It's true. Although the real concern in climate science is average concentrations of carbon dioxide over much longer periods of time, surpassing the 400 ppm mark, even for a day, is a historic milestone. 400 ppm was once a level we talked about avoiding altogether through mitigation of greenhouse gas emissions. Now, it's a reminder that we're not really doing anything to circumvent the steady increase in global carbon dioxide concentrations and global average temperature.
Scary CO2 Milestone Reached
At 400 parts per million, we are far beyond what's natural or even what's been seen since humans evolved.