CO2 put in the air by burning fossil fuels has a chemical signature
Carbon, like virtually all of the chemical elements, come in different varieties known as isotopes, distinguished by the number of neutrons in their atomic cores.[[Earth System Research Laboratory, Global Monitoring Division. “Frequently Asked Questions, Greenhouse Gases.”]]
Carbon dioxide, or CO2, can be made from all of the isotopes of carbon — but not all sources of CO2 have the same types of carbon atoms in them. Fossil fuels such as coal, oil and natural gas, for example, have no carbon-14, and neither does the CO2 that comes from burning them.[[Earth System Research Laboratory, Global Monitoring Division. “Frequently Asked Questions, Greenhouse Gases.”]] A small fraction of the CO2 molecules that enter the atmosphere through natural means such as the decay of plants, on the other hand, does contain carbon-14.
Because they have extra neutrons, atoms of carbon-14 are more massive than atoms of C-12, and so are the CO2 molecules they are made of. Instruments called mass spectrometers measure that difference.[[“Scripps Center for Mass Spectrometry - Mass Spec History.”]] Based on how much of the heavier CO2 they measure in samples of atmosphere, scientists calculate that about a quarter of the CO2 present today must come from fossil fuels. That conclusion is confirmed by the fact that this fraction amounts to most of the growth in CO2 over the last 250 years, when fossil-fuel burning has really taken off.
It is this increase in CO2 concentrations that is primarily responsible for the increase in global average temperatures over the past century, and especially in recent decades.