Forests play a very important part in the global carbon cycle: over the past two decades they have removed from the atmosphere about one-third of the carbon that is emitted globally from the burning of fossil fuels.  Although scientists have suspected that the terrestrial carbon (C) sink has been large in recent decades, its size and location have been poorly known until now.  Using forest inventory data and long-term ecosystem studies, a team of scientists from around the world estimated a total forest sink of 2.4±0.4 Pg C yr-1 (2.4 billion tonnes per year) globally for 1990-2007. This forest sink is equivalent in magnitude to the terrestrial sink that was previously estimated indirectly from estimates of fossil fuel emissions, land-use change, ocean uptake, and atmospheric concentrations.  This is the first analysis to estimate the magnitude of the global forest C sink using contemporary forest inventory data, which provides strong evidence for the geographic location of the C sink and the responsible processes.  Understanding the causes of the forest C sink is necessary to project the future contribution of forests to the carbon cycle, which may be affected by many factors such as drought, natural disturbances, and land use.