Anthropogenic climate warming is amplified in the Arctic, impacting the Arctic carbon cycle and its role in regulating climate and global biogeochemical cycles. In this Review, we provide a quantitative and comprehensive overview of the present-day Arctic carbon cycle across the land–ocean continuum. Terrestrial soil stocks total 877 ± 16 Pg C, with upper marine sediments containing 82 ± 35 Pg C. Overall, the integrated Arctic system is a carbon sink, driven by oceanic uptake of CO<inf>2</inf> (127 ± 36 Tg C year⁻¹) and organic carbon burial in shelf sea sediments (112 ± 41 Tg C year^–1). Terrestrial systems, including inland waters and disturbance, are a net source of CH<inf>4</inf> (38 (21, 53) Tg C year^–1) and CO<inf>2</inf> (12 (–606, 661) Tg C year^–1). The Arctic carbon sink will likely weaken under continued warming, owing to factors such as increased coastal erosion, outgassing of riverine organic carbon and enhanced nearshore carbon turnover lowering shelf sediment burial. Arctic greening and increases in terrestrial carbon sinks will be substantially offset by increases in soil respiration, disturbance from extreme events and enhanced emissions from inland waters. Future research should prioritize enhanced coverage of small catchments and nearshore regions, and inclusion of non-linear responses in biogeochemical models.