Where is the climate headed? If the world proceeds on a "business as usual" path, atmospheric CO2 concentrations will likely be more than 700 ppm by 2100, and they will still be rising. This is nearly double the current level and much more than double the preindustrial level of 280 ppm (Figure 10). State-of-the-art climate models suggest that this will result in an increase of about 3.5oF in global temperatures over the next century. This would be a rate of climate change not seen on the planet for at least the last 10,000 years. It is the combined threat of elevated concentrations of greenhouse gases and this unprecedented rate of increase that causes great concern.

What are the projected extent and pattern of warming over the globe? The higher latitude regions will warm relatively more than areas nearer to the equator. The land surface will warm more than the oceans, and there will be less variation in temperature from night to day.

Even if the rate of emissions is slowed enough to limit atmospheric concentrations to about 550 ppm, or roughly double the preindustrial level, the U.S. could experience temperature increases of 5o F to 10o F (Figure 11). These warmer temperatures would lead to soil drying in some regions, with drying estimated at 10 percent to 30 percent for the United States during the summer growing season (Figure 12).

Some modeling experiments have examined the consequences of CO2 levels well beyond 700 ppm, which are likely to occur after 2100 if current emissions trajectories are not altered. If the CO2 concentration were to continue to rise to four times the preindustrial level, or more than 1100 ppm, the estimated temperature increase for the United States would be 15oF to 20oF and soil drying could approach 30 percent to 50 percent during the growing season (Figures 13 and 14).






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