FIGURE 2.3 Temperature increases in the 21st century are very likely to be larger than those observed in the 20th century according to scenarios developed by the Intergovernmental Panel on Climate Change. For definitions of scenarios B1 and A1F1 at each end of the range, see the main text. SOURCE: IPCC (2007; Figure 3-2).

sions (Figure 2.3). A low-emission scenario, B1, assumes a mid-century peak in global population, the rapid development of a services-oriented economy, and a change toward clean and efficient energy technologies. A high-emission scenario, A1FI, assumes a mid-century peak in population, rapid economic growth, and intensive use of fossil fuels for energy. Other scenarios fall between those two extremes.

Even if all use of fossil fuels were to cease today, these models predict another 0.6-degree centigrade increase in temperature during the 21st century, Davis observed. Since all of the IPCC scenarios assume continued use of fossil fuels, all of the scenarios assume temperature increases larger than that amount.

The most positive scenario (B1) results in model predictions of a 1.5- to 2-degree centigrade (2.7- to 3.6-degree Fahrenheit) temperature increase over the 21st century. This scenario is almost certainly overoptimistic, according to Davis. With a more balanced mix of assumptions (scenario A1B), an additional 1-degree centigrade (1.8-degree Fahrenheit) increase occurs. As a result, the models predict a 2- to 3-degree centigrade (3.6- to 5.4-degree Fahrenheit) increase in global temperatures.



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