|September 21st, 2017|
We recommend four widely applicable high-impact (i.e. low emissions) actions with the potential to contribute to systemic change and substantially reduce annual personal emissions: having one fewer child (an average for developed countries of 58.6 tonnes CO2-equivalent (tCO2e) emission reductions per year), ... we find that ten high school science textbooks from Canada largely fail to mention these actions ...
There are a lot of reasons that this isn't a good way to look at the question of having kids, the biggest of which is that it ignores that people have many other effects other than emitting greenhouse gases. People earn money (that we can tax), people consume services, people create things that benefit others, people pollute in non-warming ways, etc, not to mention that people have their own internal experience that has value. Whether it would be better for people to have more children or fewer children in general is not at all a settled question, and looking only at emissions is an incredibly limited way to try and answer it.
But even then, let's think about what this 58.6 tCO2e means. The US EPA estimates  a "social cost of carbon", which tries to put a dollar value on the harms associated with additional emissions. Taking their "3% Average" estimate for 2015 of $36/tCO2e, then 58.6 tCO2e/year has a social cost of just $2,110/year. Even their 95th percentile estimate of $105 gives us $6,153/year.
Or we could take the 2016 Giving What We Can estimate for the cost-effectiveness of Cool Earth:
we estimate that Cool Earth reduces greenhouse gas emissions through direct protection of forests at a rate of $1.34 per tonne of CO2-equivalent. Despite the uncertainty which this figure is subject to, we estimate that this figure will be no higher than $1.87/tCO2eq and no lower than $0.65/tCO2eq.
Taken literally, this would be $80/year for 58.6 tCO2e. Now, I'm not that confident in these numbers, since charity evaluation is very hard to do well, but even taking 10x the top of their range gives us just $1,096/year.
This paper is advocating having fewer children, in a country  with a per-capita income of over $40k/year, to avoid somewhere between $80 and $6,000 in yearly emissions! Definitely put thought into whether to have children, and consider what else you could do with the money and time, but emissions should be a very small factor in the decision.
(This is an expansion of comments I made in this thread. I've seen people give this argument before informally, but looking quickly I didn't see anything I could link to so I wrote this up.)
 Well, used to estimate. Trump directed them to stop.
 Update 2017-09-21: Specifically, Canada. In the US it would be 120 tCO2e, so approximately double all costs above, and a per-capita income of $58k. I didn't realize that the US and Canada numbers were so far apart in writing this; in retrospect I should have read the paper more carefully, found the US number, and used those.