Question

what is released when fossil fuels are burned?

Answer 1

carbon dioxide, carbon monoxide

Answer 2

Fossil fuels are primarily hydro-carbons. Whenever hydro-carbons are burnt, you can have two possible outcomes.

In the presence of high levels of oxygen, hydrocarbons such as petrol [octane] burn to give carbon dioxide and water, like so :
\[C8H18 + O2 \rightarrow CO2+H2O\]
However, hydrocarbons also burn sometimes to give carbon monoxide and water, in the presence of moderate or low oxygen.
\[C8H18 + O2 \rightarrow CO+ H2O\]

Carbon Monoxide is not a greenhouse gas, however it is lethal to humans in high concentrations. Carbon dioxide is a greenhouse gas. Water is the third product.

Note : Soot can also be a product [which is basically Carbon].