Four from 40 scenarios are designated as marker scenarios that are characteristic of the four scenarios families. Together with the two additional illustrative scenarios selected from the scenario groups in the A1 family, they capture most of the emissions and driving forces spanned by the full set of the scenarios. There is no single central or best guess scenario, and probabilities or likelihood are not assigned to individual scenarios. Instead, the writing team recommends that the smallest set of scenarios used should include the four designated marker scenarios and the two additional illustrative scenarios selected from the scenario groups in the A1 family. Distinction between scenarios that envisage stringent environmental policies and those that include direct climate policies was very difficult to make, a difficulty associated with many definitional and other ambiguities.
All scenarios describe futures that are generally more affluent than today.
Many of the scenarios envisage a more rapid convergence in per capita income ratios in the world compared to the IS92 scenarios while, at the same time, they jointly cover a wide range of GHGs and SO 2 emissions. Emissions profiles are more dynamic than the IS92 scenarios, which reflects changes in future emissions trends for some scenarios and GHG species. The levels of GHG emissions are generally lower than the IS92 levels, especially toward the end of the 21 st century, while emissions of SO , which have a cooling effect on the atmosphere, are significantly lower than in IS92. Alternative combinations of main scenario driving forces can lead to similar levels of GHG emissions by the end of the 21 st century. Scenarios with different underlying assumptions can result in very similar climate changes.