evidence: In the latest bit of bad news for a planet beset by climate change, the concentration of carbon dioxide in Earth’s atmosphere has climbed to a level last seen more than 3 million years ago — before humans even appeared on the rocky ball we call home. [...] The level of CO2 in the atmosphere has risen an average of 2.5 ppm per year over the past decade, reaching 400 ppm in 2013 — and the level appears likely to go higher from here. [...] 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."