In response to an article about a controversy surrounding an art installation at the University of Wyoming, I typed out the following thoughts on our reliance upon long-dead plants for pretty much everything. I'm reposting them here because, well, I can.
We have 300 years, 500 at the outside, before our fossil hydrocarbon supplies are effectively exhausted. We rely on them for fuel, materials, and fertilizer. They took tens of millions of years to develop. We'll have them drained in less than ten thousand years, the majority used in less than a thousand. Their overuse is likely causing noticeable climatic changes at a much faster rate than even we can adapt to, never mind the other multicellular organisms we live among and consume. Barring sudden extinction, we're going to be around after the coal and oil are gone.
What are we going to do? Is anyone thinking that far ahead?
Easy way to check my rough estimate: locate info on the estimated available volume of current oil/coal sources and their extraction rate. Calculate the time to exhaustion. Allow an extra 50% increase in that extraction rate, and 1.5 - 2x the known volume (there may be more than known at first). Run those numbers as well.
When I did that for the Alberta tarsands, exhaustion came in between 300-500 years. The Ghawar field in Saudi Arabia will likely be done in the next hundred (it could run dry tomorrow; the Saudi government treats its estimates and facts as state secrets). The coal will be extracted just as quickly, even in places that have tried to stop its use, as long as we are still reliant on fossil carbon supplies for so much. Even allowing for massive, global increases in use efficiency (good luck as long as short-term financial profit is the primary motive for extraction and use) and a doubling of accessible sources, we have no more than a thousand years before we're stuck with what has been extracted. Sooner or later, we have to close as many of our resource cycles as possible. The only energy sources we can be reasonably sure will continue operating are the thermonuclear fusion reactor we orbit, and to a lesser extent the gravitationally-driven tidal forces and geothermal output. As for materials, we rely so heavily on plastics made from fossil carbon supplies that exhaustion would instantly force us to recycle and reprocess that which we toss away now as waste, and come up with methods to grow fresh sources (at which point we have to balance that with our food & water supplies). First person to say "b-b-but extraterrestrial resources!" gets to figure out how to obtain them using what we have available, and how those activities will affect the place we all, for now, still have to live on.
Barring extinction, we're going to be here a long, long time. We can try to plan ahead and think really long thoughts now, or we can just keep putting it off with procrastinative excuses like "someone will develop technology" or "the market will take care of it". There's an inherent assumption underlying those statements that our descendants won't excuse us for making if we turn out to be wrong. They might forgive us; they'll at least be stuck accepting what situation they're born into and doing what they can, but all the rationalizations we make now are choices that they will have to deal with the consequences of, and they'll judge us by what we leave for them.