According to a story in Tuesday’s New York Times:
The new study, published in the Proceedings of the National Academy of Sciences, projects that if carbon dioxide concentrations peak at 600 ppm, several regions of the world — including southwestern North America, the Mediterranean and southern Africa — will face major droughts as bad or worse than the Dust Bowl of the 1930s. Global sea levels will rise by roughly three feet by the year 3000, a projection that does not factor in melting glaciers and polar ice sheets that would probably result in significant additional sea level rises.
Even if the world managed to halt the carbon dioxide buildup at 450 ppm, the researchers concluded, the subtropics would experience a 10 percent decrease in precipitation, compared with the 15 percent decrease they would see at 600 ppm. That level is still akin to mega-droughts such as the Dust Bowl. The already parched U.S. Southwest would probably see a 5 percent drop in precipitation during its dry season.
Moral: 450 is bad, 600 is terrible.