When I was living at home, some 60 years ago, the air was heavy with foundry slag, it smelled as if you were in the factory, some areas were so polluted if you drove through them, the crap in the environment was so thick you had to wash your car. But that was before global goreing so I guess the air was better then. 
One of the most impressive things about Gary, Indiana when I first drove through, was the brown dome you could see for miles over the city. But that was 45 years ago, and between shutting down industry and moving target emissions standards, the last time I was in the area, the dome was gone.
Having put in standards or closures that eliminated the real and readily visible pollutants, the EPA, in order to continue expanding, and existing at the previous levels of staffing, created by decree other pollutants. SMOG, as we knew it, was gone, eliminated by new emissions requirements, and that was good. No more gray snow in Pittsburgh, no brown haze over the hives of humanity, even if that industry was now elsewhere (along with at least some of those emissions). But the NIMBYs of environmental regulation needed (desperately if they were going to keep their jobs) a new boogeyman.
Now it had to be something that was common (or more so) than cat crap, something already in the atmosphere and that virtually everything alive either required for life or exhaled as a matter of respiration. Something you couldn't see, because like invisible friends, invisible enemies are the bestest.
It wasn't until the trailing indicator of CO2 levels in the earth's atmospheric history could somehow be presciently blamed for the weather, that it could be declared a pollutant, and well, here we are, trapped in a nonsensical spiral of human self destruction for "the planet" (which will continue to exist, even if devoid of life on the outside, like the first couple of billion years it was here).
But then, it isn't about the environment, nor the ecology (which is a balance), nor even preserving dynamic systems in stasis, it is about the preservation of Power, the Power to determine who and what wins or loses to the accumulation of wealth by a few, who are calling for the rest of us to die off, plants included.
