For generations, the world's great forests stood as silent partners in humanity's relationship with the atmosphere, drawing down carbon and steadying the climate's pulse. A comprehensive analysis spanning 1988 to 2021 now reveals that this partnership is fracturing: moist tropical and boreal forests have crossed a threshold, releasing more carbon than they absorb since the early 2000s. The Amazon's most intact and ancient stands — once the most resilient — are now the primary drivers of this reversal, their losses increasingly mirroring spikes in atmospheric CO2. What was a buffer has become a