I’ve written a post showing you how individual choices affect global warming. You really should read that first. This is a long supporting post telling you about the research, the (frequent) misunderstandings of it in the media and then the adaptations needed to get meaningful warming numbers.
I’m nervous writing about this topic. Research suggests that when people feel climate change science is critical of their choices, they reach by attacking the science or attacking scientists’ own lifestyles.
I don’t feel I’m in a position to criticise anyone’s choices or lifestyle. For many people, how many children to have is too personal a choice for any external factors to matter. I’m absolutely fine with that. I really don’t like telling people how to live their lives. If you aren’t playing a piano next door, and you aren’t telling me how to live my life, I’m likely fine with whatever you’re doing.
What I’m not fine with is people being shown distorted science. I get mad that criminal incompetence with statistics has made people think Toxoplasma gondii infections change people’s behaviour. I get mad when a popular author misleads people into thinking childcare choices make little difference to their children’s wellbeing. And I get mad when the same journalists who rail against climate change denial reject the scientific consensus on the links between population and climate change.
So, please read this if you want to know what the research says, and don’t read in any subtext. If you share the view of the 38% of Americans between 18 and 29 who ‘believe a couple should consider the negative effects of climate change when deciding whether or not to have children,’ reading this may be informative. If not, please don’t take research findings as personal attacks on your worldview.
As for what the research actually says…
That graph shows you different actions someone in a developed country could take, and the area of a circle is proportional to the emissions reduction from an action. The estimate for having a child applies in each year they are alive, not just the year you have them.
To be clear, there’s a range of plausible values for the actual emissions/year from having a child, just as there’s a range of plausible values for the effect of emissions on temperature, and I happen to think the most widely cited estimate of 58.6 tons/yr is much too large. But just as there’s a consensus that man-made emissions affect temperature, there’s a consensus that having a child has a larger impact on emissions than any other action. A professor of climate science puts it starkly:
Unless you will ever contemplate lighting a forest fire, the decision to reproduce is probably the biggest carbon choice you will ever make.
How Bad are Bananas, Mike Berners-Lee (2013)
To emphasise: I know that when people tell you that driving/flying/eating meat/having a child causes a lot of emissions, they’re almost always trying to get you to stop. I’m not. I think the job of scientists is to give people their findings as clearly and accurately as possible, without exaggerations and without omissions, and stop there. Science isn’t a weapon for manipulating people.
Enough. I’ll return to frequent misunderstandings of the research in a minute, but first we need to do see what that research actually says.
58.6 tons per child per year
The figure 58.6 tons of emissions per child per year is widely quoted in mainstream media articles. It and the other numbers in the Guardian infographic above were taken from a paper called (The) Climate Mitigation Gap. That paper draws on an earlier paper, Reproduction and the Carbon Legacies of Individuals, which I’ll call Carbon Legacies. The authors of Climate Mitigation Gap also produced a follow-up FAQ.
There’s a chain of the Telephone game going on here, which takes a bit of work to unravel. First, Carbon Legacies says that
our goal is to quantify the consequences of the childbearing decisions of an individual
To that end, they adding up half the child’s emissions, a quarter of any grandchildren’s emissions, and so on, up until an (unspecified) end date. They model different future emissions trajectories. Under the constant emissions trajectory, they find emissions of 9441 tons for the US, 2498 tons for Russia and 2026 tons for Japan.
Next, Climate Mitigation Gap takes these figures and produces this infographic:
The leftmost column is the key one — just focus on that. To obtain the per-country figures marked on that left-most column, they divided the figures from Carbon Legacies by the life expectancy of a woman in the US/Russia/Japan. Then (I think) they average the figures for the USA (~120), Russia (~40) and Japan (~20) to get the final figure of 58.6 tons/year, which is the height of the green bar in the graph.
It should be clear at this point that 58.6 tons per child per year isn’t one we can use verbatim. There are several issues:
- Emissions are added up until an end date, which isn’t specified in Carbon Legacies. The choice of date makes a big difference to the final figure.
