On the other hand, the example of the United Kingdom inspires more optimism. Two centuries ago, the UK topped the world in manufacturing; it mined 80 percent of the world’s coal and most of its iron. British wealth and population have only grown since then. Yet Britain’s consumption of materials peaked in 2001, according to government statistics, and is today a third less than it was 20 years ago. Not coincidentally, the UK now pollutes less, too, with greenhouse-gas emissions down 38 percent since 1990.

The UK story gives some economists hope that, once a country has completed construction of its major infrastructure and attained a high standard of living, its citizens can become increasingly efficient in their use of materials. Britons are still getting richer, but they are spending more of their income on yoga classes, fitness trainers, and nice restaurants—and more of their working and leisure hours online, consuming only electricity.

So the question for the twenty-first century, and perhaps for the future of humanity, is: Which version of modernity is set to triumph on our increasingly crowded planet? Is it the orgy of construction, consumption, and manufacturing now ripping up China—trashing the water and air, destabilizing the climate, and degrading the ecosystems on which our global civilization ultimately relies? Or is it the dematerializing society now glimpsed in Britain and some other parts of the industrialized world, where innovations in technologies such as 3-D printing, robotics, and hydroponics allow standards of living to continue rising while consumption of material goods falls? Could technology give developing nations a shortcut past the environmentally ruinous road to riches taken by their predecessors?

Ehrlich’s Simple Formula for a Complex Future

Every nation pursues economic growth as a primary goal, and that seems unlikely to change. For centuries, environmental pessimists have warned that this pursuit, amplified by population growth, must inevitably end in grief. From Robert Malthus in the late eighteenth century, to William Vogt in the 1940s, to Paul Ehrlich’s 1968 bestseller The Population Bomb, demographic doomsayers have prophesied famines, plagues, resource wars, and ecological devastation as the price humanity will ultimately pay for its overpopulation and overconsumption.

On the other side of the debate, optimists have countered with appeals to human nature and progress in science and technology. Philosopher William Goodwin argued that moral improvement would “eclipse the desire for sex” and head off demographic disaster. Scientists such as Norman Borlaug, begetter of the agricultural “green revolution,” have drawn hope from high-yielding crops and innovations that cut pollution and boost efficiency. Economists such as Julian Simon have touted the power of free markets to solve shortages.

So far, the optimists can point to significant wins. Steven Pinker documents in his recent book Enlightenment Now how famines, undernourishment, and childhood stunting have all fallen dramatically in recent decades, with just 13 percent of people in the developing world undernourished today, compared to half in 1947. Deaths from infectious, maternal, and nutritional diseases have plummeted by nearly a third since 1990. And Simon famously won the ten-year bet he made with Ehrlich about whether key commodities would become scarce. Yet the jury is still out on this argument because population, incomes, and consumption all continue to grow—and consequences such as climate change, air pollution, and overfishing continue to worsen.

Arguably more important than any of Ehrlich’s alarmist predictions was his framing, in The Population Bomb and in a 1971 article in Science, of the pressure our species exerts on the planet—which remains correct. Humanity’s impact on the biosphere, he observed, is governed by a simple formula that multiplies three crucial factors: the number of people, times the stuff consumed by each person, times the environmental impact of each unit of stuff.

Of these three, population growth has turned out to be the least worrying, because the population bomb is being defused. Fertility rates are now below 2.5 children per woman on average, roughly half of what they were in the 1970s, when Ehrlich told Johnny Carson on the “Tonight Show” that billions were going to die from starvation in the 1980s. Fertility has actually been falling faster than expected in many of the world’s most populous regions. So even though our number may rise from 7.3 billion today to 10 billion or so, a peak seems likely before century’s end.

If I Were a Rich Man . . .

With population growth slowing, experts have focused more attention on the second factor in Ehrlich’s equation: consumption per capita. That trend is easiest to study in rich countries that have detailed and reliable economic data, such as the US and the UK. And in those advanced economies, analysts—including some who have a record of environmental concern—have found reasons for optimism.

Jesse Ausubel of Rockefeller University in New York City, for example, helped get climate change on the global agenda in the 1970s and masterminded the Census of Marine Life. Yet he has argued since 2008 that modern societies are “dematerializing” as increasingly wealthy consumers spend less of their income on staples. In a 2015 report titled “Nature Rebounds,” he asserted that “Slowing population growth, moderate economic growth, changes in consumer taste and behavior, and technical progress … are combining to create a new reality in resource use.”

Yes, industrialization has so far necessitated a cheap-and-dirty phase when production is inefficient, resources are wasted, and pollution is rampant. But countries gradually decouple production from pollution and reach a tipping point, he concluded, beyond which “our economy no longer advances in tandem with exploitation of land, forests, water and minerals.”