The Congressional charge that called for assessments of global change indicated a desire to have information that looked ahead 25 to 100 years, projecting major trends in climate and its consequences. In laying out these guidelines, Congress carefully chose the word "project" rather than "predict," recognizing that making a specific prediction or forecast would not be credible, given how rapidly the world is changing and the many possible outcomes. However, as is done by businesses, the military, and many other groups involved in high-stakes planning, developing scenarios is an approach that can provide plausible alternative futures (e.g., assuming a product will sell, considering the need to defend an interest somewhere in the world).

These plausible alternative futures can be used to project what might happen in the future under a particular, evolving set of assumptions. Done carefully, scenarios can provide a starting point for examining questions about an uncertain future and can help us visualize alternative futures in concrete and human terms. The National Assessment followed a scenario-based approach.

To investigate the potential consequences of climate variability and change for the environment and society, a number of scenarios were used to provide context for carrying out the analyses:

1. Climate change scenarios were developed to indicate how temperature, precipitation, sea level, and other climatic variables may change through the course of the 21st century. For this Assessment, climate change scenarios were generated in three ways: (a) by assuming 21st century climate would be like 20th century climate; (b) by assembling and processing results from simulations using global climate models; and (c) by examining where thresholds and breakpoints might occur.
2. Ecosystem change scenarios were needed to indicate how natural ecosystems, in the absence of human intervention, would change. For this assessment, ecosystem change scenarios were generated by use of ecosystem models that were driven by application of the global climate model scenarios.
3. Socioeconomic scenarios were needed to provide a context for considering the relative importance of the changes that are considered. For this assessment, socioeconomic scenarios were generated using well-established planning models assuming a range of key socioeconomic factors (such as rate of population growth).