If you’re concerned about sustainable living, you probably pay close attention to your “carbon footprint.” We all have one: the amount of climate changing carbon we emit to the atmosphere through our energy intensive lifestyles. Some of us even calculate our household’s footprint with one of the many carbon calculators available online.
It helps to have your old bills handy if you use the Household Emissions Calculator created by the U.S. Environmental Protection Agency. You’ll need to enter how many kilowatt-hours of electricity and therms of natural gas or gallons of heating oil you’ve used in the past year. Add miles traveled in the car, whether or not you recycle your trash or turn down the thermostat at night and myriad other details, and the calculator will tell you how many tons of carbon emissions you can call your very own. The per-person average for 311 million Americans, according to the EPA calculator, is about 10 tons per year.
However, missing from these numbers is a factor that, in the long term, can overwhelm our efforts to live more lightly on the planet: the choice to reproduce. “It’s probably the most basic of all biological urges,” Paul Murtaugh told a Corvallis audience in February. “To hint that there might be some benefit to controlling that urge is very controversial.” Last year, Murtaugh personally found out just how strongly people feel about it.
All in the Family
The Oregon State University professor of statistics and OSU colleague Michael Schlax published a paper in the journal Global Environmental Change describing an individual’s “carbon legacy” — the amount of carbon likely to be emitted in the future by one’s descendants. Parents, they reasoned, could be held accountable for one half of their children’s emissions, one quarter of their grandchildren’s and declining portions down through the generations.
Murtaugh and Schlax took a mathematical approach to understanding the consequences. They estimated how large a parent’s carbon legacy might be by creating a model based on per capita carbon emissions and a country’s population trends (fertility and mortality rates and average longevity). They used data from the Intergovernmental Panel on Climate Change and the United Nations to compare the carbon legacies of parents in 11 of the world’s most populous countries.
Since family size and longevity vary widely even within the same population, they ran the model thousands of times country by country. Each time, the model chose a random parent with a specific number of descendants who lived and reproduced until the lineage ran out. From those hypothetical examples emerged an average parent’s carbon legacy for each country.
Murtaugh and Schlax didn’t stop there. On top of these calculations, they added another factor: future changes in annual carbon emissions. While there are wide disparities between rich and poor countries, today’s global average is about four metric tons (as carbon dioxide or CO2) per person. If new carbon-free energy technologies take hold, emission rates could drop. If not, they could stay the same or rise. So, looking forward to the year 2100, they used scenarios that were “optimistic” (drop to 0.5 tons), “constant” (stay at four tons) and “pessimistic” (50 percent increase to six tons).
The results were clear. “If you accept our method of accounting, a decision to have a child amplifies a single parent’s lifetime emissions by three or four times in most countries, by virtue of the carbon legacy that perpetuates through generations,” says Murtaugh. “Remember these emissions accumulate over hundreds of years and many generations.”
The effect was most dramatic in the U.S. On average, the additional emissions per child — about 9,400 tons under the “constant” scenario — was almost six times the amount of carbon emitted by a parent over his or her own lifetime. Not only that, it was 20 times more than the amount that could be saved over an 80-year lifetime by the energy conservation measures included in the EPA’s emissions calculator.
Nevertheless, Murtaugh calls those short-term reductions essential. “It’s not that we can solve the global warming problem by reducing the number of children that we have,” he adds. “It will help immeasurably in the long term, but in the short term, it’s essential that we reduce our per capita emissions right away.” In fact, under the “optimistic” emissions scenario, the drop in per capita emissions reduces future impact per child by almost 90 percent.
Slowing population growth can also help, but “the savings from these reductions in fertility aren’t going to be that meaningful unless we get our per capita emissions under control,” Murtaugh adds.
After the paper was published and featured in an OSU news release, news of their analysis splashed across international headlines and caught the attention of environmentalists, newspaper columnists and conservative bloggers. Some critics responded to a misperception that Murtaugh and Schlax had called for government policies to curb an individual’s right to have children. “We said nothing in our paper about policies,” says Murtaugh. “We just did the calculations and laid them out there for people to think about. But most of the people had obviously never seen the paper. We were called Nazis and Eugenicists.”
One individual phoned Murtaugh and suggested that he consider killing himself to reduce his own carbon emissions. The caller then proceeded to reach every member of the OSU statistics department demanding that Murtaugh be silenced. “I began to fear for my safety. Fortunately the blogs, calls and emails stopped after a few weeks,” Murtaugh adds.
More than climate and reproductive rights are at stake. Rapid population growth affects other species (think ivory-billed woodpecker, passenger pigeon, blue whale and Fender’s blue butterfly) and exhausts the planet’s carrying capacity, Murtaugh says. At current levels of production, it has been estimated that it would take 1.4 Earths to maintain today’s population into the future. “In other words,” he concludes, “we’re living off the capital now.”
The United Nations Population Division expects the global population to reach 7 billion in October.