Climate change of the magnitude and time frames projected by the world's leading climate scientists poses fundamental questions of human security, survival and the stability of nation states. While state weakness and destabilising internal conflicts are a more likely outcome than inter-state war, climate change will be a stress multiplier for all nations and societies, especially those already at risk from ethnic and religious conflicts, economic weakness and environmental degradation. Prudence and sensible risk management suggest that policymakers need to take this issue far more seriously. And strategic planners ought to include worst-case climate-change scenarios in their contingency planning, as climate change is set to rank with terrorism, pandemic diseases and major war as one of the principle challenges to security in the twenty-first century.