Some are simply variants of others generated for a particular purpose. Many new scenarios are designed to track existing benchmark scenarios. A good example is the set of IS92 scenarios, especially the central IS92a scenario, which was often used as a reference from which to develop other scenarios. A further consideration is that not all scenarios are created in an equal fashion. Some are based on extensive formal models, while others are generated using simple spreadsheets or even without any formal tools at all.
Numerous factors influence future emissions paths in the scenarios.
Clearly, demographic and economic developments play a crucial role in determining emissions. However, many other factors are involved also, from human resources, such as education, institutional frameworks, and lifestyles, to natural resource endowments, technologic change, and international trade. Many of these important factors are not documented in the database, and sometimes not even in the respective scenario reports and publications. Some are neither quantified in the scenarios nor explicitly assumed in a narrative form. For this analysis, a simple scheme is used to decompose the main driving forces of GHG emissions.