Humankind's ever-expanding activities have caused environmental changes that reach beyond localities and regions to become global in scope. Disturbances to the atmosphere, oceans, and land produce changes in the living parts of the planet, while, at the same time, alterations in the biosphere modify the atmosphere, oceans, and land. Understanding this complex web of interactions poses unprecedented intellectual challenges. The atmospheric concentrations of natural trace gases-carbon dioxide (C0 ), methane (CH. ), nitrous oxide (N0), and lower-atmosphere ozone 2 2 (Os)-have increased since the...