Run out of oil? You do realize that we are constantly running out of oil, and the oil companies have just gotten really good at finding new sources? The latest breakthroughs are twofold:
- Hydraulic fracturing, or "fracking", making accessible broad swaths of previously unharvestable shale oil.
- New reserves opening up in the Arctic, primarily opening up due to longer summers and milder winters melting the ice caps.
How long do you think before these sources dry up, like every other source before them already has? Pennsylvania, Texas, and California all used to be world-class suppliers of oil. Saudi Arabia is believed to have reached its peak oil extraction and is declining, although they won't admit to it. And do we really want to count it as a good thing that global scale climate change has opened up more fossil fuels with which we can induce even more global scale climate change?
And as for trees, deforestation in northern Europe had major economic consequences and contributed to the constant stream of instability and war over the last 3000 years. The Amazonian rainforest and the African jungles are our largest remaining reserves of trees, and the Brazilians are in the process of systematically clearing them for farmland. But we won't run out of trees. Unlike fossil fuels or water, it is possible to grow enough of certain trees (especially pine) to sustain forest ecosystems and supply human needs.
Water, at least, is a bit more abundant, but still not renewable. The Earth has a total of ~1.67x10^21 kg of water, and assuming each individual consumes 60 kg of water per year (as another Slashdotter calculated), and assuming we could actually access all of that water, that gives us roughly 4 billion years of fusion power before we run out of water. While that is less than the remaining lifetime of our planet (based on the remaining lifetime of the sun, between 5 and 7.6 billion years), I cannot imagine the human race surviving for so long, or what we would look like in even a tenth of that time.