegetation models, which typically attribute greening to increased maximum greenness. Our study provides a critical process-based examination of global greenness change and highlights the key role of autumn phenology in driving Earth’s greening.</p>