Fossil fuels cannot be relied on. With the increase in depletion rates around the world, fossil fuels are not the only sources of energy that people should fall back on. With regular usage, these are now becoming hazardous sources of energy and are also becoming extremely expensive and scarce to find.
All sources of thermal power will eventually run out. The sources are coal, nuclear, geothermal, waste incineration, and natural gas. Even solar thermal electric will run out when the sun dies, (although that is in 5 billion years.)