Well, I don't know what to tell you but CO2 emissions from fossil fuels accumulate in the atmosphere. There is this weird thing happening that since 1960 the CO2 content in the atmosphere has risen from 300ppm to 400ppm. It's up to you to believe whether that is a coincidence. Random googling tells me we emit 36 billion tonnes per co2 per year. Even more random googling tells me there are 3 trillion tonnes of CO2 in the atmosphere. It's not hard to put them together and conclude that humanity responsible for growing the CO2 in the atmosphere by 1% every year right now. Now lets say these numbers are complete garbage and inaccurate (which they certainly are) and say they are off by 70% (which they certainly are not) and the a highly conservative number is closer to 0.3%. After 30 years 9% of CO2 in the atmosphere is man made assuming we start off with 0% man made CO2 (obviously not true).

The point of this comment is not to prove that there is a certain percentage of man made CO2 in the atmosphere because I don't have accurate numbers. The point is that humanity is powerful enough to reach an arbitrarily high percentage of man made CO2 in the atmosphere and it is certainly possible for 50% of the CO2 in the atmosphere to be of human origin if we wait long enough and do nothing.