We know that the levels of CO2 have increased by 40% since the beginning of the industrial revolution and that they did so at the same time as the emissions of CO2 from fossil fuel burning started.
We know this 40% rise was due to man's burning of these fuels by logic, simple math and things like isotope analysis.
We know this 40% rise was due to man's burning of these fuels by logic, simple math and things like isotope analysis.