There are quite a few problems with fossil fuels, but they all centre around future availability and the difficulties of disposing of their combustion products. There are three principal fossil fuels:
Fossil fuels produce a lot of carbon dioxide when burnt, with coal as the worst offender. Burning any fossil fuel causes global warming because all produce carbon dioxide when they are burnt. Oil and natural gas also produce water vapour as a combustion product. This is not bad in itself, but unfortunately its often produced in places where we really do not want it. Here are the numbers:
Fuel Composition CO2 H2O Natural gas CH4 2.75 2.25 Petrol C8H18 3.09 1.42 Jet Fuel A C12H26 3.11 1.38 Fuel oil C24H50 3.12 1.33 Coal Carbon 3.67 0
This table shows the tonnes of carbon dioxide (CO2) and water (H2O) produced when one tonne of fossil fuel is burnt.
The combustion products pose problems when it comes to disposing of them: