FIGURE 5-1 Global CO2 emissions (billion tons of carbon per year) from all sources for the four scenario families (A1, A2, B1, B2) from the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES). The four scenario families had different assumptions about population, technological and land-use changes, and gross domestic product (GDP) growth rates. Each colored band represents the range of results for each scenario from the six emissions models used in the SRES report. Within any one scenario family, the emissions models all used the same scenario-specific global input assumptions for population and GDP growth. SOURCE: Nakićenović et al. (2000).

climate be affected if society takes no further policy steps to stem emissions and allows the driving forces behind emissions to take their course? It appears that none of the names are fully satisfactory and without ambiguities (Nakićenović et al., 2000). For example, “business as usual” does not convey that scenarios address economic and technology changes over a century time frame. Noninterventionist, the currently preferred term, also requires clarification because society is currently taking some steps, such as vehicular mileage standards and hybrid car marketing and research, that could be interpreted as climate policy.

Emissions scenarios that take account of policy intervention include those in which greenhouse gas concentrations are stabilized and cost-benefit scenarios. Stabilization scenarios are associated with atmospheric targets that policy makers often consider as long-term goals (IPCC, 1996;

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