I think the other person may be attempting to argue that it's not a pollutant because it is, in part, naturally occurring, However, it's not in natural occurring levels that is the issue here. Human CO2 emissions have exponentially increased the amount so much that it's had significant, harmful impacts globally, thus CO2 in such excessive amounts is a pollutant. When an imbalance occurs, there's usually a negative impact and the source needs to be curtailed. Deer are natural but they need to be routinely culled otherwise they rapidly overpopulate and have a negative impact on the surrounding ecosystem. Alcohol occurs naturally and can be helpful, too much in one's system results in alcohol poisoning. It makes no sense to get hung up on semantics, the fact remains CO2 levels have reached dangerous levels and is a global problem.