No, Having One Child Does Not Add 60 Tons Per Year to Your Carbon Footprint

This chart is making the rounds, and has been picked up by  Grist, the Guardian, and countless other outlets. It makes a good point – enviros tend to focus on small impact personal mitigation measures like recycling or changing light bulbs, and ignore the big impact personal choices, like flying and having a car dependent lifestyle.

But it also claims that having one child adds 60 tons annually to the carbon footprint of an individual in the developed world.  This makes it sound like one of the most basic aspects of human existence — having a family – is the most climate irresponsible thing you can possibly do. To put that 60 ton number in context, the average per capita carbon footprint in the United States is  about 20 tons – the highest in the world. The global per capita carbon footprint possible consistent with holding climate change to 2 degrees Celsius is somewhere between two and four tons per year, depending on whose numbers you use.

So how does one child born in the developed world add three times the US per capita carbon emissions every year?  Does each newborn come with a Lincoln Navigator and a United Airlines Premier Frequent Flyer Status?

The answer is that the one child does not have such an outsized footprint. The 60 ton number in this chart misrepresents the underlying study – which accounts for the carbon impacts of all future generations going out over 500 years into the future. The chart making the rounds incorrectly attributes these centuries of impacts to the life of the current parent.

I looked into the paper underlying the chart and the sources for that paper, so let me try and explain the error the authors make.  The chart is part of a well-intentioned paper by Seth Wynnes and Kimberly Nicholas appearing in Environmental Research Letters, titled “The climate mitigation gap: education and government recommendations miss the most effective individual actions.” The Supplemental Materials for this paper cite one source for the calculation of the carbon impact of having one child in developed nations, a 2009 paper by Paul Murtaugh and Michael Schlax entitled “Reproduction and the carbon legacies of individuals.”

The important thing to understand about the Murtaugh and Schlax paper is that it does not just calculate the carbon impacts of that one child’s lifetime — rather it looks at the carbon impacts of that child, plus the impacts of that child’s child, plus the impacts of that child’s grandchildren, great grandchildren, etc,  out for almost a millenium, with a diminishing share attributed to that irresponsible 21st century ancestor.  The chart below, from the Murtaugh paper, illustrates how the calculation includes future centuries of carbon impacts:

Actually, now that I look at it, the calculation does not just extend for centuries into the future, it extends “out to infinity.” So the Murtaugh carbon footprint calculations are not carbon emissions that will occur during any individual’s lifetime.

The Wynnes paper cites the Murtaugh paper as its only source for the carbon impacts of child rearing, and give the annualized carbon impacts of having one child in the developed world as falling into a range between 23,700 kg CO2e (Russia) and 117,700 kg CO2e (US) per year, both in its table one and the following chart:

But none of the number used by Wynne actually appear in the Murtaugh paper!  The Murtaugh paper does include the following chart for all time combined not annualized out to infinity carbon impacts of one child and all of that child’s children’s children’s children’s children’s descendants:

Wynne appears to have taken these until-the-end-of-time-totals, converted tons to kg, and divided by average life expectancy to come up with their “annualized numbers” – I can’t get the exact same numbers, but it’s pretty close:

So Wynnes, et al, come up with their “annual” CO2 impacts of  having “one” child by taking the cumulative attributable impacts of your children until the end of time and acting as if those impacts occur on an annualized basis during the parent’s lifetime. This is highly misleading, at least.

The decision to have children- and how many – is obviously a highly personal choice. But I think having two children per couple – slightly less than zero population growth – is completely defensible as a climate choice. Any argument that the solution to climate change requires the elimination of future generations in order to avoid a backwards attribution of their impacts will not win any converts to the cause.

One last note: the Murtaugh numbers used by Wynnes are based on the assumption that current per capita carbon emissions in each country will continue unabated until the end of time.  There is not enough fossil fuels on the planet for that assumption to be remotely plausible.