The planet’s got 99 problems, but exponential growth isn’t one

8th February, 2024

If Christmas is (alas) a time for a materialist blowout, January is often a time for rueful reflection. Should we really have bought all that soon-to-be-landfill for each other? The answer, as I have written a dozen times, is . . . probably not.

Thankfully, people have stopped sending me emails declaring that economists don’t understand Christmas. Now they are sending me emails declaring that economists don’t understand exponential growth, and that as a result, the planet is doomed. This grates, because saying that economists don’t understand exponential growth is like saying that accountants don’t understand double-entry book-keeping, or that poets don’t understand metaphor. Consider the bait taken.

An old illustration of exponential growth remains the best. Legend has it that the genius who invented chess was asked to name his reward by a delighted monarch, and requested a modest-sounding payment: one grain of rice for the first square of the chessboard, two for the second, four for the third . . . doubling each time. This doubling is an exponential process, and most people are surprised when they first hear that the 64th square would require more than any harvest could produce.

Less intuitive yet, each square contains more rice than all the previous squares put together. Whatever square you pick, and no matter how dramatic the pile of rice might seem, what comes next will make it all seem trivial. Now substitute energy consumption or carbon emissions for rice, and you can see the environmental catastrophe looming.

If rice on the chessboard is the most famous illustration of exponential growth, the most famous essay on the topic was published in 1798 by Thomas Malthus. Malthus warned that human population would always threaten to outpace agricultural output. No matter how quickly agricultural productivity grows, if that growth is arithmetical — 10, 20, 30, 40, 50 — then it will inevitably be overtaken by the exponential progress of human population growth — 2, 4, 8, 16, 32, 64. No sustained prosperity is possible: humans will inevitably breed themselves into poverty in the end.

There can be no arguing with the mathematics here. The flaw in Malthus’s argument lies in its assumption of exponential population growth. Global population is flattening off; the number of children in the world under the age of five peaked in 2017. This is a reminder that mathematics only takes us so far and should prompt everyone worried about the planet to ask: what else do we assume is growing exponentially, and is not?

A glance at the UK, one of the world’s first developed economies, is instructive. This industrialising heart of empire once burnt vast quantities of atmosphere-warming, lung-shrivelling coal. But as Hannah Ritchie notes in her thoughtful new book Not the End of the World, per capita coal emissions in the UK peaked more than 100 years ago. Some of that fall represents the offshoring of industrial processes, with the coal choking someone else, but most of it reflects the use of cleaner, more efficient technology.

In the UK, CO₂ emissions per person have halved during my lifetime. Globally, CO₂ emissions per person peaked in 2012. Although the world still faces huge environmental challenges, there is nothing about these numbers that suggests exponential growth.

Economic growth does continue — maybe not exponentially, but it’s exponential-ish. Fortunately, the planet simply does not care about numbers in the national income accounts. What matters for our environment are flows of energy, pollutants and other physical quantities.

One might assume that economic growth must mean growth in pollution and energy use, but the data suggests the situation is more hopeful than that. So does a bit of introspection: if you won £1,000 on the lottery, you might turn up the heating in your home. That does not mean that if you won £1mn, you would boil yourself alive. Not every penny spent must be ripped from the soil of our planet.

There are other glimmers of hope. For example, although deforestation is still happening on a worrying scale, it was much worse for most of the 20th century — and in many rich countries, the forests are returning. Agricultural land use peaked globally about 25 years ago, and Ritchie argues that we might also be at or near a peak in fertiliser use.

But not every indicator is so reassuring. Ed Conway, in his book Material World (2023), points to some unnerving numbers on the sheer quantity of stuff — sand, water, earth — that we move around. “In 2019,” he writes, “we mined, dug and blasted more materials from the earth’s surface than the sum total of everything we extracted from the dawn of humanity all the way through to 1950.”

This is partly because of growing demand, and also because we have picked the fruit in easy reach. Copper is the nervous system of our electronic age, but miners have had to squeeze ever more out of ever sparser ores — the largest and most famous copper mine in the world, Chuquicamata, had seams that were up to 15 per cent copper in the late 19th century. Today, they are less than 1 per cent. Our devices are getting smaller and lighter, but the gargantuan trucks of Chuquicamata are not.

Conway worries that we take for granted the hidden industrial processes underpinning our everyday comforts. Ritchie is concerned that we are so disheartened by prophecies of doom that we may miss the chance to become the first truly sustainable generation in the modern world.

Both are right. We depend on a huge variety of natural resources; there are both alarming and encouraging trends. We need the right policies now, and to embrace them means setting aside thought experiments about exponential growth, and looking instead at what the data shows us about the challenges and opportunities ahead.

Written for and first published in the Financial Times on 12 January 2024.

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