That's a logical fallacy:

Just because something is present at a small concentration doesn't mean that nearly doubling its concentration won't have consequences.

As an example: The stress hormone cortisol is normally present at a concentration of 250 nM, much lower than the CO2 concentration in the atmosphere. According to you argument, we should not pay attention to cortisol, because of its low conc.

However, doubling the cortisol concentration has serious consequences, among others a constantly-stressed state and weight gain.

So, if you want to advocate against AGW, please use other arguments, but not that CO2 is present at low concentration and, therefore, of no consequence.