"We are using 50% more resources than the Earth can sustainably produce, and unless we change course, that number will grow fast—by 2030, even two planets will not be enough," says Jim Leape, director general of the World Wide Fund for Nature International (formerly the World Wildlife Fund).
To get the right answer, ask an economist. They will point out that when a good gets scarce, its price increases, which gives consumers an incentive to conserve or find substitutes, and producers an incentive to find more of it.
Until about 10 years ago, it was reasonable to expect that natural gas might run out in a few short decades and oil soon thereafter. If that were to happen, agricultural yields would plummet, and the world would be faced with a stark dilemma: Plow up all the remaining rain forest to grow food, or starve.
But thanks to fracking and the shale revolution, peak oil and gas have been postponed. They will run out one day, but only in the sense that you will run out of Atlantic Ocean one day if you take a rowboat west out of a harbor in Ireland. Just as you are likely to stop rowing long before you bump into Newfoundland, so we may well find cheap substitutes for fossil fuels long before they run out.