How do we know that the atmospheric build-up of CO2 is due to human activity?
The nuclei of carbon atoms in carbon dioxide emitted by combustion of fossil fuels differ from the nuclei of carbon atoms in carbon dioxide emitted under natural conditions, in that the carbon dioxide emitted from natural sources on the Earth’s surface retains a measurable radioactive portion, while the carbon dioxide emitted by fossil fuel combustion does not. As carbon dioxide has been emitted through fossil fuel combustion, the atmospheric levels of CO2 have increased, but the radioactive fraction of carbon in the atmosphere has decreased.
