Population and Environment
According to U.N. estimates, the global population now exceeds 8 billion. November 15 was arbitrarily chosen as the date we reached this milestone. The media took note, opinions were aired, and public discourse quickly moved on...
Many of my newsletters describe scenarios in which statistics are misrepresented in news reports, often inadvertently, sometimes maliciously. This newsletter is different, because I'll be suggesting that in stories about the 8 billion milestone, journalists sometimes did a better job than experts do at describing the environmental impacts of population growth.
This is not to say that science journalists have greater expertise than the experts, or that journalists always get it right. Rather, debates among the experts have been so heated that some of them downplay or simply ignore data that doesn't fit their views. As a result, it's sometimes the journalists who offer nuanced summaries while experts quarrel and oversimplify.
The challenge
It's not easy to parse out the environmental impacts of global population growth. For example, compare the two graphs below:
The similarity between these graphs might tempt us to conclude that population growth causes an increase in atmospheric concentration of CO2 (which drives climate change). However, correlation doesn't prove causation. Since 1750 there have also been dramatic increases in the burning of fossil fuels for transportation, electricity production, industry, and so on. Of course, part of the reason we burn more fossil fuels is that there are more people around to use them. It's challenging to distinguish the impact of population growth per se from the impact of fossil fuel consumption and other closely-related changes such as new agricultural practices and deforestation.
Statistically speaking, we say that population growth and fossil fuel consumption are collinear, meaning that they're different things, but closely related. And yet, no matter how close the relationship, we need to tease them apart, for scientific as well as practical reasons, since greenhouse gas emissions such as CO2 are the main cause of climate change in recent centuries.
A focus of ongoing, passionate debates among experts concerns the impact of population growth on climate change and environmental degradation (depletion of resources, habitat destruction, pollution, etc). Opinions range from the fear that population growth is a mounting global crisis, to the reassurance that population growth is mostly a great thing and we need more of it. The actual data call for more nuanced perspectives somewhere between these extremes.
In the next two sections I'll sketch in some historical context for the current debates, then illustrate how the experts sometimes do more poorly than journalists at representing the data.
Environmental Malthusianism
Exactly 50 years ago, I was sitting in my 6th grade social studies class, Iistening to Mr. McKenzie rant about overpopulation. There are 4 billion people in the world, he said. A billion of them are going to starve to death soon, wars are coming, governments will fail...
Back then it was relatively easy for teachers to get away with crazy talk, but Mr. McKenzie was right about one thing: Global population had been increasing exponentially, and by 1972 there were an estimated 3.84 billion people. What I didn't know back then is that Stanford biologist Paul Ehrlich had published a book in 1968 called The Population Bomb that got a lot of folks, including Mr. McKenzie, agitated about environmental degradation and other disasters that were being caused by population growth and would soon get worse.
Ehrlich in turn had built on the views of English clergyman Thomas Malthus, who argued in 1798 that food production can't keep pace with population growth. As a population expands, food resources will be depleted, resulting in disasters such as famine and war that reduce the population back to sustainable levels. (Malthus concluded that governments need to implement population control strategies, rather than relying on disasters to keep the population in check.)
There are many variants of the Malthusian view, including the one sensationalized by Ehrlich (and picked up by Mr. McKenzie). My focus is on environmental Malthusianism – i.e., the view that global population growth per se damages the environment. Proponents don't claim that closely-related developments, like fossil fuel consumption, are benign. Their point is simply that we need to worry about population growth too.
Newer perspectives
In the half century since Mr. McKenzie scared the crap out of me, many experts have shifted away from the view that global population growth per se poses a major threat to the environment. Here are three of the reasons for this shift:
1. The gloomiest predictions of Malthus, Ehrlich, and others (including Mr. Mckenzie) didn't pan out, thanks in part to innovations in agriculture and medicine.
2. Concerns about global population growth diminished as it became clear that the overall rate of growth is slowing. The world's population is now expected to level off and begin to decline around the beginning of the next century. In the meantime, in some countries and regions, the population isn't growing rapidly enough to support the future economy.
3. Traditionally, beginning with Malthus, concerns about population growth motivated population control strategies, and the outcomes were often grim. Some experts openly admit to shifting their focus away from population growth owing to this dismal historical record. Perhaps the best known example is China's controversial one-child policy, formally implemented in 1980 (and phased out in 2015 owing to its perceived effectiveness at curtailing population growth). Apart from ethical concerns about governments deciding how many children citizens can have, China's policy was especially repugnant in practice owing to inequities (e.g. families could have more than one child if they could afford to pay the fine) as well as human rights abuses (e.g., some women who already had a child experienced forced abortions and/or sterilizations).
Given that population growth now tends to be most rapid in less affluent parts of the world (e.g., sub-Saharan Africa), the concern some scholars raise is that treating population growth as a threat to the environment (or anything else) risks supporting discriminatory political policies.
