An ecosystem model, ecosys, has been used to examine the effects of recent warming on carbon exchange in higher latitudes of North America. Model results indicated that gradual warming during the past 30 years has increased net ecosystem productivity (NEP) and leaf area index (LAI). Spring increases in LAI advanced by 2.3 days decade−1 and decreases in autumn were delayed by 5.0 days decade−1 from 1982 to 2006. These advances and delays were corroborated by similar trends observed in the normalized difference vegetation index. NEP modelled during this period increased at an average rate of 17.6 Tg C decade−1. Increasing carbon losses modelled with soil warming in autumn, when thaw depth was greatest, offset 34% of increasing carbon gains modelled in spring. If autumn warming continues, carbon losses in this season may further offset enhanced carbon sequestration in spring.