Ramesh Murthy gazes up at a stand of cottonwood trees growing in the high Sonoran Desert 35 miles northeast of Tucson, Arizona. Planted as 6-inch stumps, the 20 to 25-foot-tall trees offer shade from the warm midday sun. Rather than growing along some desert stream, however, these cottonwoods stand in neat rows within a futuristic-looking, glass-and-steel enclosed structure at the Biosphere 2 Center in the Santa Catalina Hills near Oracle, Arizona. They were planted not to restore a natural habitat or to save an endangered species, but for a series of experiments to test how ecosystems might respond to rising carbon dioxide levels and other climate changes associated with global warming.