Climate warming is resulting from disruption of the global carbon (C) cycle. The Arctic is already warming significantly and the region governs some critical feedbacks in global change, including release of huge C stores from high-latitude soils, shift in albedo due to changes in vegetation and snow cover, and potential effects on the thermohaline circulation as a result of alterations in river discharge to the Arctic Ocean. However, the links between climate, the C cycle, energy balance, and hydrology are complex and our understanding of them – and their impact on Arctic ecosystems – is poor. ABACUS aims to provide answers to these critical questions.