Fossil fuels are relatively easy to use to generate energy because they only require a simple direct combustion. However, a problem with fossil fuels is their environmental impact. Not only does their excavation from the ground significantly alter the environment, but their combustion leads to a great deal of air pollution. For example, carbon dioxide ( CO2 ) from burning fossil fuels is the largest source of greenhouse gases from human activities. Extra greenhouse gases we are putting into the atmosphere are causing global warming and climate change. Besides, these are fuels that are being consumed more rapidly than they are being replaced. That means that someday we could run out of these fuels.