Whether global warming is happening and what the effects will be is a scientific issue. But what we need to do to reduce carbon dioxide emissions is to change energy policies, so that is a political issue. It's just the same as with CFCs eating away the ozone layer and sufur emissions causing acid rain. If no political action had been taken, those would still be problems.
Ironically, most of the people who argue against the science of global warming are opposed to what to do about it. They argue we should not destroy the economy and go back to an agrarian lifestyle. But using LED light bulbs (and doing other things to use energy more efficiently) and generating power from solar, wind, and nuclear are the actual proposed solutions, not lifestyle changes. In effect they're taking a politcal issue and trying to argue it in the scientific arena, which will never work.
It's not even a matter of whether a particular substance is a "pollutant" or "toxic". Many necessary substances can be harmful if present in high concentrations. You can die just by drinking too much water. That doesn't mean that water's a pollutant, even though too much can kill you. The argument that carbon dioxide is not a pollutant because plants need it is similarly confused -- too much of a good thing can be harmful.
To get to the heart of the matter, the EPA considers any harmful emission to be a pollutant, even if the substance emitted is necessary for life.