The carbon balance of tropical, temperate and boreal forests is a critical factor in the global carbon cycle. Forests are major carbon reservoirs and contributors to global primary productivity. The carbon balance of forests is determined by processes of carbon acquisition and loss, and small changes in these processes can significantly impact the global carbon cycle. This paper discusses the climatic influences on the carbon dynamics of boreal, temperate and tropical forests, using a synthesis of micrometeorological, ecophysiological and forestry data from three case-study sites. Historical changes in the carbon balance of each biome are reviewed, and the evidence for a carbon sink in each forest biome and its likely behavior under future global change are discussed. The paper concludes that there have been significant advances in determining the carbon balance of forests, but there are still critical uncertainties, particularly in the behavior of soil carbon stocks. The increasing concentration of carbon dioxide in the atmosphere since the industrial revolution is one of the most significant human influences on the global environment. The source of this carbon dioxide is primarily attributed to the use of fossil fuels, cement production and deforestation. However, only a fraction of the estimated emissions remain in the atmosphere, with the rest being transferred into the oceans or stored in terrestrial 'missing sinks'. The paper discusses the historical and current state of forests, the expansion of agriculture since 1850, logging since 1850, and forest degradation and fragmentation. It also examines the carbon cycle of intact forests, the influence of climate on carbon dynamics, and the seasonal cycles of carbon uptake and loss. The paper highlights the differences in carbon dynamics between boreal, temperate and tropical forests, with boreal forests having a higher proportion of carbon in soil organic matter, while tropical forests have a more even distribution between vegetation and soil. The carbon balance of forests is influenced by a variety of climatic and environmental variables, including temperature, moisture availability and frequency of disturbance. The paper also discusses the impact of human activities on forest carbon balance, including deforestation, logging, and land-use changes. The study concludes that forests have acted as an indirect negative feedback on the rise in CO2 concentration since deglaciation, and that the carbon balance of forests is a complex and dynamic system influenced by a range of factors.The carbon balance of tropical, temperate and boreal forests is a critical factor in the global carbon cycle. Forests are major carbon reservoirs and contributors to global primary productivity. The carbon balance of forests is determined by processes of carbon acquisition and loss, and small changes in these processes can significantly impact the global carbon cycle. This paper discusses the climatic influences on the carbon dynamics of boreal, temperate and tropical forests, using a synthesis of micrometeorological, ecophysiological and forestry data from three case-study sites. Historical changes in the carbon balance of each biome are reviewed, and the evidence for a carbon sink in each forest biome and its likely behavior under future global change are discussed. The paper concludes that there have been significant advances in determining the carbon balance of forests, but there are still critical uncertainties, particularly in the behavior of soil carbon stocks. The increasing concentration of carbon dioxide in the atmosphere since the industrial revolution is one of the most significant human influences on the global environment. The source of this carbon dioxide is primarily attributed to the use of fossil fuels, cement production and deforestation. However, only a fraction of the estimated emissions remain in the atmosphere, with the rest being transferred into the oceans or stored in terrestrial 'missing sinks'. The paper discusses the historical and current state of forests, the expansion of agriculture since 1850, logging since 1850, and forest degradation and fragmentation. It also examines the carbon cycle of intact forests, the influence of climate on carbon dynamics, and the seasonal cycles of carbon uptake and loss. The paper highlights the differences in carbon dynamics between boreal, temperate and tropical forests, with boreal forests having a higher proportion of carbon in soil organic matter, while tropical forests have a more even distribution between vegetation and soil. The carbon balance of forests is influenced by a variety of climatic and environmental variables, including temperature, moisture availability and frequency of disturbance. The paper also discusses the impact of human activities on forest carbon balance, including deforestation, logging, and land-use changes. The study concludes that forests have acted as an indirect negative feedback on the rise in CO2 concentration since deglaciation, and that the carbon balance of forests is a complex and dynamic system influenced by a range of factors.