Besides human activities like fossil fuel combustion, several natural processes produce large quantities of CO2. These include:

• Volcanic eruptions, which release CO2 from the Earth's interior.
• Respiration by animals and plants, where CO2 is released as they convert oxygen and glucose into energy.
• Decomposition or decay of organic matter, such as dead plants and animals, which releases CO2.
• Ocean outgassing, where CO2 dissolved in ocean water is released back into the atmosphere.
• Naturally occurring wildfires that burn vegetation and produce CO2.
• Weathering of carbonate rocks, where chemical reactions release CO2.

These natural sources of CO2 have historically been balanced by natural sinks like photosynthesis and ocean absorption, maintaining atmospheric CO2 levels within a relatively stable range before the industrial era. However, human activities have disrupted this balance leading to increased atmospheric CO2 concentrations.