Burning Fossil fuels release More CO2 (Carbon Dioxide) in the atmosphere which makes it hard for oxygen breathing organisms to breathe easily. Also Burning Fossil Fuels releases Nitrogen Oxide which causes smog to accumulate in the air. Fossil Fuels contribute to green house gasses in the ozone layer which causes climate change. Climate change then leads to The warming of the earths surface which endangers animals and other life on land that are not used to warmer weather. 

Citation:

“Why Are Fossil Fuels Bad for the Environment?” Reference, IAC Publishing, www.reference.com/science/fossil-fuels-bad-environment-ed81a473564fab02.

https://socratic.org/questions/why-are-fossil-fuels-harmful-for-the-environment