- Carbon Legacies considers several emissions reduction trajectories, but they are pretty arbitrary, not grounded in the data.
- Dividing the Carbon Legacies figures by life expectancy is not really meaningful…
- and nor is averaging over three randomly chosen developed countries.
I don’t mean to be too damning of the papers, by the way. Climate Mitigation Gap was primarily trying to make the point that in a developed country, having a child has a far greater impact on warming than any other action, and it succeeds in that.
How Bad are Bananas
Berners-Lee provides his own calculation for the emissions from having a child; in the 2011 edition of How Bad are Bananas, he cites 688 tons for an average US child and 373 tons for the UK, based on the assumption that emissions will drop by 3.9% a year.
3.9% a year is the reduction rate needed to hit the UK’s old Paris goals for 2050. As before, I find that emissions reduction trajectory somewhat arbitrary; we should be working forwards from the data to find likely trajectories, not backwards from targets set by politicians. 3.9% is well outside the range of projections.
The other issue with Berners-Lee’s analysis is that (for ‘simplicity’) it doesn’t consider grandchildren, etc.. I think that as with Climate Mitigation Gap, he is mainly trying to point out how large the numbers are here, but leaving out grandchildren makes a big difference — if you have a child in 2021, they will have grandchildren around 2050, and those grandchildren will produce 50 years of emissions before 2100.
There haven’t been other quantitative estimates of the warming resulting from a child, at least in the 350 or so papers that cite Climate Mitigation Gap. There is plenty of related work, though. Everything I have found concurs that having that having children has a very large impact on warming.
This webpage has an alternative calculation of the emissions from having a child, finding that it increases an individual’s annual emissions by about 40%. (It’s not peer-reviewed, but that’s no reason not to take it seriously; we care about the argument, not the arguer.) Unfortunately it assumes the US will fully decarbonise in 2080, which is incredibly optimistic: the US Annual Energy Outlook for 2020 projects total US emissions in 2050 to be about equal to those in 2020. Greener policies may well shift that down a little, but carbon neutrality in 2080 is not remotely likely.
Science versus Journalism
If you read Climate Mitigation Gap and see what they’re actually claiming to do, all they say is that they consider individual choices and
calculate their potential to reduce greenhouse gas emissions in developed countries.
As part of this, they find that choosing to have fewer children has far more effect on emissions than any other action. As we saw above, that hasn’t been contested by any subsequent research.
That ought to be the end of the story. Unfortunately, what happened was that ‘the recommendation to have ‘one fewer child’ was picked up in the media and subject to considerable controversy’. Publications from National Review and Breitbart on the right to the Guardian and Vox on the left attacked politicised versions of the research. I want to dispense with their assorted straw-men here, so you don’t inherit their misunderstandings. (You can skip this section if you just want to see the calculation.)
Having fewer children will wipe out the human race
If you literally assume that everyone does the exact same as you, any action you take will have devastating consequences. If you don’t have children, the human race is wiped out. If you don’t drive, the automotive industry collapses and there’s a massive recession. And so on. The sane way to assess individual actions is at the margin; your individual choice not to have a child reduces the fertility rate by a tiny amount, rather than wiping out humanity.
We need more workers to support the elderly in 50 years
That assertion doesn’t actually contradict the research findings? But for what it’s worth, fertility has less impact on GDP than people assume because the working-age population have to support both the elderly and children: taxes go to pensions but also to schools. A higher fertility rate means fewer pensions per worker but more schools.
To be more quantitative: this paper weighs the factors against each other and finds that the socially optimal policy involves taxing people for each child.* NB. no-one’s seriously suggesting doing that; it’s just an economic analysis that shows that the ‘more workers’ argument is not too convincing.
*Socially optimal policy is an economics term I’d need way too much space to unpack.
People have a right to have children
Yes. I know. The authors of Climate Mitigation Gap weren’t questioning that, any more than they were advocating sending the secret police into homes to force people to recycle. One of the authors said:
‘I knew this would be a sensitive topic to bring up. Certainly it’s not my place as a scientist to dictate choices for other people. But it is my place to do the analysis and report it fairly.’
