The most important decision any government has to make about climate change is one of priority: how much effort to expend on countering it, relative to the effort that must be spent on other issues. This risk assessment aims to inform that decision. CONCLUSIONS OF THE RISK ASSESSMENT: A climate change risk assessment must consider at least three areas: the future pathway of global emissions; the direct risks arising from the climate’s response to those emissions; and the risks arising from the interaction of climate change with complex human systems. Each of these areas contains large uncertainties. From our assessment, we draw the following conclusions about the most significant risks. DIRECT RISKS: The risks of climate change are non-linear: while average conditions may change gradually, the risks can increase rapidly. On a high emissions pathway, the probability of crossing thresholds beyond which the inconvenient may become intolerable will increase over time. SYSTEMIC RISKS: The risks of climate change are systemic. The greatest risks may arise from the interaction of the climate with complex human systems such as global food markets, governance arrangements within states, and international security. VALUE: Valuing these risks is essentially a subjective exercise. Any valuation of the risks of climate change will involve subjective judgments, most notably with regard to the importance attached to the wellbeing of future generations. Such judgments should be made transparently, so that they may be publicly debated.