Biodiversity promotes ecosystem productivity and stability, a positive impact that often strengthens over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO2) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over multiple decades. Using a multidecadal grassland biodiversity-global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over multiple decades, CO2 and nitrogen enrichment can gradually erode the positive effects of biodiversity on ecosystem stability. As elevated CO2, N eutrophication, and biodiversity loss increasingly co-occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long-term grassland stability.