The article presents an estimate of the net CO2 exchange between the terrestrial biosphere and the atmosphere in North America from 2000 to 2005, derived from a state-of-the-art data assimilation system called CarbonTracker. The system uses 28,000 CO2 mole fraction observations to produce surface fluxes that are consistent with recent atmospheric CO2 trends. The estimated net carbon sink for North America over this period is −0.65 PgC/yr, with uncertainties ranging from −0.4 to −1.0 PgC/yr. The sink is primarily located in deciduous forests along the East Coast (32%) and boreal coniferous forests (22%). The study also highlights the sensitivity of carbon sinks to climate extremes, as terrestrial uptake decreased to −0.32 PgC/yr during the 2002 drought. CarbonTracker's results are in good agreement with the first North American State of the Carbon Cycle Report (SOCCR) and are freely available online. The article discusses the principles and methods behind CarbonTracker, its evaluation using independent observations, and its potential for monitoring the carbon cycle in detail.The article presents an estimate of the net CO2 exchange between the terrestrial biosphere and the atmosphere in North America from 2000 to 2005, derived from a state-of-the-art data assimilation system called CarbonTracker. The system uses 28,000 CO2 mole fraction observations to produce surface fluxes that are consistent with recent atmospheric CO2 trends. The estimated net carbon sink for North America over this period is −0.65 PgC/yr, with uncertainties ranging from −0.4 to −1.0 PgC/yr. The sink is primarily located in deciduous forests along the East Coast (32%) and boreal coniferous forests (22%). The study also highlights the sensitivity of carbon sinks to climate extremes, as terrestrial uptake decreased to −0.32 PgC/yr during the 2002 drought. CarbonTracker's results are in good agreement with the first North American State of the Carbon Cycle Report (SOCCR) and are freely available online. The article discusses the principles and methods behind CarbonTracker, its evaluation using independent observations, and its potential for monitoring the carbon cycle in detail.