Final answer:
Humans affect the carbon cycle by burning fossil fuels, using fertilizers, and breathing.
Step-by-step explanation:
Humans affect the carbon cycle unnaturally by burning fossil fuels which release large amounts of carbon dioxide into the atmosphere. This contributes to the greenhouse effect and climate change. Additionally, the use of fertilizers containing nitrogen compounds also accelerates the release of nitrous oxide, another greenhouse gas. Finally, breathing is a natural process that releases carbon dioxide into the air.
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