There's clear evidence that even now, the world's population is living beyond its collective means. The World Wildlife Fund's "Living Planet Report 2006," which publishes biocapacity (i.e., the potential productivity of cropland, forests, fisheries, and fresh water) and per-capita ecological footprint data generated by the Global Footprinting Network provides the most comprehensive existing data.
By their estimate, our total consumption of 13.685 billion global hectares (gha) represents a 22-percent overshoot of Earth's total biocapacity, which is estimated as 11.22 billion gha. This overshoot is possible only because we can (temporarily) substitute or supplement our biological resources by exploiting our finite supply of fossil fuels.
Fossil fuels (coal, oil, and natural gas) provide concentrated energy derived from the sun over millions of years. Renewable energy is essentially harvested in real time--directly through photovoltaic technology or indirectly via wind and wave action. Because renewables are intrinsically far less energy dense than fossil fuels, they cannot replace even the current world power requirement, let alone the requirement that will result from a more overpopulated planet. Furthermore, the more promising sources of renewable energy (wind and solar) can only be harnessed intermittently. It'll be impossible for renewables to satisfy the energy needs of high- or even medium-income countries after fossil fuel and uranium resources have been depleted. Even if self-sustaining nuclear power becomes viable, a world without artificial fertilizers, air transport, and other essential petroleum products will be incapable of sustaining even today's population. It follows that a sustainable future is only possible through a combination of reducing consumer numbers and per-capita consumption. Indeed, the situation is now so serious that it makes little sense to talk about slowing population growth unless in the context of taking the first step toward reversing it.
As global population increases, the maximum sustainable (by which we mean indefinitely supportable) ecological footprint each person can have is proportionally reduced. For a sustainable footprint of 6.4 gha per person, which is the mean ecological footprint for residents of high-income countries, maximum world population is 1.8 billion. (To put that figure in context, U.S. citizens consume 9.6 gha each on average; Europeans consume 4.8 gha.) Around 6 billion people (90 percent of today's population) could be supported at 1.9 gha per person, the mean ecological footprint for middle-income countries, and around 14 billion people could scrape by at a marginal existence of 0.8 gha per person, which is the average biocapacity consumed by inhabitants of low-income countries. The latter implies utterly unacceptable poverty and is less than each African consumed in 2003 (1.1 gha per person).
These figures signify maximum values; any drop in biocapacity through climate change (such as desertification or flooding) will reduce them.
The proportionality [PDF] of these numbers demands that higher footprint lifestyles at one end of the spectrum force lower footprint lifestyles at the other. If we want everyone on Earth to live at the moderate mean consumption level represented by a footprint of 4 gha per person (comparable to Portugal, South Korea, and Kazakhstan), then we must plan--through applying our contraceptive technology wisely, democratically, and compassionately--for a maximum world population of 2.8 billion. If we fail, then we must prepare for nature to do it through viruses, violence, starvation, and climate-change-intensified disasters such as hurricanes, droughts, and rising sea levels. It's sobering to think that it would take more than five planets to support everyone on Earth at the average footprint level of U.S. residents. If we're ever to achieve worldwide equity, we must look at reducing not only consumption, but also population, which is often a taboo subject. The years ahead will reveal just how intelligent we are as a species.
In her comments here, Betsy Hartmann perpetuates some infamous myths about people who have a quantitative concern about human population. The first is that such concern diverts attention from the need that she rightly highlights, to take action on climate justice and related humanitarian issues. But why the dichotomy? Can't we be concerned about human needs as well as human numbers?
The second myth is that being concerned about population leads intrinsically to coercion. Why so? At the Optimum Population Trust, despite our well-justified anxiety about human numbers on a finite planet, we reject coercion--for many good reasons that can be found in our paper, "Youthquake" [PDF]. Countries as varied as Thailand, Tunisia, and Iran have cut their average family size in half simply by removing a range of barriers to meeting women's family planning needs--and they did so as quickly as China, where coercion played a role.
As Fred Meyerson rightly says, one-third of global pregnancies are unintended. Or as we put it, "Too many accidents are caused by humans, and too many humans are caused by accidents." These accidents are avoidable through making the choice of voluntary family planning available and accessible to women in every bedroom in every society, so that every child might be a wanted child.