TeXes Science Practice Test

Which of the following is the most abundant source of carbon dioxide in the atmosphere?

1. Respiration from living organisms

2. Burning of fossil fuels

3. Dying plant decay

4. Photosynthesis by plants

The correct answer is: Burning of fossil fuels

The most abundant source of carbon dioxide in the atmosphere is indeed the burning of fossil fuels. This process releases significant amounts of carbon dioxide as a byproduct. Fossil fuels, such as coal, oil, and natural gas, are primarily composed of carbon and when they combust, they react with oxygen, producing carbon dioxide and water. This process occurs on a large scale in industries, power generation, and transportation, leading to a substantial increase in atmospheric carbon dioxide levels. While respiration from living organisms also contributes carbon dioxide to the atmosphere through the metabolic process of breaking down sugars for energy, its contribution is relatively small compared to that of fossil fuel combustion. Additionally, dying plant decay releases carbon dioxide, but this process also generates a much smaller amount when compared to the extensive emissions from burning fossil fuels. Photosynthesis by plants, on the other hand, is critical for reducing atmospheric carbon dioxide since it converts carbon dioxide into glucose and oxygen. However, during the overall carbon cycle, photosynthesis does not produce carbon dioxide; rather, it absorbs it, further emphasizing that burning fossil fuels is the most substantial contributor to the atmospheric carbon dioxide levels.