What are the two main human activities that are altering the carbon cycle according to the chapter?

The two main human activities altering the carbon cycle are burning fossil fuels and deforestation.

Burning fossil fuels releases carbon dioxide into the atmosphere, increasing its levels. Deforestation contributes to this issue by reducing the number of trees that can absorb carbon dioxide through photosynthesis, and it also adds carbon dioxide to the atmosphere when trees are burned after being cut down.