Just before the turn of the millennium, I met a man who had recently invested a fortune in wind power. He said he wanted to do all that he could to slow the course of climate change. He was also convinced that, as the world began to run out of oil, alternative sources of energy would offer a unique entrepreneurial opportunity. “Oil prices will fluctuate for a while,” he told me. “But, eventually, they can only move in one direction. Up. Oil is a finite resource and, as supplies dwindle, the costs will have to rise. That will make alternatives like wind power much more attractive.”

That sounded sensible, and, for the many people who have long argued that our addiction to oil and gas is destroying the planet, so did the much discussed concept of “peak oil’’—the theoretical moment when half the world’s oil reserves had been consumed and fossil fuels began to become scarce. The date of peak oil is hard to pin down, but most suggest we passed that point a decade ago. In a Times column titled “The Finite World,’’ Paul Krugman said that the magic moment had arrived in 2010.

High oil prices would force governments, corporations, and consumers to find another way to power the world. It was a nice dream, but it’s over now. We are awash in cheap oil. Propelled largely by a boom in domestic production, due to hydraulic fracturing, or “fracking,’’ and horizontal drilling, oil prices fell below $70 a barrel on Thursday—from a high in June of $112.12. Prices have fallen nearly every day for the past two months, and some economists predict that we will soon see oil selling for less than fifty dollars a barrel.

That’s good news for consumers; it means that they will have more disposable income. More gas means more travel and more spending, which our sluggish economy clearly needs. But the costs may be enormous. In 2008, a barrel of oil cost $147, more than twice today’s price.

Many economists and geologists predict that prices are unlikely to rise again soon. Fracking technology has transformed the United States from an importer of gas and oil to an exporter. In fact, the International Energy Agency has predicted that the United States will produce more oil next year than Saudi Arabia; we might even pass Russia, which, at ten million barrels a day, is the world’s biggest producer. (We already produce more natural gas than Russia.)

Fracking is not a magic fix, or even a benign one: the technology poses clear risks to the environment. It would be hard to make the case that it would be better to rely on Syria, Russia, Iraq, or Saudi Arabia to fuel the nation, as we have in the past. We have endured unpredictable oil prices since OPEC imposed an embargo on the U.S. soon after the 1973 Arab-Israeli war. That punishment led to the first oil “shock,’’ and, since then, prices have been influenced by politics as much as by demand. But the cheaper fossil fuels become, the more challenging it will be for cleaner forms of energy—like solar and wind power—to become competitive.

Many environmentalists had assumed that if neither fear nor reason helped us to lessen our reliance on oil, then at least we could count on scarcity. But scarcity is not an economic or environmental policy. Humans have long had a habit of expecting the sky to fall. Yet from Malthus to Paul Ehrlich, predictions that the planet was on the verge of starvation have never come to pass (or at least not as broadly as expected). Nonetheless, the drop in oil prices comes at a terrible moment. Last month the Intergovernmental Panel on Climate Change reported that our only chance to halt the rising temperature of the Earth, and to prevent the calamity that rise will cause, would be to eliminate fossil-fuel emissions by the end of the century.

A plan to end U.S. fossil-fuel dependence would be an unlikely goal in any case, but, if oil remains easily accessible, it becomes politically impossible. “It is technically feasible to transition to a low-carbon economy," Youba Sokona, the co-chair of one of the I.P.C.C.'s working groups, says. "But what is lacking are appropriate policies and institutions. The longer we wait to take action, the more it will cost to adapt and mitigate climate change."

It seems churlish, at best, to lament easy access to one of the world’s most vital commodities. But cheap gas has become like an industrial form of crack. It doesn’t really matter how much damage it causes, because we simply don’t have the power to walk away.