What is is a disadvantage of using fossil fuels to generate electricity?

Firstly, there is only a limited quantity of fossil fuels on the planet, and they are only being replenished on a geological timescale, meaning over millions of years, so if unless we lose our dependence on them there will be a crisis when we run out.

Second, fossil fuels consist mostly of hydrocarbons, which, when burnt, produce carbon dioxide, which is in large quantities causing global warming.