Note: The solid lines indicate the four illustrative marker scenarios of these four families, with the coloured bands showing the range of emissions scenarios within each group. For the A1 scenario, the two illustrative scenarios A1FI and A1T are also shown (dashed lines).
Source: Nakicenovic and Swart 2000: figure 3.
Text description of figure
The emissions of the four scenario families are shown in four graphs. The first, for the A1 family, has curves for the A1F1, A1B, and A1T scenarios. All peak and decline, the A1F1 peaking around 2080, A1B peaking around 2050, and A1T around 2035. After about 2030 the A1F1 scenario has much more rapid growth in emissions than the other two, with emissions at the end of the century close to the peak emissions, around 30 Gigatonnes of carbon per year. The A1B scenario has a slower growth and faster decline, ending the century with emissions about 15 Gigatonnes of carbon per year, or half those of the A1F1 scenario. The A1T scenario has a still lower peak and faster decline, ending the century with emissions around 5 Gigatonnes of carbon per year, or less than the 1990 values of around 8 Gigatonnes of carbon per year. It is notable that within the A1T sub-family, the scenarios with the lowest early growth have the lowest peak but also decline most slowly, ending the century with the highest emissions within this group, slightly above the 1990 values.
The second graph shows the A2 scenarios. These have much slower initial growth in emissions than the A1 family, but the growth is monotonically increasing. At the end of the century the emissions are very similar to those of the A1F1 scenarios, around 30 Gigatonnes of carbon per year, but they are increasing rapidly.
The third graph has the B1 family and shows they are broadly similar to the A1T scenarios but include scenarios which peak higher and later (the family peaks between 2020 and 2050 and between 10 to 19 Gigatonnes of carbon per year). Final values, between 4 and 10 Gigatonnes of carbon per year, are comparable with the A1T sub-family.
The fourth and final graph, of the B2 family, shows these scenarios have slower early growth than many others and that this growth slows very gradually with time, rather than increasing as for the A2 scenarios but such that some scenarios in the family are still growing at the end of the century, others are in gentle decline. At this time the emissions range from about 11 to about 22 Gigatonnes of carbon per year.