Vol. 339 no. 6125 pp. 1276-1277
Approaching the Limits
One of the foundational principles of biology is that a population cannot grow forever in a finite ecosystem—a progressive system feedback of starvation, predation, and disease limits uncontrolled growth.
The global human population has now nearly tripled since 1950, and economic activity increased tenfold, leading many to suggest that humanity is heading toward a population and consumption overshoot (resource depletion and correction, as economists would say).
In Harvesting the Biosphere, Vaclav Smil traces the historical development of human consumption of biological resources and evaluates whether we could be approaching important global limits. Smil (an economist at the University of Manitoba) has written several books on global energy and other resource issues; here, he focuses on human consumption of the plant and animal life and whether current trends are sustainable.
One cannot assume that all of global NPP is potentially available for human use. Some regions of the Amazon or Siberia, for example, are too remote for harvest. More important, do we really want to plow and clear the whole world? Most of us want to preserve some natural systems for biodiversity, ecosystem services (such as water and air purification), recreation, or aesthetic beauty. Human settlements and infrastructure, termed impervious surfaces, presently cover only 0.44% of Earth’s continental surface, whereas agriculture and grazing lands cover about 40%. Although global NPP currently appears stable, Smil suggests the great potential for pollution, exhaustion of soil nutrients, and irrigation depletion to substantially reduce the future NPP available for humanity. In addition, bioenergy is emerging as a massive new demand on NPP. Should fossil fuels become scarce, expensive, or unwanted, biofuels could, if allowed by policy and economic strategies, consume all remaining available NPP (2).
The future limits of HANPP become an urgent policy issue when one considers the 40% increase in global population expected over the next three or four decades and the expansion in living standards aspired to by the under-developed world. Smil expects that current policies will lead to a2-3 fold increase in HANPP demand in the next half century, and he rightfully asks if this increase is possible.
Scholars around the world have been asking roughly this same question since 1972, when the landmark Limits to Growth book appeared (3). More recent analyses—such as the global human footprint, planetary boundaries, and Gaia—address the question from various angles. Each has indicated that another half-century of the current trajectory of human development, consumption, and economic aspirations does not appear possible (4–7).
Smil’s final recommendations echo others: global population must be stabilized at or below 9 billion; agriculture has to become sustainable, no longer relying on fossil-fuel–based fertilizers and mining groundwater for irrigation; meat consumption must be moderated; and food storage and processing must be improved and wastage minimized. Crucially, the rich nations have to share global resources more equitably with emerging countries, as simply growing more does not appear possible.
Full of recent references and statistics, Harvesting the Biosphere adds to the growing chorus of warnings about the current trajectory of human activity on a finite planet, of which climate change is only one dimension. One can quibble with some assumptions or tweak Smil’s calculations, but the bottom line will not change, only the time it may take humanity to reach a crisis point.
Systems ecology teaches that the human population and consumption trajectories need a stronger feedback control than currently exists. Either we are smart enough to craft that feedback mechanism ourselves, or the Earth system will ultimately provide it.
Unfortunately, the tragedy of the commons suggests that collective international actions to voluntarily reduce consumption are contrary to human nature.
1. P. M. Vitousek, et al. Human Appropriation of the Products of Photosynthesis. Nearly 40% of potential terrestrial net primary productivity is used directly, co-opted, or foregon because of human activities. Bioscience 36, 368 (1986).
2. W. K. Smith et al, Global Bioenergy capacity as constgrained by observed biospheric productivity rates. Bioscience 62, 911 (2012).
3. D. H. Meadows et al., The Limits to Growth: A Report for the Club of Rome’s Project on the Predicament of Mankind (Universe, New York, 1972).
5. J. Lovelock. The Vanishing Face of Gaia: A Final Warning (Basic, New York, 2010).
6. J. Randers. 2052: A Global Forecast for the Next Forty Years (Chelsea Green, White River Junction, NH, 2012).
7. A. Wijkman, et al. Bankrupting Nature: Denying Our Planetary Boundaries (Routledge, London, ed. 2, 2012)