"Twenty thousand years ago," Caldeira says, "carbon-dioxide levels were lower, sea level was lower--and trees were in a near state of asphyxiation for lack of carbon dioxide. There's nothing special about today's carbon-dioxide level, or today's sea level, or today's temperature. What damages us are rapid rates of change. Overall, more carbon dioxide is probably a good thing for the biosphere--it's just that it's increasing too fast."