This is a cycle because it continues forever. Cycles like for example, a human. A human starts off as a baby, becomes a toddler., then ages into a kid then a teenager, afterwards an adult. And this cycle continues. It happens everywhere in nature and everywhere in the world today with living things including: Plants, Humans, and animals. I hope this helps It might be wrong and I'm sorry if it is:(
1. Fracking 2. Air pollutants. 3. Decomposition of organic matter e.g. in landfills. 4. <span>Chlorofluorocarbons </span>mainly in refrigerants and air conditioners.