PARIS — Some figures will make your head spin. Since the end of World War II, the number of inhabitants on the planet has increased nearly three-fold. And in that time, people used more resources than were consumed during the entire history of mankind prior to 1945.

Even more alarming is how quickly consumption continues to accelerate. In 1990, the world needed les than 43 million tons of metal to function; in 2012, it needed 91 million, according to the World Bank. China, whose share went from 4% to 45% is partly responsible.

At this rhythm, our civilization could quite simply collapse in our time. This is the theory put forward by two researchers, Pablo Servigne, an agricultural engineer, and Raphaël Stevens, an environmental advisor, in their essay "Comment tout peut s'effondrer" (How it Could All Collapse), published last spring.

These "collapsologists," as they refer to themselves, are positive that if we run out of oil, gas and coal, our system simply won't be able to function. "Almost everything we consume depends on it: transportation, food, clothing, heating etc." they write.

Servigne and Stevens believe we are reaching an energy production peak, and that the main minerals and metals could be heading the same way. One of the materials concerned, of course, is oil, which continues to be depleted even though public concern over its supply has waned as a result of the shale oil boom in the United States.

The real question, at any rate, isn't about the quantity of black gold left beneath our feet, but at what price we're ready to extract it. The operation becomes increasingly expensive because it becomes more and more complex: matter needs to be collected deeper and deeper, or extracted by other means, from bituminous sands in the Arctic, for example.

Saudi petroleum costs less than $20 per barrel to extract. U.S. shale oil costs between $60 and $80. And oil from Kazakhstan is set to reach $125 per barrel. The problem? Beyond a certain price, between $120 and a maximum of $150, demand goes down.

"Forty years ago, they said there were 40 years of oil left," says Nicolas Meilhan, a consulting engineer at Frost & Sullivan. "We're still told the same thing today. The difference? Forty years ago, it was 40 years of $10-oil. Now it's 40 years of $100-oil, and in 40 years, it'll be 40 years of $1,000-oil. But we probably won't have the means to extract it."

Even if the physical limit is not reached, the economic limit, which can of course vary according to technical progress, is very real. The end of the "cheap" oil extraction, at the very least, is getting closer.

One depends on the other

The same thing goes for many metals, some of which risk shortages within about 15 years. Aluminum and iron are abundant in the earth's crust. But other major industrial metals such as copper, zinc and nickel, specialty metals such as tantalum or tungsten, and even precious metals are more and more complicated to extract from the soil, technically and economically.

"Geologists will tell you there aren't any resource problems. In reality, it's important to take into account the interaction between energy and metals," says Philippe Bihouix, an engineer specialized in metals and author of L'Age des low tech (The Age of Low Tech).

Today, miners need to dig up 125 tons of rock to produce 1 ton of copper. A century ago the ratio was 50:1. In South Africa, gold mines can go as deep as almost 4 kilometers. Bihouix also points out that nearly 10% of the global primary energy is used to refine metals.

The inverse is also true. Accessing the material needed to produce energy requires metal. And the harder that material is to access, the more metal we need. Already 5% of the world's steel is used in the oil and gas sectors alone. Renewable energies are also "metal-consuming," as are connected objects or electric cars, which can contain three times as much copper as do diesel cars.

Of course, some metals can be successfully substituted. Apart from metals such as copper and lead, the current recycling rates are still very low. Rare earths are recycled at less than 1%. But recycling at 100% will never be possible.

This is why we need new economic models. Some advocate restraint, others degrowth, returning to local economies. Some even call for regulating births.

Throughout history, serious crises have enabled radical changes of consumption. In this way, the huge smog that suffocated London in late 1952 and killed thousands of people pushed Great Britain to pass a revolutionary law on air quality. After the 1973 oil shock, the whole world started finding other ways of producing electricity. In 2011, after the Fukushima disaster, Japan, suddenly deprived of nuclear energy, took unprecedented economic measures and encouraged its population to change their everyday habits.

But apart from these cases of major crises, our societies evolve very slowly. Perhaps we'll have to start mining space, as it holds infinite quantities of minerals and metals. The exploration has already begun.