And in case that sounds too clinical, she continues:
“Having a child […] is a vote of hope,” she says. “It’s a vote that the world is going to be a better place and we can actually tackle this challenge.”
[…] in her own life, she and her fiancé are deciding whether they want to cast that vote of hope for themselves.
“Because we care so much about climate change, it is a factor we’re considering. But it’s not the only one.”
This is blaming the developing world for problems caused by the developed world
Climate Mitigation Gap is only about the impact of individual choices in developed countries. It says so in the second sentence of the abstract.
To be absolutely clear, the research is unequivocal in showing that the impact of having a child in a developing country is far smaller than in a developed one.
Don’t blame parents for their children’s emissions; that’s double-counting
Words like ‘blame’ and ‘responsibility’ don’t turn up even once in Climate Mitigation Gap! It only talks about the effect of individual choices on emissions. This straw man particularly annoys me because the Vox piece that popularised it is from an author who rails against science denial. (He also completely misses that the study is only about developed countries.)
Journalists love blame; it sells copy. But it’s totally unscientific. Why? Because for every unit of emissions, there are many, many people who caused that unit of emissions to be released.
If you have to fly for work, that creates emissions. Are you to blame, for choosing that job, or is it your boss’s fault for sending you abroad? Or maybe we should blame the owners of your company? Actually, if you have a pension, you own a chunk of all the big companies out there, so we need to blame you again.
All of those people can act to reduce those same emissions. That’s why Climate Mitigation Gap and other good research talk about actions, rather than blame.
Global warming is the fault of neoliberalism/capitalism/big business/fossil fuel companies and individual action can’t impact it
I have looked really hard and I cannot find any research to support this common sentiment, interpreted widely. Best I can determine, government policies and individual actions can have about the same effect on the warming rate. (Details for another article.) For now, I hope the ‘Little Planet’ formulation in the main article makes it clear that individual actions in the developed world can have a large impact on warming.
Emissions and Sins
Hidden in many of these straw men is the idea that emissions are sins. If there’s global warming happening, someone, somewhere has done something bad and we need to point a finger at them and jump up and down. This idea leads naturally to the viewpoints discussed above: if your children’s emissions are their sins, they can’t be your sins.
If there’s one thing I want you to take away from this article, it’s that thinking of emissions as stemming from sins is just factually incorrect. Setting aside indirect emissions via children, the research says that for the US ‘tolerable’ lifestyle changes can only give a 30% reduction in energy use and emissions; a 50% reduction ‘would require dramatic changes which we believe would be unacceptable to most people’. (Other research suggests that individual actions can only reduce emissions by 6–16%, but 30% is consistent with the numbers in my main article.)
One reason that larger reductions are hard is that you pay tax, and the government spends that in ways that produce emissions — not because government is evil but because it’s giving you healthcare, education and so on. Another is that emissions are entrenched in everything you consume and do, often in ways that are hard to see. For example, typical use of reusable nappies will result in higher emissions than disposable ones, because washing them uses emissions-rich electricity.
What this means is that however green your life is, you will have a large impact on global warming. If you have children, then however green their lives are, they will have large impacts on global warming.
Of course that doesn’t mean you shouldn’t have children. For many people, having children is more important than anything else, and that’s natural. But for 38% of young Americans climate change is a factor in the decision. If you don’t share their view, please let them read about the research without jumping to science denial.
Calculating the Impact
I’m not comfortable using any of the numbers above, because they all depend on very arbitrary assumptions about how fast emissions will decrease and about how many generations to consider. So we’ll redo the Climate Mitigation Gap calculation with those issues fixed.
The US, the UK, and other developed countries are reducing emissions per person, if more slowly than most people think. I ended up writing a whole other (long) article on this, which has all the sources. Here are some highlights for per-capita emissions:
- UK consumption emissions dropped 1.2% a year.
