The last time levels of atmospheric carbon dioxide
were this high came during the Pliocene Epoch, which extended from
about 5.3 million to 2.6 million years ago. During that period, average
sea levels were about 50 feet higher than they are today and forests
grew as far north as the Arctic, said Rob Jackson, a professor of earth
system science at Stanford University. “Earth was a very different
place,” he said. “You would hardly recognize the land surface, and my
gosh, we don’t want to go there.”