What Is the Earth’s Carrying Capacity?

The “carrying capacity” of the Earth is the academic term for how many humans the Earth can support without going awry. 

What can make things go awry? The most obvious limits are space, food, water, and energy, but we already saw in the previous article that these factors would not prevent us from reaching 100B people. Yet the Earth could have many more limits: global warming, lack of phosphorus, too much nitrogen, not enough timber, diseases, waste disposal, the destruction of biological diversity… What makes this analysis difficult is that there could be millions of such limiting factors. How can you prove that none of them are really limiting? That’s what we’re going to attempt. 

The “Experts” of Carrying Capacity

The way to do this is by analyzing the arguments of all the experts who think the Earth has a limited carrying capacity. And there are a lot of these experts!

The first thing you should notice is that, weirdly, most analysts believe the Earth is already at its carrying capacity! Look at the graph above: the mode¹ is at 8B people! WHAT A COINCIDENCE, SHERLOCK.

What is the likelihood that we’re just at the limit of the Earth? Very low. My immediate reaction to this is: Most analysts simply lack imagination. They see the current world, notice that there are some problems, and conclude that we’re hitting our limits.

The poor quality of these studies was highlighted 30 years ago in this paper, which found six different approaches to assessing the carrying capacity:

• Geographic Regional Division: The academics divided the Earth into regions and assumed maximum population densities for each. The problem with that is that they didn’t detail their assumptions and also used fixed numbers, rather than assuming that technology keeps improving and we keep finding better ways to do things!

• Curve Fitting: Analysts who just consider past growth and assume it will continue. Of course, you can’t do that because fertility rates and life expectancy change!

• Single Constraint: Focus on one limiting factor—usually food. But as we have seen, food is not a limiting factor! Are there others?

• System Models: Academics create complex computer models incorporating multiple interdependent factors using complex equations. But these models have lots of untested assumptions, and they seldom pinpoint what exactly will be the proverbial straw that breaks the camel’s back.

Take as an example The Limits to Growth, a 1972 report that raised the alarm about the Earth’s population growth rate, and how it meant humanity would run out of many resources as it grew. Alarmism works, so the authors sold plenty of books and made a living off of their pessimism. But criticisms of the work were brutal, and reality has caught up with its authors: This was 50 years ago, and so far, humans have not run out of a single resource. Let’s repeat this, because many people don’t internalize it.

Think about it! Can you name a single one? Go search for it if you don’t believe me!

In fact, we keep finding more and more resources! Whenever a resource runs out, it becomes very expensive, and some clever people focus all their efforts on finding more of it. Case in point: lithium. A few years ago, there was a widespread fear that batteries would consume all the known lithium on Earth. What happened?

Lithium prices dropped precipitously after a steep climb that peaked in late 2022 (Trading Economics)

Why? Because of news like this:

In this article, one of the authors of The Limits to Growth updates her thinking. It’s an illuminating article to understand how the authors think. Most of it is handwringing about complex models, but when the rubber meets the road, their arguments are pretty weak:

Another limit to food production is water. In many countries, both developing and developed, current water use is often not sustainable. In an increasing number of the world’s watersheds, limits have already been reached. In the U.S. the Midwestern Ogalallah aquifer in Kansas is overdrawn by 12 cubic kilometers each year. Its depletion has so far caused 2.46 million acres of farmland to be taken out of cultivation. In an increasing number of the world’s watersheds, limits have already, indisputably, been exceeded. In some of the poorest and richest economies, per capita water withdrawals are going down because of environmental problems, rising costs, or scarcity.

As we learned in the desalination articles, this thinking is static! It assumes that the problems of today and yesterday can’t be solved, without even looking at the places that have already solved them, like Israel and Saudi Arabia!

Since there are an infinite number of stupid arguments to make against the growth of humanity, I can’t address them all. Instead, we’ll examine the single strongest study on carrying capacity, see if it has any merit.

Planetary Boundaries

A team of 28 academics have come up with the Planetary Boundaries, a series of nine processes that threaten to collapse under the weight of humanity’s impact. From what I’ve seen, it’s the most serious attempt to quantify what can go wrong. According to the team’s latest report in 2023, six of the boundaries have already been transgressed.

Let’s look at each of the six transgressed boundaries.

1. Climate Change

The report mentions CO₂ and radiative forcing² as the main issues.

