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CO2 for dummies #3

Fossil fuels

When a living being died, all the carbon used to form its tissues was slowly released into the air in the form of CO2. But not all the carbon returned to the atmosphere:  sometimes an organism died and its body was trapped in layers of ground which were impermeable to the air and other organisms. In these cases the carbon remained trapped between the rocks and the first deposits of coal, oil and natural gas were formed. This was the start of the Carboniferous period, around 300 million years ago. It took an average of 100 million years for these fossil fuels to form. So even they are renewable energy in their own way, if you have time to wait...