EVERGLADES NATIONAL PARK, Fla. — Amid lurking alligators and the steamy heat of Florida’s Everglades, President Barack Obama on Wednesday sounded the alarm about damage from climate change he said was already wreaking mayhem in Florida and across the United States. In an implicit rebuke to Florida’s governor and other Republicans, Obama accused those who deny the man-made causes of climate change of sticking their heads in the sand.