4. New evidence suggests that environmental degradation and other problems formerly attributed to global population growth are more strongly influenced by variables such as fossil fuel consumption and affluence.
Expert bias
This new evidence has led some experts to dismiss global population growth as a substantial threat to environmental health. However, this evidence typically only suggests that population growth is less influential than other variables, not that it's devoid of influence.
The experts in question are guilty of a logical fallacy known as causal oversimplification (or causal reductionism), where a phenomenon with many causes is treated as if it only had a single cause, or the importance of one cause is overemphasized at the expense of others.
Causal reductionism: First example
A typical example of this fallacy can be found in a classic, frequently cited 2009 review by David Satterthwaite, in which the author shows that population growth in recent decades has been greatest among countries with the lowest per capita emissions. The paper is a masterpiece of waffly language. In some places increasing emissions are said to be caused strictly by consumption, not population growth. In other place, population growth is said to have "little" impact on emissions. Well, which is it? Does population growth have no impact, or only a little impact?
Satterthwaite's own data suggest that population growth has at least a small impact. However, he didn't directly address the collinearity problem that I mentioned earlier. Rather, he simply showed that population growth and emissions don't always coincide. For example, when looking at the figure below (which presents data for 1980 through 2005), he wants you to notice that among low income nations, population growth is vastly greater than growth in emissions, while among high-income nations, the pattern is reversed:
What I want you to you notice about this figure is that all of the bars are positive. In other words, in all four groups of countries, population has been increasing and so have emissions. This tells us that population growth and emissions are correlated during the time period under study, but it's unclear by how much, or what would happen if income were controlled for.
Satterthwaite's paper is one of many in which it feels like ideology is driving methodology, because we're told that population growth doesn't matter, but the data suggest that it might. (The data are correlational, so we can't say anything stronger than "suggest".) In the end, the only thing that's clear from this paper – and this is widely agreed upon now – is that more affluent countries create more emissions than less affluent countries do, and that population growth is greater in less affluent countries. This suggests that affluence impacts emissions more strongly than population growth does, but it doesn't show that population has no effect. If anything, the figure above suggests that it does.
Causal reductionism: Second example
Even in papers that acknowledge the role of population growth, the statistical logic can be questionable. For example, in a 2022 article, David Samways claims that economic growth "must have played a bigger role than population growth in the increase of CO2 emissions", because "the two have not increased proportionally."
"Indeed, taking 1850 as our starting point, world population increased by a little over sixfold to 2019 (1.6 – 7.71 billion)....while, over the same period, anthropogenic carbon emissions have increased by a multiple of more than 180 (0.1969 – 36.42 billion tonnes per annum)...."
Samways argues that because the percentage increase in carbon emissions is so much greater than the percentage increase in population size, population growth can't have contributed much to the increase in emissions. The logic here is unclear. A person is not a carbon emission. Population size and carbon emissions are measured on different scales. There's no guarantee that the two scales have a simple linear relationship. The data that Samways cites is consistent with the possibility that population growth is the sole cause of increased emissions. In other words, it might turn out that each 1% of increase in population results in a roughly 30% increase in emissions over a 170-year period, regardless of economic growth.
I'm not saying the latter is true. It's surely not. My point is simply that causally-related variables don't necessarily change at the same proportional rate. Meanwhile, CO2 emissions are known to be influenced by many variables that aren't considered or controlled for here.
Causal reductionism: Third example
The reductionist problem can also be seen when experts intentionally try to bridge the gap between scientists and the general public. An example is a July 2022 statement released by the Union of Concerned Scientists (an organization that does important, deeply admirable work and which I have no quarrel with). Here's a key excerpt:
"We’re sometimes asked “Isn’t population growth driving climate change?” But that’s the wrong question—and it can lead to dangerous answers.
"A misplaced focus on population growth as a key driver of past, present, and future climate change conflates a rise in emissions with an increase in people, rather than the real source of those emissions: an increase in cars, power plants, airplanes, industries, buildings, and other parts of our fossil fuel-dependent economy and lifestyles. Implicit in this faulty framing is the notion that all people contribute significantly to heat-trapping emissions. In fact, data show that that the richest 10 percent of the world’s population contributes 50 percent of annual global warming emissions."
My concerns about this excerpt pertain to the wording more than the substance.
First of all, asking whether population growth drives climate change may not be the "wrong" question. It's just a question. If the answer is "no", you can say so. What's troubling about the phrasing here is that some researchers do suggest that population growth does contribute to climate change. The fact that fossil fuel consumption might have a greater influence doesn't mean that population growth has a negligible impact.
Second, an increase in cars, power plants, etc. is driven in part by population growth, and so it seems misleading to say that these objects are the "real source" of emissions. All you can say is that they're the most proximal source. Exhaust comes out of the tail pipe of a car, not out of a person. But the person is driving the car. Given that more cars are produced as populations grow, it seems misleading to argue that cars are the real source of emissions but people aren't. That's almost like saying bullets are really what kill people, not the shooters.
