Figure SPM-4: Total global cumulative CO2 emissions (GtC) from 1990 to 2100 (SPM-4a) and histogram of their distribution by scenario groups (SPM-4b). No probability of occurrence should be inferred from the distribution of SRES scenarios or those in the literature. Both figures show the ranges of cumulative emissions for the 40 SRES scenarios. Scenarios are also grouped into four cumulative emissions categories: low, medium-low, medium-high, and high emissions. Each category contains one illustrative marker scenario plus alternatives that lead to comparable cumulative emissions, although often through different driving forces. This categorization can guide comparisons using either scenarios with different driving forces yet similar emissions, or scenarios with similar driving forces but different emissions. The cumulative emissions of the IS92 scenarios are also shown.