That day will come: the life-changing moment when renewable energy—wind, solar, geothermal and others still in development—replace fossil fuels as the principal source of world energy. Most analysts insist, however, that this day will not arrive for many decades to come—certainly well past the middle of the century. Fossil fuels are too entrenched, it is said, and renewables too costly or impractical to usurp existing systems. According to the International Energy Agency (IEA), the share of global energy provided by renewables—a mere 14 percent in 2012—will increase only slightly between now and 2040, rising to just 19 percent. But there are good reasons to believe that the transition to renewables will occur much faster than previously assumed, pushing that percentage higher and higher. Indeed, recent increases in wind and solar installations have been running at nearly twice the rate of the IEA’s projections for long-term capacity growth, suggesting that its projections of renewables’ share of global energy are much too low.

It is hardly surprising, of course, that many experts say we will witness a relatively drawn-out transition from fossil fuels […]