What is the main source of carbon dioxide in the atmosphere?

The correct answer identifies the complete combustion of carbon-containing fuels as the main source of carbon dioxide in the atmosphere. This process occurs when fossil fuels such as coal, oil, and natural gas are burned to produce energy. During complete combustion, carbon from these fuels reacts with oxygen, resulting in the release of carbon dioxide as a byproduct.

This process contributes significantly to the increase of carbon dioxide levels in the atmosphere due to the large scale at which fossil fuels are utilized in transportation, electricity generation, and industrial processes. This contribution is particularly notable given the global reliance on these energy sources.

While deforestation does also play a role by reducing the number of trees that can absorb carbon dioxide, the immediate and substantial emissions from the combustion of fossil fuels outweigh the impacts of deforestation on atmospheric carbon dioxide levels. Similarly, photosynthesis acts to reduce atmospheric carbon dioxide, and the respiration of animals contributes to carbon dioxide levels but on a much smaller scale compared to the emissions from burning fossil fuels. Therefore, complete combustion stands out as the primary contributor to the carbon dioxide concentration in the atmosphere.