Increased early-season productivity drives earlier peak of vegetation photosynthesis across the Northern Hemisphere

Changes in vegetation carbon uptake are largely influenced by the timing and magnitude of the peak of the growing season (POS), when vegetation photosynthesis reaches its maximum. However, the factors controlling the timing of POS remain poorly understood, leaving us uncertain about its future trajectory. Using satellite observations and carbon flux measurements, we show that, in recent decades, increased early-season carbon uptake has been driven by both an earlier onset of the growing season and higher temperatures. In 93% of northern (>30 degrees N) vegetation, these increases in early-season carbon uptake were associated with an advancement of POS. This ongoing shift suggests a developmental constraint on seasonal productivity, potentially limiting carbon uptake later in the season. Our findings provide a mechanistic explanation that reconciles previous observations linking earlier growing season onset, rising temperatures, and shifts in POS timing, and suggest a decrease in late-season carbon uptake with climate warming.