We’re officially living in a new world.

Carbon dioxide has been steadily rising since the start of the Industrial Revolution, setting a new high year after year. There’s a notable new entry to the record books. The last station on Earth without a 400 parts per million (ppm) reading has reached it.

Carbon dioxide officially crossed the 400 ppm threshold on May 23 at the South Pole Observatory.
Carbon dioxide officially crossed the 400 ppm threshold on May 23 at the South Pole Observatory. NOAA

A little 400 ppm history. Three years ago, the world’s gold standard carbon dioxide observatory passed the symbolic threshold of 400 ppm. Other observing stations have steadily reached that threshold as carbon dioxide has spread across the planet’s atmosphere at various points since then. Collectively, the world passed the threshold for a month last year.

In the remote reaches of Antarctica, the South Pole Observatory carbon dioxide observing station cleared 400 ppm on May 23, according to an announcement from the National Oceanic and Atmospheric Administration on Wednesday. That’s the first time it’s passed that level in 4 million years (no, that’s not a typo).