In this paper we investigate how the CO₂ exchange between the land vegetation and the atmosphere varies from year to year. We quantify the relation between variations in the CO₂ exchange and variations in air temperature. For this quantification, we use long-term measurements of CO₂ in the air at many locations, a simulation code for the transport of carbon dioxide through the atmosphere, and a data set of air temperature. The results help us to understand the mechanisms of CO₂ exchange.