Finally, all people do in fact contribute to heat-trapping emissions. Whether they contribute "significantly" is hard to say, because it's unclear what that term might mean. I would replace the term "significantly" with something like "equally", to indicate that affluence predicts a disproportionately large amount of emissions rather than the entirety.
In fairness, the Union of Concerned Scientists wants to prevent countries from using population control as a means of combatting climate change. This is understandable given the checkered history of population control and skepticism that it is either effective or could mitigate climate change. But clear descriptions of data are needed too.
Media reports: A pair of dogs
Not surprisingly, you can easily cherrypick awful, ideologically-motivated news reports about the 8 billion milestone.
For example, on November 28, Fox News published an article entitled "Global population hits 8 billion and we need to keep growing…" The writers, a pair of ideologues hawking a new book, argue in essence that population growth is an unqualified good, assuming free markets and freedom of expression, and problems like environmental degradation will eventually go away because the more people there are, the more innovative solutions there will be. There are many problems with this thesis, including the fact that population growth is currently most rapid in poor countries where it's unclear whether additional growth would do more than exacerbate existing misery.
Most egregious is a November 15 article in the Wall Street Journal, which consists of miscellaneous data followed by a one-sentence conclusion:
"For now, the U.N. warned that as the population grows, there will be more people to release harmful greenhouse gases."
This is one of the poorest summary statements I've seen all year in any major news outlet, including Fox News. What the U.N. did say, among other things, is that population growth is increasingly concentrated among the world's poorest countries and creates challenges for those countries. Meanwhile, emissions are highest in more affluent countries where consumption is greatest. Environmental health, and the well-being of the poorest countries, can be protected mainly by changing patterns of production and consumption in the wealthier countries. Slower population growth could help too, according to the U.N.
Media reports: Better examples
As I mentioned at the outset, some media reports on the 8 billion statistic are quite good and seem to better represent the scholarly work than what some of the scholars have written.
One of the best reports in my view is a BBC article addressing the question of how many people the earth can handle. There you'll find a lengthy historical review and many sides of the story.
An article in Wired also provides a close look at population growth and decline at different times in different places. Stories like this provide nuanced context for discussions of broad population trends in some studies (and in this newsletter).
Excellent summaries of the U.N.'s assessment (see above) can be found in sources such as the New York Times, AP News, and others. By noting the environmental impacts of population growth while acknowledging that variables like consumption play a stronger role, these articles seem to portray the data more clearly than some of the academic works I cited earlier.
Finally, with respect to climate change, the organization Population Matters sums up the role of population growth in an especially concise way:
"Multiple factors contribute to climate change, and multiple actions are needed to address it. The number of people on our planet is one of those factors. Every additional person increases carbon emissions — the rich far more than the poor — and increases the number of climate change victims – the poor far more than the rich.
"Population growth is also important because it affects the Earth’s ability to withstand climate change and absorb emissions, such as through deforestation as land is converted for agricultural use to feed a growing human population."
Conclusion and Implications
Global population growth contributes to climate change and environmental degradation. Although the extent of impact is unclear and not as great as that of greenhouse gas emissions, population growth does seem to play a role independent of closely-related changes such as economic growth and consumption practices.
I draw this conclusion in part because researchers who claim that population growth plays no role, or that it's not a "real" factor, typically report data that contradicts these statements (or at least fails to support them).
I also base this conclusion on studies, which I alluded to in this newsletter but didn't discuss, in which population growth seems to cause environmental damage after being statistically distinguished from related variables such as economic growth. These impacts may be greater in some parts of the world than others.
Deriving policy implications from this conclusion is tricky, in part because experts who agree with it don't agree among themselves about the extent or ways in which population growth per se damages the environment. In addition, ethical concerns can be raised about anything that smacks of population control. According to a study published last year, environmental concerns prompt some people to forego having children. I think it's important to support these people and avoid making anyone who is childless, for whatever reason, feel like they've fallen short in some respect (Rebecca Solnit has written brilliantly about this topic). I think it's also important to ensure adequate sex education, access to family planning resources, and legal protections for safe and affordable abortions (all areas in which the U.S. arguably falls short). Beyond that, I would not advocate anything else to someone concerned about population growth in a country such as ours.
I'd close on a different note if I lived in a country like Japan or the Ukraine, where the populations are shrinking because annual death rates now exceed the birth rates. Although in my view it's never advisable to pressure people to have babies, if the future of your economy depends on increasing birth rates, there are many ways to support parents, such as more generous parental leave policies, subsidized child care, and so on. Some experts are actually concerned about declining rates of population growth in the U.S. My response is that Americans might have more babies if parents received more support for raising them. (Here's a statistic for you: America is the only first-world country that fails to offer nationally-guaranteed, paid parental leave. In Japan, where the underpopulation problem is currently one of the severest in the world, new parents receive 52 weeks of paid leave at 67% and then 50% of their current income.)
Thanks for reading!