Final answer:
Carbon dioxide is released into the atmosphere by volcanic eruptions at subduction zones, where the subducted plate melts and releases various gases. Although these natural occurrences do contribute to atmospheric CO₂, the main source today is the burning of fossil fuels.
Step-by-step explanation:
Carbon dioxide is released back into the atmosphere by volcanic eruptions related to subduction zones. These zones are areas where one tectonic plate moves under another and sinks into the mantle as they converge. The subducted plate moves down into high pressure and temperature regions, eventually melting and causing the release of gases that were trapped in the crust or that resulted from the melting of carbonate rocks.
These gases include CO₂, H₂O, and SO₂. Subduction zones are thus crucial in recycling carbon through geological processes. While volcanoes play a significant role, they are not the primary source of CO₂ in our atmosphere today; that title goes to the burning of fossil fuels. However, volcanic activity, especially at subduction zones, is a natural contributor to the atmospheric CO₂.