It's true, at least for today. 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. Happy Friday!