Fossil fuels are non-renewable, we are going to run out.
2. When fossil fuels are used (i.e. burned) there is air pollution.
3. Some delicate ecosystems may be disturbed/destroyed in man's zeal to obtain evermore fossil fuel.
4. Cost of fossil fuels is relentlessly going up, eating into common people's ability to pay for other necessary items
Source: Boundless. “Environmental Problems Associated with Fossil Fuel Use.” Boundless Chemistry. Boundless, 13 Nov. 2014. Retrieved 29 Nov. 2014 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/thermochemistry-6/energy-use-and-the-environment-62/environmental-problems-associated-with-fossil-fuel-use-291-1025/