mspohr writes from an article on The Washington Post: We haven't seen this much CO2 added to the atmosphere in 66 million years: "If you look over the entire Cenozoic, the last 66 million years, the only event that we know of at the moment, that has a massive carbon release, and happens over a relatively short period of time, is the Palaeocene-Eocene Thermal Maximum (PETM)," says Zeebe. "We actually have to go back to relatively old periods, because in the more recent past, we don't see anything comparable to what humans are currently doing." [New research suggests, even the drama of the PETM falls short of our current period, in at least one key respect: We're putting carbon into the atmosphere at an even faster rate than happened back then.] "The anthropogenic release outpaces carbon release during the most extreme global warming event of the past 66 million years, by at least an order of magnitude," writes Peter Stassen, an Earth and environmental scientist at KU Leuven, in Belgium, in an accompanying commentary on the new study. "Given that the current rate of carbon release is unprecedented throughout the Cenozoic, we have effectively entered an era of a no-analogue state, which represents a fundamental challenge to constraining future climate projections," the study concludes.