evidence: Earth’s orbit is eccentric, meaning it has changed repeatedly over time. Nudged by the gravitation of Jupiter, Mars, Venus and other planets, our world’s axial tilt and precession are always slowly shifting. And its orbit slips between circular and elliptical paths in complex cycles across millennia. One cycle in particular, with a duration of 405,000 years, helps geologists calibrate planetary dynamics using sediment records: like clockwork, when this cycle brought Earth closer to the sun, the climate warmed, leaving behind evidence laid down in rock. [...] Whatever was responsible for the surge of carbon dioxide that preceded the PETM, the event—and the rise in global temperatures that followed—are the best analogue in the rock record for current human-caused global warming. That does not mean orbital forcing is playing a role in anthropogenic climate change, Zeebe says. In fact, humans are releasing carbon into the atmosphere far faster than it occurred at the time of the PETM, meaning its impacts could be more severe. "Obviously, orbital configurations today are very different than they were 56 million years ago," he says. "And in terms of future climate change, there’s very little expectation that orbital forcing will reduce or mitigate [it]."