The recent warming in high latitudes is affecting a broad spectrum of physical, ecological, and human/cultural systems in this region. Some of these changes may be irreversible on century time scales, and have the potential to cause rapid changes in the earth system. The response of the carbon cycle in northern high latitude regions is a major concern. The release of large stores of carbon in land and ocean systems of the Arctic has the potential to substantially increase the concentration of carbon dioxide and methane, which would act as a positive feedback to climate change with worldwide consequences. To address this concern we have conducted the Arctic Carbon Cycle Assessment, an activity sponsored by the Arctic Monitoring and Assessment Program (AMAP), the Climate and Cryosphere (CliC) Project, and the International Arctic Science Committee (IASC). Our overall goal in this assessment is to clarify key uncertainties and vulnerabilities in the response of the carbon cycle in northern high latitude regions to projected climate change. In this assessment we (1) provide a contemporary picture of the stocks and fluxes of the carbon cycle in northern high latitudes, (2) identify the role of northern high latitudes in the contemporary global carbon cycle, and (3) present our current understanding of the sensitivities of the carbon cycle in this region to climate change and the implications of responses for the global carbon cycle. The assessment has identified that there is between 1000 and 2000 Pg (1E+15 g) C stored in northern high latitudes that is potentially vulnerable to climate change over the next century. Currently, it appears that northern high latitude regions are a sink for atmospheric carbon dioxide of approximately 0.5 Pg C per year and a source of atmospheric methane of approximately 50 Tg (1E+12 g) C per year. While these estimates have substantial uncertainty, they do indicate that northern high latitudes are important in the global carbon cycle. A substantial release of volatile carbon stored in and below permafrost of terrestrial ecosystems of northern high latitudes has the potential to impact global climate change significantly. The rate of carbon release will depend on both the rate of warming and whether the landscape becomes wetter or drier. The greatest potential for marine systems in the Arctic to influence the global carbon cycle is through substantial methane release to the ocean and atmosphere from the warming of marine methane hydrates and seabed permafrost.