Forecasts of changes in future climate due to emissions from human activities begin with the development of emission scenarios. These scenarios are not predictions, but represent plausible future conditions under particular assumptions.
Emissions scenarios are estimates of future greenhouse gas emissions released into the atmosphere. Such estimates are based on possible projections of economic and population growth and technological development, as well as physical processes within the climate system.
Such scenarios are indispensable to compare between them the results of the various climate models, by looking at what are the different results (depending on the models) with the same hypothesis.