390 parts per million. Do you know how small that is?

I see, over and over again, the claim that CO2 is less than 400ppm (0.04%) of the composition of the Earth's atmosphere. That's entirely correct. The conclusion we are meant to draw is that such a tiny fraction of a sliver of a gas can't possibly have a noticeable effect on the climate.

QED, right? Can a very small amount of a substance dramatically alter the behavior of a complex system?

200-300mg of cyanide constitutes a lethal dose. A large human weighs perhaps 100kg. So the lethal dose of cyanide is about 3ppm.

0.1 mL of dimethyl mercury can be fatal. An adult has perhaps 30 L of volume, which figures out to about 33ppm.

The LD50* of ricin is 22 parts per *billion*, or 0.02ppm.

So such an argument, standing by itself, carries zero weight.

* the dose that kills 50% of the subjects it's given to.