It's not historic. Real scientists tell us the level has been much higher in past millenia. The announced "scary level" is simply an insignificant amount, especially considering it's a natural component of the atmosphere...NOT a toxic pollutant.
Here's a little grade-school math:
Four hundred One millionths = 400/1,000,000
Divide by 100....400/1000000 = 4/10,000ths
Divide by four 4/10,000 = 1/2500th
1/2500 = 0.0004 = .04%
AKA: Four hundredths of a percent.
That's not even close to one percent.
For comparison sake, Oxygen is 21% of the atmosphere, Nitrogen is 78%.
HELL-freaking-LOW!
These "scientists" manipulate the numbers in order to try and scare the uninformed.