Mauna Loa Observatory - Another CO2 Record

[Durian Fiend]

.. 422 ppm, now double the CO2 at the start of the Industrial Revolution...

I believe it's 50% higher now than back then.    

To find the last time earth was over 400 ppm takes a trip back in time to 16 million years ago.

[TomK]

I think that's more accurate and the right place to start since there wasn't any real evidence atmospheric CO2 levels were rising significantly before the Industrial Revolution. Before that time, there were around 280 parts per million but in just about a couple of hundred years they are now well over 400 and the rise is consistent with the amount of CO2 being pumped into the Earth's atmosphere by human activity. There aren't any other sources of CO2 that can explain this.