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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.

Chart showing atmospheric levels for the last 500,000 years. Prior to the industrial revolution, levels bounced between approximately 180 ppm and 300 ppm, with a peak every 100,000 years on average. In the last 60 years we have gone from 300 ppm to 400 ppm.

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.


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