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As others have stated it's less than 0.04% And no, I honestly didn't need to look that up.

And while you may not believe that 6.2 trillion tons of CO2 in the atmosphere is significant since you are only concerned about percent I can assure you it is. While comprising only 0.04% of the atmosphere, it accounts for over 20% of the greenhouse effect and is in fact 2nd only to water vapor in it's contribution.

Nonetheless, if you still insist that 0.04% is too small a number to give any attention or is an insignificant figure then I would like you to take my test:

(And feel free to Google this of course)

Look up the term Batrachotoxin if you are not familiar with it. I would like you to tell me in percent body weight, how much Batrachotoxin would it take to kill a bull elephant. Seriously, see if you can calculate the answer. If you can't, I'll run the numbers and post it up. Let me know.