This article in Environmental Research Letters by Seth Wynes and Kimberly Nicholas calculates the ways various personal choices affect greenhouse gasses. The paper has received a lot of attention in the media (e.g., see here or here). At the heart of the issue are the results from this figure in the original paper showing the relative effect of different actions on greenhouse gas emissions.
The findings led to headlines like this one in The Guardian, "Want to fight climate change? Have fewer children." The findings are interesting on a number of fronts. For example, I regularly see stories suggesting that the most impactful thing one can do to fight climate change is eat less meat. That strategy shows up as a mere 7th on this chart, and way, way behind having children (not having a child has more than 60 times the emissions impact as moving to a more plant based diet).
The implication that we should have fewer children raises a number of thorny issues that have long been debated. Since at least Malthus, folks have been worried about a growing world population. Stanford biologist Paul Ehrlich has raised alarm since the 1960s about the dire consequences of a "population bomb." At the heart of this thinking is the premise that an extra person is a kind of threat: a threat to food security, a threat to the climate, a threat to the environment. There's even a bit of a hint of this thinking in the common mantra of many agricultural organizations that we need to do what we can so that we can feed nine billion people by 2050. The extra people that will arrive in the next 30 years or so are placing a burden on us today to increasing productivity.
An alternative perspective, one often attributed to Julian Simon, is that an extra person is a blessing rather than a curse. People aren't just consumers of resources but are are sources of ideas, creativity, and ultimately new resources. An extra person isn't a threat but an opportunity.
Regardless of where one falls in this debate, it should be noted the above graph looks at just one side of the equation: the cost of an extra human in terms of extra greenhouse gasses. What is ignored is the potential benefit of an extra human. What is the opportunity cost of a foregone Einstein, Edison, or Jobs? Going further, who are the folks most likely to heed the advice to forego children for the climate, and what would their would-be kids have been like?
It is also worthwhile mentioning that there is no guarantee that population will continue to grow, particularly if the world continues to develop and incomes rise. Even the UN projections place some probability on a population decline in 30 years time. One writer of Wired magazine, Kevin Kelly, when asked what we should be worried about (but presumable are not), fretted about about an underpopulation bomb. Here's what he writes:
“Here is the challenge: This is a world where every year there is a smaller audience than the year before, a smaller market for your goods or services, fewer workers to choose from, and a ballooning elder population that must be cared for. We’ve never seen this in modern times; our progress has always paralleled rising populations, bigger audiences, larger markets and bigger pools of workers. It’s hard to see how a declining yet aging population functions as an engine for increasing the standard of living every year. To do so would require a completely different economic system, one that we are not prepared for at all right now. The challenges of a peak human population are real, but we know what we have to do; the challenges of a dwindling human population tending toward zero in a developed world are scarier because we’ve never been there before. It’s something to worry about.”
A couple final thoughts. The above graphs shows several possible mitigation options, but I haven't heard much discussion of tradeoffs. Surely the end goal in life isn't to focus all our individual energies on activities to reduce carbon emissions. So, given my preferences for driving, eating meat, or being a parent, the figures above suggest useful ways of thinking about this problem. Maybe I don't want to have a more plant-based diet, but at least in terms of greenhouse gas impacts, I can "offset" that and more by foregoing my trip to Paris this year.
More broadly, we don't normally worry about impacts of our various consumption choices on availability of other resources like steel or fertilizer or labor. Why? Because the market price for the goods should reflect the relative scarcity of these items. One upside to a carbon tax is that we could forego all the moralizing and all these sorts of "consumption advice" types of papers and simply allow consumers to make choices they want given the relative prices of different goods. But what about children? The above graph would seem to justify a large carbon tax on having kids. I'd guess that is a highly unpopular idea, which suggests to me that most of us are more likely to be in the Simon camp than the Ehrlich one when thinking about our own offspring.