I wouldn't say it's any one solitary thing but a mixture of things. For instance, there are simply more people now than before. People use O2, produce CO2. So, that would mean more CO2 in the air. Second, fewer trees due to development. Trees use CO2, produce O2. Fewer trees to produce CO2, more CO2 still around. Though I am not quite sure of the chemical reactions in cars and the plants, I do believe it is similar with them. They use O2 and produce CO2. More of them around means more CO2 in the air. And, while all of this is going on, there simply hasn't been as much work on "building" O2-producing systems, like planting more trees.