China has cut exports of rare-earth metals to Japan, Europe, and the United States, undermining high-tech manufacturers that rely on the minerals for wind turbines and missile-guidance systems. The People’s Republic controls more than 90 percent of rare-earth production and now claims that their reserves may be exhausted in the next 20 years. Extinct plant and animal species notwithstanding, has the Earth ever run out of a natural resource?

Sort of. According to the U.S. Geological Survey, we have no remaining reserves of cryolite, a mineral that is used in the processing of aluminum. The last active cryolite mine, located in Greenland, closed in the 1980s, and manufacturers now rely on a synthetic alternative. But that doesn’t mean the planet has run out of this resource. When we talk about natural reserves of a mineral, we’re generally not referring to every last ounce of that mineral thought to exist. Rather, we mean only those deposits that would be worth extracting, given their present value and the cost of mining them. Veins of cryolite are still scattered around the globe, but they’re too small to justify the expense of a mining operation at current prices.

Since the availability of a given resource depends on its present value, along with the quality of the technology used to extract it, the total “known reserves” of something can actually increase over time. Indeed, that’s been the case for most commercially important minerals . In 1950, for example, the USGS estimated global reserves of zinc at 77 million tons. Yet exploration and improved mining techniques allowed humans to dig up more than 293 million tons of the stuff over the next half-century. In 2000, the government announced that zinc reserves were up to 209 million tons. Tin, copper, iron ore, and lead have all experienced similar increases. In 1970, researchers thought we had only 30 years of oil left. By 1990, the estimate had risen to 40 years, where it has remained. While many believe oil could one day become commercially nonviable, few in the industry think all the wells will have run dry by 2050.

Resource exhaustion used to be a hot topic among economists. Thomas Robert Malthus predicted in 1798 that land shortages would lead to famine and population collapse. In 1865, William Stanley Jevons predicted that Britain would soon run out of coal, bringing the economy crashing down, and others soon joined the gloomy chorus. These days, though, few economists lose sleep over the prospect of absolute exhaustion of any particular resource. For most of the 20th century, coal and oil prices remained constant, and mineral prices dropped in real dollars, indicating confidence in long-term supply. Most experts assume that mankind will continue to discover new deposits and devise substitutes for any resources that become scarce. Of far greater concern than actually burning our final gallon of gasoline or mining the last pound of copper, they say, are the environmental and geopolitical risks associated with resource extraction.

So where will we go for rare-earth metals if China’s supply runs low? California has deposits, although production ceased in the 1990s when China started mining the minerals for less money. The moon also has some rare-earth resources, which might be mined within a century or two using some version of a space elevator.

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Explainer thanks George Brimhall of the University of California at Berkeley, Rod Eggert of the Colorado School of Mines, Kate Johnson of the U.S. Geological Survey, and Michael Shara of the American Museum of Natural History.

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