Are humans really behind the extra CO2 in the atmosphere?

Why are we so sure that humans are responsible for all that extra carbon?

In this video essay about climate science [Simon Clark will] talk about why we know that the extra carbon in the Earth’s atmosphere is from human activities. In particular, the changing carbon isotope mix of the atmosphere tells us that the extra CO2 in the atmosphere is from fossilised organic matter.

Text above is from original video description.