Problems of risk and uncertainty arise throughout the assessment of climate change and the development of climate policy. Both the climate system and the economic system are characterized by long time lags and complex causal connections; over long periods of time, both climate and economic outcomes are necessarily uncertain. 

In view of the centrality of risk and uncertainty to climate assessment, it will be helpful to begin by clarifying the meaning of these concepts. Recommended approaches to risk and uncertainty are addressed in the final section in this document.