When you break down the sources of CO2 emissions, you'll actually see that humans are one of the least contributing parties. Decaying plantlife takes the crown at 30% or so, and we're only at something of 2%.
However, our emissions are still more than what the delicate balance of the Earth had been. The oceans are basically a giant CO2 sink, and we have forests like the Boreal Forest in Canada which provides a huge amount of oxygen back into the atmosphere, and it worked for a very long time, getting gradually warmer since we started agricultural civilizations some 11,000 years ago. (And before that it went through warming and cooling periods for various other reasons.)
The Earth is constantly changing, only now it's affecting us, so it's a "problem".