- UK emissions are projected to drop 1.7% a year to 2035.
- US consumption emissions dropped 0.54% a year.
- US emissions are projected to drop 0.71% a year to 2050.
For reasons I explain in the article, I’m projecting an average annual drop of 1% to 2% in per-capita emissions. Here’s what that means for total emissions:
For the main article, I’ll assume 1.5% a year. But I’ll also show you below how changing this number affects the carbon impact of each child.
Grandchildren and great-grandchildren
Because we’re looking at the temperature at a particular date (2100), there’s an objectively correct way to decide how many generations to track. We want to consider all the emissions before 2100, and only those. In practice, if you decide to have a child now,* you’ll likely have grandchildren around 2051 and great-grandchildren around 2081.
*Or, well, 2021, unless you’re some kind of supermutant.
So we need to sum up emissions from:
- your child from 2021 to 2099
- your grandchildren from 2051 to 2099
- your great-grandchildren from 2080 to 2099.
Let’s firm that up. In an ideal world I’d find the probability distribution for parental ages at the times children are born, and use it to find the probability distributions for the years in which children, grandchildren and great-grandchildren were born. Unfortunately I can’t find that data. We’ll have to approximate things using the average ages of mothers and fathers when their children are born.
Those graphs tells us that the average age of US mothers and fathers when they have children are around 28 and 31. (NB. we’re averaging over all children, not just firstborns.) I could average those to give 30.5, but it won’t make a difference in practice if we go with 30. So, as I suggested at the start of this section, we’ll assume all grandchildren are born in 2051 and all great-grandchildren in 2081.
How many children does each US adult have? On average each woman will have 1.728 children; that’s called the total fertility rate (TFR). The UN projects that it will stay very close to 1.8 through the next century, so I’ll use that figure. Remember that’s children per woman; since the gender ratio is very close to 50–50, we get 0.9 children per adult.
That means on average, having one child means you will end up with 0.9 grandchildren and 0.81 great-grandchildren. This is a shorthand for saying that there’s some probability you’ll have no grandchildren, some probability you’ll have one grandchild, etc, and that if you average over all the possible ways things could go, the number of grandchildren will be 0.9. This matters because it means that if you have one child, the average (or expected) emissions from your grandchildren in a year will be 0.9 times the emissions one person produces in that year. As everywhere else in emissions calculations, that expected figure is the one to use.
That was all for the US. For the UK, the average ages are 30.4 for mothers and 33.4 for fathers, averaging to 31.9. TFR is projected to be between 1.77 and 1.78 between 2050 and 2100; that’s about 0.89 children per adult. All of these figures are close enough to the US ones that I’ll stick with those — given the wide range of estimates for emissions, trying to get things to be overly precise is meaningless.
The Final Calculation
We’re now ready to do the final calculation. As a reminder, we’re computing the impact of one child; we assume 0.9 grandchildren in 2051 and 0.81 great-grandchildren in 2081. Rather than using a fixed annual emissions reduction, I’ll show you the effect of a range of parameters. For comparison, I’ll also add your ‘direct’ emissions, assuming you’ll live another 50 years.
I could use current estimates for UK/US emissions to give the final carbon impact of each child in (mega)tons, but there’s a more convenient approach which lets us handle both countries at once. I’m going to measure the impact of each child as a multiple of current (2020) annual emissions. So e.g. for the US, where 2020 emissions are 21 tons a year, 100 years would mean 2100 tons, i.e. 2.1 kilotons, of emissions.
Here we go…
The emissions reduction rate is on the x-axis, and the emissions from your children on the y-axis. The green lines indicate the historical/projected emissions decrease rates from the last section.
With a 1.5% rate (‘UK Historical’), your descendants will emit almost exactly twice the emissions that you do — i.e. the red line is twice the height of the blue line. (The exact number is 2.02x, but we’re not working to that level of accuracy.) And changing the rate doesn’t change that by very much; at 1.0% they emit 2.2x your emissions, and at 2.0% they emit 1.8x your emissions.
