How can we predict world population growth?
Posted by Dave on March 2, 2010 | 3 Comments
Predicting the future is always difficult. Who could have known in the year 1775 that 100 years from then, ships and trains powered by coal would allow people to circle the earth in weeks rather than years? Who could have predicted that in another 100 years, the human voice—and moving images—would be able travel that distance in less than a second? Now we have much more sophisticated tools for prediction, but prognostication remains elusive.
But never has predicting the future had stakes as high as it does now. Precisely because of the technological advances that have transformed the world in the past two centuries, we need prediction to ensure our technology doesn’t kill us. We already had one near miss, with the near destruction of the ozone layer due to chlorofluorocarbons and other pollutants. In his book The Weather Makers, Tim Flannery estimates that 50 percent of the UV protection provided by the ozone layer for people living in middle latitudes would have been lost by 2050 if we hadn’t taken steps to stop it. Instead, the ozone layer will likely be fully restored by that date.
But population involves so many other variables that predicting it more than a decade or so into the future is quite difficult, and models for future population changes vary widely. Today I’m going to consider just one model for population growth and its impact on the economy and carbon emissions.
A team led by Salvador Puliafito took a look at global population data from 1850 to the present and then tried to develop a model that would predict population through 2150. They based their model on a set of equations originally developed for tracking predator-prey relationships in ecological systems. You may have seen it in your high school biology textbook; it’s called the Lotka-Volterra model and it looks like this (source: Wikipedia):
The idea is simple. Imagine an island like Isle Royale in Lake Superior, where wolves feed primarily on moose. If there are lots of moose in a given year, there’s plenty of food for the wolves, and the wolf population will grow. But eventually the wolves will kill off the moose, and the moose population declines. Naturally, with less food, the wolves die off too. With fewer predators, the moose population recovers, and the cycle starts over again.
Puliafito’s team argues that human population and gross domestic product (GDP) work the same way: Eventually a growing population depletes resources so much that GDP declines. This results in a declining population, and so on. But because the human lifespan is so long, and because technological advances affect both lifespan and GDP the cycle doesn’t look the same as it does for animals. Instead, they claim, it looks something like this:
The researchers only extend their model to 2150, but as you can see, it’s got a very optimistic estimate for total population, topping off at just under 10 billion. So even though our population has more than tripled in the last 140 years, this model says shouldn’t increase more than about 40 percent in the next 140, with things leveling off in just 65 years.
Needless to say, the Puliafito model isn’t the only one out there. Many other models exist, as demonstrated by this graph from their paper:
Two of the models predict population as high as 15 billion by the year 2100—and even those models reflect a slower population growth rate than we’ve experienced in the last 100 years. Puliafito’s team’s model reflects a lower GDP increase and higher energy output than many of the other models out there, with the result being a middle-of-the-road carbon emissions projection—but still one that’s three times as great as our current output by the year 2110.
The researchers emphasize that their model can be tweaked with slightly different assumptions to match nearly every other model out there. In other words, we still don’t have enough information to accurately predict population growth. Regardless of which population scenario you choose, there are different impacts on the world. Even today, while some parts of the world have growing populations, other areas are in decline. What happens if population declines worldwide—or if the climate changes and previously populous areas are rendered uninhabitable? I’ll explore some of these issues in future posts.
Puliafito, S., Puliafito, J., & Grand, M. (2008). Modeling population dynamics and economic growth as competing species: An application to CO2 global emissions Ecological Economics, 65 (3), 602-615 DOI: 10.1016/j.ecolecon.2007.08.010
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March 2nd, 2010 @ 5:18 pm
How can we predict world population growth? http://goo.gl/fb/kpYA
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March 2nd, 2010 @ 7:05 pm
How can we predict world population growth? http://goo.gl/fb/FsYi
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March 2nd, 2010 @ 8:27 pm
Reading: How can we predict world population growth? http://is.gd/9zeLE
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