It’s hard to ignore the headlines these days, with all their warnings about ecological breakdown. Last year brought troubling news on everything from plastic pollution to soil depletion to the collapse of insect populations. These crises are worsening as our demands on the Earth intensify. Right now, virtually every government in the world is committed to pursuing economic growth: ever-expanding levels of extraction and consumption year on year.

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And the more we grow, the more we eat away at the web of life on which we all depend. We have known about this problem for decades now, but we’ve been told not to worry: As technology improves and becomes more efficient, we’ll be able to keep growing the economy while nonetheless reducing our impact on the natural world. The technical term for this is “green growth,” which requires absolute decoupling of GDP from material use. According to the theory, we can speed this process along by incentivizing innovation; if we tax carbon emissions and material extraction, we can spur companies to invest in more efficient tech. It sounds great, it’s promoted at the highest levels by tech billionaires like Elon Musk and international organizations like the World Bank and the United Nations, and it sits right at the center of big global plans like the Paris Climate Accord and the Sustainable Development Goals. We’re all hanging our collective future on this hope. But is it really possible? Here’s the magic number: 50 billion tons. That’s how much of the Earth’s materials and life forms we can safely use each year. That includes everything from wood to plastic, fish to livestock, minerals to metals: all the physical stuff that we consume. Right now, we’re using about 80 billion tons each year–way over the limit. So for growth to be green, we need to somehow get back down to 50 billion tons despite expanding the GDP. When green growth theory was first proposed, there was no evidence on whether it would actually work–it was purely speculative. But over the past few years, three major studies have set out to examine this question. All have arrived at the same rather troubling conclusion: Even under best-case scenario conditions, absolute decoupling of GDP growth from material use is not possible on a global scale. It was a team of scientists led by Monika Dittrich that first pointed this out. They ran a model showing that under business-as-usual conditions, growth will drive global resource use to a staggering 180 billion tons per year by 2050. At more than three times the safe limit, that means game over for human civilization as we know it.

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Then the team ran the model with the optimistic assumption that every nation on Earth immediately adopts best practice in efficiency, with all the best available technology. The results were a bit better: We would end up hitting 93 billion tons per year by 2050. But that’s not absolute decoupling, and it’s a far cry from anything approaching green growth. A second team of scientists tested the same question again in 2016, and found that even aggressive measures like a carbon price as high as $250 per ton and a doubling of technological efficiency don’t do the trick. If we keep growing the global economy by 3% each year, they found, we’ll still hit about 95 billion tons by 2050. No absolute decoupling. No green growth. Finally, last year the United Nations itself weighed in on the debate, hoping to settle the matter once and for all. It modelled a carbon price rising to a whopping $573 per ton, added a material extraction tax, and assumed rapid tech innovation spurred by strong government policy. The results? We hit 132 billion tons by 2050–even worse than the two previous studies found. Worse because this time the scientists included the “rebound effect” in their model. As gains in efficiency reduce the cost of commodities, demand for those commodities goes up, cancelling out some of the reductions in material use. And let’s not forget: All three of these models use radically optimistic assumptions. We’re a long way from even testing a global carbon tax, much less a tax of $573 per ton; and we’re not on track to double our efficiency. In fact, quite the opposite: Right now our efficiency is getting worse, not better. Why the bad news? The main reason is that tech innovation just doesn’t work the way most of us assume. We know that Moore’s law says that chip performance doubles about every two years–but this doesn’t apply to material use. There are physical limits to material efficiency, and once we start to reach them then the scale effect of growth drives material use back up in the long run. For instance you might be able to produce a wooden table more efficiently, but you can’t produce a table out of nothing. In the end you’ll need a minimum amount of wood, and once you reach that limit, then any growth in table production is going to come along with a corresponding growth in wood use. It would be hard to overstate the impact of these results. Right now, our only plan for dealing with the ecological emergency that’s staring us in the face is to hope that tech innovation and green growth will mitigate the coming disaster. Yes, we’re going to need all the wizardry we can get–but that alone is not going to be enough. The only real option is in fact much simpler and more obvious: We need to start consuming less.

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