What is the scientific evidence that burning fossil fuels is bad for the environment?

If you mean global warming the answer is Zilch. The main justification is a computer programme that was discredited as soon as its algorithm was made public. For the rest, various climatic changes are blamed on CO2 increases without any evidence. Remember CO2 is only a tiny portion of the greenhouse gases in the atmosphere. I think the figure is about 350ppm compared with about 2% (average) of water vapour.