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

Myth:

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 (C-14), as this decays over time (indicating that the carbon has been in the ground for millions of years). CO₂ from fossil fuels contains only Carbon-12 (C-12) and Carbon-13 (C-13). It is therefore not difficult to determine what proportion of the CO₂ in the atmosphere is a consequence of human activity. [A C-12 atom has 6 protons and 6 neutrons. A C-13 has 6 protons and 7 neutrons. A C-14 has 6 protons and 8 neutrons.]

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.