There’s one last thing we need to correct for. Suppose I decide to have a child, and my wife decides to have a child. 1 + 1 = 2, so that’s two extra children, right? Well, no. We’ve double counted — statistically speaking, a decision to make a child is most often jointly made by two adults. If you like, you can think of your descendants as splitting their time between your little planet and your partner’s little planet. To compensate for that, we should halve the numbers above. That gives us this rule of thumb:
Half the carbon impact of a child and their descendants (to 2100) is close to your carbon impact, assuming you’ll be around until 2070.
More concretely, if we assume that emissions reduce by 1.5% a year, as in the methods post, we find that half the carbon impact of having a child is equal to 35.7x your 2020 emissions. That’s assuming your child has the same lifestyle that you do; let’s look at that assumption next…
I’ve been told that a large family with a ‘green’ lifestyle might result in the same emissions as an small family with an average lifestyle. That’s prima facie plausible, but I just can’t square it with the research I’ve read. There are two problems.
First, as noted above, even the greenest lifestyle can’t reduce individual emissions by nearly as much as most people think. The food calculations gave us some inkling of this; you can halve your food emissions by going vegan, but that’s pretty much the limit. Another reason is that more people means more schools, more hospitals, more roads, and generally more government, all with associated emissions. Unless you teach your children not to pay taxes,* they have no control over those emissions. Per this paper, ‘tolerable’ lifestyle changes can only give a 30% reduction in energy use and emissions; a 50% reduction ‘would require dramatic changes which we believe would be unacceptable to most people’.
Second, I can’t find any evidence that you can have much long-term impact on your children’s emissions, let alone your grandchildren and great-grandchildren. The research tends to looks at older teenagers, and so doesn’t shed much light on long-term behaviour. That said, this finds a correlation of only 0.13 for saving electricity, which is a good proxy for reducing emissions; it’s likely to go down after they leave home.
It’s also worth noting that if you have a large family then your children are also slightly more likely to have large families (correlation of 0.12–0.15), increasing your descendants’ emissions.
Putting these two effects together, I can’t see that how you bring up your children could shift your descendants’ expected emissions by more than 10%. (Using the 0.13 and 30% figures above would only suggest 4%, but if you assume parents have total control over young children’s emissions, you get a somewhat higher figure. I’ll go with 10% in the main article, but the real figure may be a little lower.)
On the Little Planet
If you’ve read the main article, you’ll know we’re turning emissions figures into something more comprehensible by illustrating their effect on a Little Planet which is 7.8 billion times smaller than Earth.
We noted above that half the carbon impact of having a child is equal to 35.7x emissions in 2020. (Remember, the child splits their time between two Little Planets, so we only want half their emissions.) In 2020, the average person in the US produced 21 tons of emissions, and the average person in the UK produced 13 tons of emissions. That means having a child in the US generates 750 tons of emissions, and a child in the UK generates 464 tons.
Now, a Little Planet warms by 2.96 °C for every 1000 tons of emissions. That tells us:
- For an average US person, each child increases their Little Planet’s 2100 temperature by 2.2 °C.
- For an average UK person, each child increases their Little Planet’s 2100 temperature by 1.4°C.
If you’ve seen other sections, you’ll note these are much larger numbers than anything else we consider.
It would be lovely if the numbers weren’t so harsh, but science isn’t often that kind. It’s just a depressing fact that for those of us in the developed world, the number of children we choose to have is the main way we’ll affect warming in 2100. After all our calculations and cross-checks, this picture turned out to convey the right idea, even if our numbers aren’t the same:
One final point I should be clear on. I noted in the last section that the research suggests that encouraging children to live a ‘green’ lifestyle can’t reduce their climate impact by a large percentage — maybe 10% or so. But 10% of 2.2 °C is 0.22 °C, which is quite a lot compared to other choices, like going vegetarian yourself.
So how you bring up your children will make a difference to the temperature in 2100; it just won’t counteract as much of the warming from having children as you might have hoped.