It is a brilliant hot morning in the desert north of Tucson, Ariz., and the sun blazes down on a dozen or so people as they wend their way through a savanna and around a marsh, an ocean, and miles of pipes, channels, steel struts and glass panels. Half factory, half unkempt-looking greenhouse, the great glass structure that is Biosphere 2 is open to the public, to students and to scientists conducting climate change experiments. And on this day the rain forest, usually closed to visitors, is also open.

The group moves from behind the scenes--from the concrete underbelly of a man-made mountain and its 55-foot waterfall--out into the steamy forest, walking along wooden boards, sweltering in 85 degrees Fahrenheit, 95 percent humidity. The Arizona desert is no longer visible through the tangle of sugar palms, banana and Kapok trees (the latter have to be trimmed so they don't burst through the glass ceiling), and other vegetation. William Young, a guide, explains that researchers have just finished subjecting the plants to 30 days of drought followed by seven days of rain over several months to learn how these conditions affect carbon dioxide intake. Because Biosphere 2 is a closed system, rainfall and atmospheric makeup can be regulated and measured, allowing scientists to conduct controlled experiments.

   
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