February is one of the first months since before months had names to boast carbon dioxide concentrations at 400 parts per million.* Such CO2 concentrations in the atmosphere have likely not been seen since at least the end of the Oligocene 23 million years ago, an 11-million-year-long epoch of gradual climate cooling that most likely saw CO2 concentrations drop from more than 1,000 ppm. Those of us alive today breathe air never tasted by any of our ancestors in the entire Homo genus.

Homo sapiens sapiens—that’s us—has subsisted for at least 200,000 years on a planet that has oscillated between 170 and 280 ppm, according to records preserved in air bubbles trapped in ice. Now our species has burned enough fossil fuels and cut down enough trees to push CO2 to 400 ppm—and soon beyond. Concentrations rise by more than two ppm per year now. Raising atmospheric concentrations of CO2 to 0.04 percent may not seem like much but it has been enough to raise the world’s annual average temperature by a total of 0.8 degree Celsius so far. […]