400 parts per million. That's 1 part in 2,500. That's 4/100ths of 1 percent.
Let's go a little further...
In 1958, when the Mauna Loa site - that is a volcano, right? - started collecting data, the numbers averaged 315 ppm. Now (I'll round up) we're roughly at 400 ppm. That's a difference of 85 ppm - in 56 years.
So somehow a change of 1 part in 11,764 over 5+ decades (0.0085%) is responsible for all this gloom and doom? I'm hardly versed in the atmospheric sciences, but I'd need somebody to explain this to me.
Money. Lots and lots of money.