Three different population trajectories that correspond to socio-economic developments in the storylines were chosen from recently published projections. They share the lowest trajectory, increasing to 8.7 billion by 2050 and declining toward 7 billion by 2100, which combines low fertility with low mortality. The B2 scenario family is based on the long-term UN Medium 1998 population projection of 10.4 billion by 2100. The A2 scenario family is based on a high population growth scenario of 15 billion by 2100 that assumes a significant decline in fertility for most regions and stabilization at above replacement levels. It falls below the long-term 1998 UN High projection of 18 billion.
All scenarios describe futures that are generally more affluent than today .
The scenarios span a wide range of future levels of economic activity, with gross world product rising to 10 times today s values by 2100 in the lowest to 26-fold in the highest scenarios. A narrowing of income differences among world regions is assumed in many of the SRES scenarios. Two of the scenario families, A1 and B1, explicitly explore alternative pathways that gradually close existing income gaps in relative terms. Technology is at least as important a driving force as demographic change and economic development. These driving forces are related.