John Denver is worried that the sunshine on his shoulders these days may be a sign of global warming. "In this decade, we're going to determine whether or not the human race can sustain itself on this planet," the singer said Thursday in Grand Rapids, Mich. He later gave a concert to benefit Grand Valley State University's Water Resources Institute, which works to improve area waterways. Denver, 47, who has been involved in many environmental activities, said he's most concerned about ozone depletion and global warming. "Those are the ones that have the greatest ramification for the greatest number of people," he said.