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Why The Latest Study Saying ‘Have Fewer Kids To Save The Planet’ Is Junk Science


It’s in vogue for people on the Left to look down on big families as not just socially gauche, but actually morally troubling: as if rearing up the next generation were some ethically suspect endeavor. Last week gave another instance of that trend, with an article in The Guardian discussing an academic paper which claimed having one fewer child could reduce one’s carbon footprint by 58 metric tons per year.

It’s easy to see the politically motivated reasoning in why the piece went viral: reading between the lines, it argued that the way to be a truly environmentally conscious person is to be a childless vegan working a professional job in a dense urban area. Quelle surprise.

But for someone like me who works on demographic and economic issues all the time, it was particularly frustrating because the basic mathematical logic of the article makes no sense. When you strip apart the bad reasoning that informs the more-kids-more-emissions story, it becomes clear that in the vast majority of cases, how many kids you have just won’t affect any environmental outcomes.

Now, to be clear, for those who don’t believe that humans affect the climate anyways, this whole argument is a moot point. I do think that human-sourced greenhouse gases play a measurable role in observed climate trends, but we don’t need extreme policies to fix that, least of all deeply anti-human policies and scientifically baseless policies aimed at restricting fertility.

These Academic Claims Are Suspect

The basic logic of the academic paper in question is simple. They take a number of different activities like replacing lightbulbs, taking a transatlantic flight, or having an additional child, and they compute the life-cycle carbon emissions from that activity. That means they don’t just look at fuel burned for a specific flight, but fuel burned to produce energy for the factory that built the airplane as well, divided among all the passengers that airplane will ever carry.

This exercise is extremely computationally challenging, so environmentalists can hotly debate the life-cycle carbon footprint of complex products. That also means that when somebody does calculate the life-cycle footprint of a product, later researchers tend to pick up that estimate and re-use it, rather than doing the intense legwork themselves. That’s what happened in this case. The paper The Guardian covered derives its estimate of the yearly impact of having one fewer child (58 fewer tonnes of carbon emitted) from another paper from 2009.

That paper, however, didn’t count the carbon footprint of, say, a child, or how adding a child to a household changes that household’s carbon footprint. Rather, they calculated the “carbon legacy” of a person. They calculated this “carbon legacy” by assessing the plausible per-person carbon emissions of future generations, then assigning a share of each of a person’s descendants’ emissions to the original parent.

So, for example, if I have three children, I would be assigned half of each child’s lifetime carbon emissions, and my wife would be assigned the other half. Then if each of those three people have three kids, I would be assigned a quarter of each of my nine grandchildren’s emissions. Then if we multiply by three again, I am assigned an eighth of my 27 great-grandchildren’s emissions, and so on.

The result is that, in the United States, having one child adds 470 life-years of carbon emissions to a person’s carbon footprint. The authors of the most recent study take those emissions, divide by lifespans in developed countries, and voila, you get about 58 tons of emissions per parent per year per child.

This measure is dishonest and morally troubling. By this calculation, the best thing that ever happened for our climate was the Black Death, which wiped out a large percentage of humanity. Mass-murderers become climate heroes while everyday people having two or three kids acquire centuries-worth of carbon-emissions debt because they had the audacity to go forth and multiply.

Curiously, the authors do not show the emissions reduction you could enact by committing suicide: it seems clear that suicide would not only prevent the carbon legacy of childbearing, but also all current consumption. Thus presumably the truly noble defender of Gaia would simply call a physician in the Netherlands and ask for a painless way out.

There Are Also Numerous Methodological Problems

Let’s walk through the basic scientific errors at work here.

First, the authors of the underlying “carbon legacy” study do not do any “carbon discounting.” That is, they treat one metric ton of carbon emitted 100 years from now by my grandchildren as equivalent to one metric ton emitted today by me. No respectable climate scientist would ever endorse this view: delaying emissions by a century would be unambiguously good for the climate if you believe any of the major climate models! Emissions now are of far greater concern than emissions in 100 years! Without discounting, we understate the importance of carbon emissions today, and overstate the importance of carbon emissions further down the road.

It’s worth noting here that one of three things will be true in 100 years. Either the climate will be about what it is now because the climate models were wrong and thus we don’t need to worry about emissions 100 years from now, or humans will have found a way to effectively manage emissions and thus we don’t need to worry about emissions 100 years from now, or the dire forecasts of climate models will come true and we will all be drowned and dead and thus we don’t need to worry about emissions 100 years from now.

Under any scenario, the common thread is that we don’t need to worry about emissions 100 years from now. We’ve got enough to worry about for today. To compare the generationally distant emissions of my grandchildren buying diapers for their kids to emissions from me joyriding out to Italy for a tomato-sampling tour is utterly preposterous.

It gets worse, however. The study in question takes the entire sum of a person’s “carbon legacy” and then divides it by the potential parent’s lifespan for an annual estimate. This figure has absolutely no mathematical meaning whatsoever; it is a fake statistic. Dividing a temporally distant “stock” of carbon by my lifespan does not yield a “flow” comparable to if I were to, say, mow my lawn twice as frequently this year.

