The discussion centers on comparing the carbon dioxide (CO2) content in the Earth's crust versus the atmosphere. While the atmosphere contains approximately 0.035% CO2, the Earth's crust holds significant carbon primarily in the form of limestone and marble, which are compounds of carbon dioxide and calcium oxide. A notable carbon store is the clathrate found on ocean floors, estimated at around 11,000 gigatons. The conversation highlights the complexity of defining "crust" and "carbon dioxide," as it can include various forms such as dissolved gases, hydrocarbons, and carbon in different oxidation states. The difficulty in quantifying CO2 in the crust arises from the need to distinguish between chemically bound carbon and free carbon dioxide in various geological contexts.