Climatic warming has greatly increased vegetation productivity in the extratropical Northern Hemisphere since the 1980s, but how long this positive relationship will continue remains unknown. Here we show changes in the effect of warming on Northern Hemisphere summer gross primary productivity for 2001–2100 using Earth system model outputs. The correlation between summer gross primary productivity and temperature decreases in temperate and boreal regions by the late twenty-first century, generally becoming significantly negative before 2070 in regions <60° N, though Arctic gross primary productivity continues to increase with further summer warming. The time when the correlation becomes negative is generally later than the time when summer temperature exceeds the optimal temperature for vegetation productivity, suggesting partial mitigation of the negative vegetation impacts of future warming with photosynthetic thermal acclimation. Our findings indicate that vegetation productivity could be impaired by climate change in the twenty-first century, which could negatively impact the global land carbon sink.