Because greenhouse gases already released into the atmosphere will linger for many years, and because the Earth is slow to adjust to these changes, the climate is expected to continue to warm
in the future. In fact, this continued warming will occur even if governments act today to reduce emissions from cars, power plants, and other sources. As Mayer Hillman explains, "Even if no additional carbon dioxide were emitted from now on, atmospheric concentrations would take centuries to decline to pre-Industrial Revolution levels."16
The most widely accepted predictions of future temperature and climate changes are found in the IPCC's 2007 report. Essentially, the IPCC says that the degree of global warming will vary depending on the level of future greenhouse emissions. So it developed six emission scenarios and, using sophisticated climate simulation programs, projected "best estimate" and "likely range" future temperatures for each scenario. Overall, the IPCC projected a range of temperature increases between 2 to 11.5 degrees Fahrenheit (1.1 to 6.4 degrees Celsius) by the end of the century.
The most optimistic IPCC scenario, for example, assumes that emissions can be dramatically lowered as a result of a rapid change toward a service and information economy and a quick shift toward clean energy technologies. Under these conditions, the IPCC predicts a likely range of 0.5 to 1.6 degrees Fahrenheit (0.3 to 0.9 degree Celsius), and a best estimate temperature rise of about 1.1 degrees Fahrenheit (0.62 degree Celsius) by 2100.
If no action is taken to reduce emissions, however, the IPCC concludes that there may be twice as much warming over the next two decades as today. And the worst-case scenario predicted by the IPCC suggests that the world might see drastically higher temperatures over the next century. This scenario assumes that there will be rapid economic growth and continued reliance on fossilintensive energy production and consumption, and thus continuing high levels of greenhouse emissions. If this occurs, the IPCC says the world would likely see temperatures soaring somewhere in the range of 4.3 to 11.5 degrees Fahrenheit (2.4 to 6.4 degrees
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