For millennia humans burned wood for cooking and heat. As long as there were a large number of trees, trees absorbed the CO2 from the fires and created new trees.
Then humans pulled coal and oil from beneath the ground and burned it. The rate of CO2 being put into the atmosphere increased dramatically. Furthermore, because trees all over the world were being cut down faster than they could be replaced, the absorption of CO2 from the atmosphere decreased. The increase in atmospheric CO2 traps more heat and thus warms the Earth overall.
Those who deny the human effect on global warming must believe that 1+1-1 equals 3!
