"Professor Ian Plimer of Adelaide University and Professor Jon Harrison of the University of Arizona accept that oxygen levels in the atmosphere in prehistoric times averaged 35 percent compared to only 21 percent today. The levels are even lower in densely populated, polluted city centers and industrial complexes, perhaps only 15 percent or lower.<quoted text>
Hey dumbass..... The percent of oxygen in the atmosphere used to be over 30% and that was a hell of alot prior to any industrial revolution.
It's 20-21% all during the industrial revolution. Plants still need Carbon Dioxide and they still produce oxygen. People don't have an effect on it!
Much of this recent, accelerated change is down to human activity, notably the industrial revolution and the burning of fossil fuels. Which means we are slowing down one process, oxygen generation, and speeding up another, carbon dioxide production."