The answer is respiration.
The burning of fossil fuels is known as combustion. It is often compared with cellular respiration.
The general equation of cellular respiration is:
glucose + oxygen → water + carbon dioxide
The general equation of combustion is:
fuel + oxygen → water + carbon dioxide
Both reactions involve breaking down of large molecules (glucose and fuel) using oxygen into water and carbon dioxide. In both reactions, chemical energy is transformed into more useful form.