Washington • As a young paleontologist, Kirk Johnson traveled to the Arctic to excavate fossils from a 50-million-year-old landscape. In that era, the air was thick with carbon dioxide and so warm that even the North Pole had no ice. Johnson and his colleagues dug up fossilized crocodiles, turtles, and palm trees.
"Palm trees!" Johnson recalled, "In the Arctic! ... It blew my mind that the Earth could change that much."
Since that expedition in the 1980s, global carbon dioxide concentrations have soared to levels not seen in the history of humankind. New temperature records are set almost every year.