Myth: We can’t tell man-made CO2 from that emitted by nature

We don’t know what proportion of the CO₂ in the atmosphere is natural and what proportion was emitted by human activity. The vast majority of it could just be natural. 

Fact:

A molecule of CO₂ from burning fossil fuels does not contain the isotope Carbon-14, as this decays over time (indicating that the carbon has been in the ground for millennia). CO₂ from fossil fuels contains only Carbon-12 and Carbon-13. It is therefore not difficult to determine what proportion of the CO₂ in the atmosphere is a consequence of human activity. [Interestingly, the existence of C-14 allows us to date plant material up to 60,000 years old, since C-14 has a half-life of about 5,700 years. This process is known as ‘radiocarbon dating’. It is regularly used in archaeology.]