Final answer:
The missing word is 'fossil fuel'. In the carbon cycle, carbon is stored in the lithosphere as organic carbon. Over time, the organic matter becomes fossil fuels, which, when burned, release carbon into the atmosphere.
Step-by-step explanation:
The missing word that fills in the blanks is fossil fuel.
In the carbon cycle, carbon is stored in the lithosphere as organic carbon. Over millions of years, the organic matter undergoes anaerobic decomposition to become fossil fuels, such as coal, oil, and natural gas. When these fossil fuels are burned for energy, carbon is released into the atmosphere as carbon dioxide, contributing to the rise of atmospheric carbon dioxide levels and climate change.