Terrestrial ecosystems and the climate system are closely coupled, particularly by cycling of carbon between vegetation, soils and the atmosphere. It has been suggested(1,2) that changes in climate and in atmospheric carbon dioxide concentrations have modified the carbon cycle so as to render terrestrial-ecosystems as substantial carbon sinks(3,4); but direct evidence for this is very limited(5,6). Changes in ecosystem carbon stocks caused by shifts between stable climate states have been evaluated(7,8), but the dynamic responses of ecosystem carbon fluxes to transient climate changes are still poorly understood. Here we use a terrestrial biogeochemical model(9), forced by simulations of transient climate change with a general circulation model(10), to quantify the dynamic variations in ecosystem carbon fluxes induced by transient changes in atmospheric CO2 and climate from 1861 to 2070. We predict that these changes increase global net ecosystem production significantly, but that this response will decline as the CO2 fertilization effect becomes saturated and is diminished by changes in climatic factors. Thus terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change.