Important aspects of the global climate system, which directly affect many people, are already seeing the effects of arctic climate change. This assessment of the most recent science shows that numerous arctic climate feedbacks will make climate change more severe than indicated by other recent projections, including those of the Intergovernmental Panel on Climate Change Fourth Assessment report (IPCC 2007). Some of these feedbacks may even interact with each other. Up-to-date analyses of the global consequences of arctic change highlight the need for ongoing critical review of the thresholds of dangerous human interference with the climate system, and demand increased rigour to stay below these thresholds through an ambitious global effort to reduce atmospheric greenhouse gases.