Hello!
Fossil fuels are Carbon-based compounds that when heated in the presence of oxygen undergo a combustion reaction to produce thermal energy and gases. The chemical equation for the combustion of a generic Fossil Fuel (C) is the following:
C + O₂ →CO₂
From this chemical reaction, we can see that CO₂ (Carbon Dioxide) is produced, so the consumption of fossil fuels has increased the Carbon Dioxide concentrations in the atmosphere.
Have a nice day!