"They aren't making more fossil fuels." -- Well, they are, just rather slowly. All the fossil fuels we use now have been created since the Cambrian and most have been created since the start of the Mesozoic. And if we are willing to wait another 300 million years, we can have yet another 2 centuries of vigorous economic growth based on fossil fuel consumption. In fact, we can probably have 200 good years, every 300 million years, for as long as the Earth lasts.
Thomas Gold thought it would last a good bit longer. He hypothesized Earth's methane is primordial, dating to the origin of the planet (as with other planets in the solar system). There would be a virtually limitless supply. http://metaresearch.org/publications/bulletin/2007issues/091...
There are hydrocarbons (methane especially) on other planets. Oil is old algae, but methane might not be - there is a lot of energy in the Earth's core than can make it.
Tell that to the algae in the ocean. Anyway we won't run out of fossils. But I am hopeful for replacement. We almost tamed production, battery tech is progressing. The only hard place for fossil fuels to be replaced if push comes to shove is air travel.
