In scenarios designed to project future GHG emissions, economic projections, e.g., changes in future income levels, will often necessarily be combined with other projections that affect emissions, e.g., future population levels. Since these future changes are highly uncertain, one approach is that of scenario analysis. In scenario analysis, scenarios are developed that are based on differing assumptions of future development patterns. An example of this are the "SRES" emissions scenarios produced by the Intergovernmental Panel on Climate Change (IPCC). The SRES scenarios project a wide range of possible future emissions levels. The SRES scenarios are "baseline" or "non-intervention" scenarios, in that they assume no specific policy measures to control future GHG emissions. The different SRES scenarios contain widely differing assumptions of future social and economic changes. For example, the SRES "A2" emissions scenario projects a future population level of 15 billion people in the year 2100, but the SRES "B1" scenario projects a lower population level of 7 billion people. The SRES scenarios were not assigned probabilities by the IPCC, but some authors have argued that particular SRES scenarios are more likely to occur than others.