Do cars or trees breathe faster?

I have a hard time believing that the number of planes, trains, and automobiles is good for the oxygen levels on our planet.  I look at mars and think earth could become that way if we continue to change the chemical composition of the elements in our atmosphere. Are we creating our own doom?  The three most basic needs of survival seem at risk....air, water, and food.  When I sit at my country home and smell auto fumes with no running cars around, I feel concern.  There has to be a better way.