As the world industrializes and consumes more energy, each new barrel of oil costs more because the cheapest and easiest oil has already been pumped. This observation gave rise to a theory called “peak oil,” which holds that world production will eventually max out and decline as oil fields deplete. Skeptics of this notion point to the technological innovations that let U.S. producers extract oil and gas from previously impermeable shale, unlocking vast new resources, albeit at greater expense; the issue isn’t quantity but cost. The other variable is demand; no one knows oil’s future as consumers grow more efficient and switch to alternative fuels such as natural gas and renewable power. Oil supplied 31 percent of the world’s energy in 2012, down from 46 percent in 1973. There may come a day when oil gets cheap because it’s unwanted. That’s the argument often advanced by advocates of divestment. They warn of a financial crisis caused by a bursting “carbon bubble” of inflated energy-company valuations after fossil-fuel prices rise to account for the costs of contributing to global warming.