Carbon dioxide hits a level not seen for 3 million years. Here's what that means for climate change — and humanity.

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.

“We’re racing toward a state very different from the kind humans evolved in and that civilization developed in,” said Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, California.

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.

“None of these specific numbers are really thresholds in the sense that anything particular happens when we cross them,” Gavin Schmidt, a climatologist who directs NASA’s Goddard Institute for Space Studies in New York City, told NBC News MACH in an email.

“But these impacts are going to persist for a very long time,” said Dana Royer, a professor of earth and environmental sciences at Wesleyan University in Middletown, Connecticut.

Even if moving to renewable energy and other measures help stanch the steady flow of carbon dioxide into the atmosphere, our descendants will likely be saddled with the negative consequences of our artificially elevated levels of CO2.

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