This point may seem abstruse, but it must be belabored: major newspapers are running a respectable study where the headline is a mathematically meaningless number. It’s like if I said, “Well, I earn $50,000 per year and have only one wife, therefore I earn $50,000 per wife.” True. That is a true statement. It is also an analytically useless one. To uncritically amplify such vapid claims elevates them from mere academic futility to journalistic malpractice.

Beyond these basic mathematical problems, there’s a deeper methodological issue. Consider what actually happens if I have a child. I will buy food, diapers, clothing; I may have to consume more services as well. Right now I could fit an additional child in my home and car, but if I had two or three more, I might need to make big new investments. All of these forms of consumption would increase my carbon footprint: having kids does boost carbon emissions.

Major newspapers are running a respectable study where the headline is a mathematically meaningless number.

But what if I don’t have a kid? Well then, there’s a swanky wine tour of the Caucus Mountains my wife and I have been eying for a while: time to buy the tickets! And if the estimates in question are true, I could take 25 or 30 international vacations every year and still harm Mother Earth less than I would if my wife became a mother.

That math is clearly untenable. It is simply not true that 25 or 30 international trips would have a lower carbon footprint than a one-year-old. Consider some basic math. One mile of air travel produces about 53 pounds of CO2. If you divide by, say, 200 passengers per mile, that makes 0.265 CO2 pounds per person per mile.

If we estimate that my average international vacation from DC is a flight from DC to London, that’s about 7,000 miles of flying per trip, so about 1,855 pounds of CO2 per person. Multiply by two to account for my lovely globe-trotting wife, and we produce 3,710 pounds of CO2 per trip. Multiply by 25 for the claimed ratio of child-emissions to air-travel-emissions and you’ve gotten to 92,750 pounds of CO2, or about 42 metric tons of CO2. The obvious implication of the estimates I’m critiquing, then, is that babies produce about 42 metric tons of CO2 per year.

Newsflash: the marginal consumption on behalf of a 10-pound child will not yield 42 tons of CO2. How do I know? Well, the entire United States only produces between 15 and 19 tons of CO2 per person per year, so unless babies are unique pollution-fiends (having changed a few diapers, I will grant they do have some foul emissions), 42 tons of emissions seems like perhaps an overestimation.

So when we consider marginal effects—that is, whether babies substitute for higher-carbon forms of consumption—it actually becomes quite ambiguous what the net effect of childbearing may be.

The Problems Run Deep

But more broadly, the basic error in climate activists’ thinking is that they misread the equation for carbon emissions. As a rule of thumb, CO2 per person is equal to total CO2 divided by population. But in this case, it does not therefore logically follow that adding an extra person will increase CO2 emissions. These formulas are accounting identities not causal mechanisms.

The marginal person added may be born into a low-carbon country, and indeed, countries with high fertility do, on average, have lower carbon emissions thanks to also being rather poor countries. The marginal person added may substitute away from higher-carbon consumption; that is, even if more people in the long run means more emissions, it may direct current resources towards lower-emissions spending: daycare instead of plane tickets.

If the problems facing humanity are as dire as the climate activists would have us believe, then surely we should want more human resources to throw at the problem.

Furthermore, many carbon emissions are fixed or fairly lumpy: adding one person does not change how many oil spills will occur. At some macro-significant level, sure, higher population necessitates new investments, but inframarginal efficiencies can be large as bigger populations find ways to enjoy better and better economies of scale.

Finally, the reality is that many technological problems facing humans and the world over which we are called to exercise dominion and stewardship remain unsolved. We don’t know how to produce cheap, efficient, cleanly produced, large-scale batteries yet, for example. These problems will only be solved by people, and largely by as-yet-unborn people.

The old adage that two heads are better than one applies as well to 2 billion, and is especially true about kids born into societies rich in educational and social resources like the United States, where most people will have a good shot at attaining their full potential. If the problems facing humanity are as dire as the climate activists would have us believe, then surely we should want more people, more human resources, to throw at the problem, especially if those people are well-educated, committed to democracy, and informed about climate risks, as is the case for most kids born in the United States?

The scientific consensus right now doesn’t even support the view that reduced fertility could prevent global warming. On the contrary, most scientists view fertility reductions as being far too delayed and as having far too small an impact to fix the problems their models predict within any reasonable timeframe.

There is no society in the world today that would be seriously threatened by lifetime fertility of married women being around three to four kids.

Now, look, extremely high fertility has its own problems, from risks to maternal health, to reducing education outcomes for women, to posing higher risks of child abandonment, especially in poor countries. Large numbers of young people, especially men, can be a recipe for political instability in poorer countries as well, which can lead to civil war and terrorism.

When we see countries, as Emmanuel Macron inartfully put it, with total fertility rates of around 7 or 8 kids per woman, there’s a real argument that helping those women get access to the right tools to steward their fertility is good. But likewise, when we see places with far-below-replacement fertility, we should see it as a harbinger of social ills to come. There is no society in the world today that would be seriously threatened by lifetime fertility of married women being around three to four kids, and such fertility rates pose no meaningful climate risk within any rigorously forecastable timeframe.

Well, actually, there is one society that might be threatened by widespread middling-to-big nuclear families of genetically related people raised by two parents who choose to view their fertility as a blessing from God, not a climate-apocalypse risk factor: the society of American progressives, to whom the celebration of family and motherhood is seen as little other than coded sexism and pining for Gilead.