Answer:
I believe it is the burning of fossil fuels.
Explanation:
The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis.
The cycle of carbon is carbon is moved through the atmosphere (carbon dioxide) to organisms. It is then moved from one organism to the next in complex molecules. Next, it is brought back to the atmosphere as carbon dioxide again.
Simple explanation is carbon passed through the atmosphere and back to animals and plants.