The invention provides methods and systems for tagging carbon dioxide to be stored in a geologic formation. In some embodiments, a method includes: providing a carbon dioxide tracer that is quantifiable and distinguishable versus non-anthropogenic produced carbon dioxide; providing carbon dioxide to be stored in the geologic formation; determining what portion of the carbon dioxide is anthropogenic produced carbon dioxide; and mixing a predetermined quantity of the carbon dioxide tracer with the carbon dioxide stored to develop a tagged quantity of carbon dioxide for storage in the geologic formation. In some embodiments, a system for tagging a stream of carbon dioxide includes a tagging module and a mixing module. Tagging module includes a carbon dioxide tracer that is quantifiable and distinguishable versus non-anthropogenic produced carbon dioxide. Mixing module includes mechanisms for containing and injecting the carbon dioxide tracer into a stream of carbon dioxide.