400 ppm

The world reached a milestone last month. Or maybe the month before. Or at the beginning of the year. Or possibly last year. But no matter how you measure it, atmospheric CO₂ is now at 400 parts per million (ppm), a level not seen in three million years. Well, somewhere between 2.2 and 3.6 million years. Or it might be anywhere between 800,000 and 15 million years. When dealing with proxy data at these time scales, there's a lot of room for uncertainty.

Two things are certain, however. We've reached the highest concentration of atmospheric CO₂ since human beings first appeared on the scene, and we're adding CO₂ at a much faster rate than we've ever seen. Neither of these bodes well for the future of humanity.