Last year will go down in history as the year when the planet’s atmosphere broke a startling record: 400 parts per million of carbon dioxide. The last time the planet’s air was so rich in CO2 was millions of years ago, back before early predecessors to humans were likely wielding stone tools; the world was a few degrees hotter back then, and melted ice put sea levels tens of meters higher.“We’re in a new era,” says Ralph Keeling, director of the Scripps Institution of Oceanography’s CO2 Program in San Diego. “And it’s going fast. We’re going to touch up against 410 pretty soon.”