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Forest ecosystems are the main terrestrial carbon sinks and therefore play a critical role in global climate mitigation strategies. However, their long-term maximum capacity to store carbon under present and future climate conditions remains uncertain. To address this knowledge gap, we investigated the limits of above-ground biomass accumulation in existing forests and non-forested areas, and predicted how climate change scenarios could impact this carbon storage capacity. We evaluate the physical constraints on carbon accumulation, using a machine learning framework that integrates climate, pedologic, and hydrological parameters with natural forest disturbances, and CO₂ fertilization effects on forest growth. The results show that relying on forests to offset business-as-usual carbon emissions is possible only to a very limited extent. Projections of future forest carbon in existing forests remain highly uncertain and critically dependent on the methods and assumptions used for the assessments. Climate model-based estimates–which are often used in international policy and IPCC reports–suggest substantial future increases in carbon storage capacity, mainly driven by a strong CO₂ fertilisation effect. On the contrary, experimental and satellite-based evidence suggests much weaker increases or even a stabilisation of the terrestrial sink. Furthermore, our results suggest that ecosystem vulnerability to climate extremes will increase the frequency and severity of natural disturbances in the tropical and mid-latitude regions, ultimately reducing the effective long-term capacity of forests to store carbon. Potential increases in the carbon storage of boreal ecosystems could partially offset these losses, but only over longer timescales that cannot compensate for near-term declines elsewhere. These results highlight the need to reframe the potential of forest-based solutions for climate mitigation: not as an offset for anthropogenic carbon emissions, but as an essential buffer that helps prevent the land sector from becoming a net carbon source. For this purpose, forest conservation and sustainable management aimed at increasing ecosystem resilience to climate should be prioritised as transitional measures that support emission reductions, rather than substitutes for them.