Scenarios for the Fourth National Climate Assessment

Scenarios are coherent, internally consistent, and plausible descriptions of possible future states of the world. Scenarios may be quantitative, qualitative, or both. The components of a scenario are often linked by an overarching logic, such as a qualitative narrative of how the future may evolve. ‘Scenario’ is one category in a typology of terms, for example as used by the Intergovernmental Panel on Climate Change (IPCC), for describing future states, which also includes ‘storyline’, ‘projection’, and ‘probabilistic future’. The U.S. Global Change Research Program (USGCRP) is mandated to “assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change.” To fulfill this mandate, the NCA assesses risks to the Nation posed by climate and global change. This entails addressing specific questions about what is at risk in a particular region or sector, aligned with different potential futures. Scenarios that span a range of plausible future changes in key risk drivers and determinants of vulnerability, such as weather and climate extremes, sea level, population, and land use, can help carry this out. USGCRP has therefore coordinated the development of a set of scenario products, accessible through this website, to support NCA4 development.