CO₂

As we noted in How Bad Is CO2?, it’s true that CO₂ levels are higher than they were two centuries ago, and that this is causing global warming. But in the grand scheme of things:

• CO₂  levels are still lower than they’ve been throughout most of the world’s history.

• Plants have been reducing atmospheric CO₂ for millions of years, at times to dangerously low levels.

• Now that the concentration is higher, plants are thriving, growing faster than ever.

• The problem with CO₂ is not its level itself, but how fast the level is rising, warming the globe too quickly for species to adapt.

• More specifically, the risk is that there are tipping points that global warming can hit, like warming up Greenland too much, which could stop the Atlantic Meridional Overturning Circulation (AMOC), throwing northern Europe into a glacial age.

So yes, it’s true that we have too much CO₂ in the atmosphere and that we are adding even more. But we already know how to solve this! In short:

• Solar energy and global electrification will dramatically shrink the emitted CO₂ in the coming decade. Within a couple of decades, our CO₂ emissions will be much lower.

• We will still have to sequester the CO₂ that is already in the atmosphere, but we know how to do it: With technologies like olivine weathering.

• In the meantime, we should reduce global temperatures, which we can do cheaply and with minimal side-effects using sulfate injection. 

Which means we should nuance the concept of planetary boundaries. Yes, there are limits to what the Earth can withstand. Yes, sometimes we pass these limits. But that doesn’t mean we can’t subsequently course-correct. In the 19th century, London was impossibly polluted, and now its air is clean. Humans need to suffer from problems before they look for solutions. 

We see this in the famous Kuznets’ curve applied to the environment:

We are seeing this play out in the evolution of CO₂ emissions per capita:

CO₂ emissions have been shrinking since 1980 in developed countries like the US and the EU, although they’re still growing in poor and middle-income countries like China and India—to the point that China now emits more CO₂ per capita than the EU, even if the EU is 2.5x richer than China on a per capita basis.

So does the CO₂ situation mean we can’t get to 100B humans? Absolutely not. It just means that, on the path to 100B, we will hit some roadblocks that we’ll need to clear. That’s it.

Radiative Forcing

The Planetary Boundary papers tell you that the heating of the Earth comes not just from CO₂ emissions, but also methane, and darkening the surface of the Earth³, while aerosols reflect heat. The balance is radiative forcing, and today it’s at 2.91 W/m² (watts per square meter) but according to the papers, 1 W/m² would be optimal. Here’s the breakdown:

• Carbon Dioxide (CO₂): +1.66 W/m²

• Methane (CH₄): +0.48 W/m²

• Other gases: +0.5 W/m²

• Aerosols: -0.5 to -2.5 W/m²

• Additional ozone in the troposphere: +0.35 W/m²

• Clearing forests: -0.2 W/m²

We can see that radiative forcing is mostly CO₂—which we’ve covered. The other big culprit is methane, which is a potent greenhouse gas, but unlike CO₂, it disappears from the atmosphere in about 12 years. And we’re getting much better at spotting it. For example, it used to be impossible to see methane leaks because the gas is transparent, but thanks to SpaceX, we can now send cheap satellites to space that can visualize these leaks. Here are orbital images from the company OSK showing methane leaks:

So as we solve CO₂, we’ll also solve methane, and it will be even easier since we just need to stop releasing it, rather than also sequestering it, like for CO₂.

In other words, climate change is not an obstacle to get to 100B humans.

2. Land System Change

We’re clearing our forests. This isn’t good, nor is it sustainable.

Source

To get where we are, the world has cleared too many wild forests and has sacrificed too much biodiversity. This is terrible; we should not have done it.

But here we are. We can’t change the past.
But we can change the future.
So why did we clear so many forests, and can we change that?

We cleared our forests for agriculture—mainly crops (52%) and grazing (37%). Eurasia did this thousands of years ago, when forests in places like Europe, the Middle East, China, or India were cut down to give space to humans and their farms. We don’t realize this because it was before history—there was no writing, so no records. We lost a tremendous amount of biodiversity then, like mammoths or saber-toothed tigers. What we are seeing now in places like the Amazon Rainforest or Indonesia’s forests is just the modern version.

Can we stop that from happening in the future, or would a path to 100B people force us to continue uprooting nature?

One of my favorite sayings is from William Gibson:

The future is already here; it’s just not evenly distributed.

To know what would happen in the future, we need to look at current trends and project them into the future.