Old-growth forests are significant global carbon sinks that continue to accumulate carbon, contrary to the long-held belief that they are carbon neutral. A comprehensive analysis of 519 forest studies across boreal and temperate regions revealed that forests aged 15–800 years typically have a positive net ecosystem productivity (NEP), indicating they act as carbon sinks. These forests, which constitute over 30% of global primary forests, sequester approximately 1.3 gigatonnes of carbon per year, contributing at least 10% of global net ecosystem productivity. Despite this, old-growth forests are not protected by international treaties, as they are often considered carbon neutral. However, the study challenges this view, showing that old-growth forests continue to accumulate carbon for centuries, with large amounts stored in roots, soil, and woody debris. The study also highlights that young forests are often sources of carbon dioxide due to higher decomposition rates of organic matter. The findings suggest that old-growth forests should be recognized as important carbon sinks in global carbon accounting. The research underscores the importance of protecting these forests to maintain their role in carbon sequestration. The study also addresses uncertainties in carbon flux measurements and provides a framework for estimating these uncertainties. Overall, the study demonstrates that old-growth forests are crucial for global carbon balance and should be considered in climate change mitigation strategies.Old-growth forests are significant global carbon sinks that continue to accumulate carbon, contrary to the long-held belief that they are carbon neutral. A comprehensive analysis of 519 forest studies across boreal and temperate regions revealed that forests aged 15–800 years typically have a positive net ecosystem productivity (NEP), indicating they act as carbon sinks. These forests, which constitute over 30% of global primary forests, sequester approximately 1.3 gigatonnes of carbon per year, contributing at least 10% of global net ecosystem productivity. Despite this, old-growth forests are not protected by international treaties, as they are often considered carbon neutral. However, the study challenges this view, showing that old-growth forests continue to accumulate carbon for centuries, with large amounts stored in roots, soil, and woody debris. The study also highlights that young forests are often sources of carbon dioxide due to higher decomposition rates of organic matter. The findings suggest that old-growth forests should be recognized as important carbon sinks in global carbon accounting. The research underscores the importance of protecting these forests to maintain their role in carbon sequestration. The study also addresses uncertainties in carbon flux measurements and provides a framework for estimating these uncertainties. Overall, the study demonstrates that old-growth forests are crucial for global carbon balance and should be considered in climate change mitigation strategies.