Overshoot and dieoff : humans beyond carrying capacity

Hooke, R; Martin-Duque, J.F. 2012. Land transformation by humans: A Review.  Geological Society of America.

Prognosis for the Future

Looking ahead a few decades, land suitable for agriculture will likely continue to diminish as

  • urban areas expand
  • soil is degraded
  • fertile soil is washed down rivers and blown away 10 times faster than it is replaced (Montgomery, 2007)
  • water tables decline in areas dependent on groundwater for irrigation (Gleick, 1993)
  • Foreseeing a shortage of arable land, global investors are, in fact, buying huge tracts in Africa and South America (De Castro, 2011).

Despite foreseeable future technological developments, agricultural productivity is likely to decrease as

  1. the supply of phosphate for fertilizer decreases (Rosmarin, 2004)
  2. petroleum (used to run farm machinery and as feedstock for fertilizer) becomes more expensive and less available
  3. pollution adversely affects pollinators, plant growth, and predators that control agricultural pests (Peng et al., 2004; supplemental data, Sec. H [footnote 1])
  4. climate changes.

Will Earth be able to support the projected 2050 population of 8.9 billion?

Wackernagel et al. (2002) estimate that, as of 1978, the land area needed to grow crops, graze animals, provide timber, accommodate infrastructure, and absorb waste sustainably, has already exceeded Earth’s available area, and that as of 2002, we needed 20% more land than is available. If this is the case, we are in a period of overshoot.


Overshoot occurs when populations exceed the local carrying capacity. An environment’s carrying capacity for a given species is the number of individuals “living in a given manner, which the environment can support indefinitely” (Catton, 1980, p. 4). Only a population less than or equal to the carrying capacity is sustainable.

A sustainable population is one that (i) consumes renewable resources at a rate less than the rate at which they are renewed; (ii) consumes non-renewable resources at a rate less than the rate at which substitutes can be found; and (iii) emits pollution at a rate less than the capacity of the environment to absorb the pollutants (Daly, 1991, p. 256).

It is axiomatic that, on a finite planet, there is a limit to growth. The question is, “Are we now bumping up against that limit?”

Several observations suggest that, with our present lifestyles, we are, indeed, now living in a state of overshoot.

We struggle to supply the food needed by the present population. Groundwater tables are declining. Our way of life is based on non-renewables like fossil fuels, phosphates, and ores, accumulated over millions of years, with no clear plan for adequate substitutes once natural sources are exhausted. We discard many chemicals (e.g., CO2, N, plastics) faster than they can be absorbed by the environment.

When the number of individuals exceeds the carrying capacity, overuse of the environment sets up forces that, after a delay, first reduce the standard of living and then eventually the population (Catton, 1980, p. 4–5). Initiation of the correction may be manifested by stagnant or negative economic growth rates, by famine and/or water shortages, by increases in disease resulting from undernourishment (Pimentel et al., 2007), and by increases in conflict. Sound familiar? Fifty-four nations with 12% of the world’s population experienced economic declines in per capita GDP from 1990 to 2001 (Meadows et al., 2004, p. xiv; World Bank, 2003, p. 64–65). Famine, disease, and conflict are frequently in the news.


The rate of change in area of cropland and pasture has decreased in the last few decades. Projected into the future, these trends suggest a peak and then a decline in the areas of both. Let’s focus on cropland, because that is the land use for which data are most robust and the one of most concern, given our swelling population. At least 3 trends are contributing to the decline in the rate of increase in cropland:

  1. Urban area is increasing, commonly at the expense of agricultural land. Between 2000 and 2030, worldwide, the loss of agricultural land to urbanization may be as much as ~15,000 km2 annually (Döös, 2002).
  2. There is a dearth of additional land suitable for agriculture. Of Earth’s land area, 70% to 80% is unsuitable for agriculture owing to poor soils, steep topography, or adverse climate (Fischer et al., 2000, p. 49; Ramankutty et al., 2002). About half of the rest is already in crops (Table 1), and a large fraction of the other half is presently under tropical forests that beneficially take up CO2. Tropical-forest soil loses fertility rapidly, once cleared.
  3. Some existing agricultural land has deteriorated so much that it is no longer worth cultivating. As of ca. 1990, soils on nearly 20 Mkm2 of land, or ~40% of the global agricultural land area, had been degraded (Oldeman et al., 1991, p. 28). Of this, over half was so degraded that local farmers lacked the means to restore it.

Partially offsetting these trends may be increases in efficiency of farming and food distribution. Rudel et al. (2009), however, could not find correlations that supported this hypothesis.


If we are in a state of overshoot, here are some ways to bring the human impact on Earth back to sustainability:

1. Reduce demand. Demand can be reduced by improving building insulation or mandating energy-efficient vehicles and appliances. Recycling reduces demand for primary materials. Tempering our impulse to buy things that we don’t really need or of which we will soon tire also reduces demand.

2. Develop technological solutions. Existing technology can mitigate our impact. Adoption of efficient building and farming practices limits degradation, and ecological restoration can partially reverse it (Rey Benayas et al., 2009). Technological breakthroughs are also possible. Simon (1996) argued that a larger population increases the likelihood of spawning the brain power needed to achieve such breakthroughs. But without well-fed bodies, brains don’t function well.

Our technological skills have enabled us to support an ever increasing population. They have also exacerbated some problems. Use of oil as an energy source in agriculture has increased efficiency, but at the expense of leaving us presently dependent on a non-renewable resource. Mechanical well drilling and pumping facilitate irrigation, but now ground-water tables are dropping unsustainably (Gleick, 1993). Given present usage, more than half of the U.S. High Plains aquifer will likely last for 50 to 200 years, but significant parts will be exhausted in <~25 years while others are already effectively spent (Buchanan et al., 2009). Use of bioengineered wheat in Punjab, India, and rice in Bali, Indonesia, increased crop yields, but also led to a variety of economic, pest, and health problems (Tiwana et al., 2007, p. xxii–xxiii; Lansing, 1991, p. 110–117).

3. Reduce the population. Increasing the availability of health care, education, and microfinancing, particularly for women in developing countries, reduces fertility. Reduced fertility reduces poverty, because available resources are distributed among fewer people. Couples worldwide can be urged to have only two children and to delay having them so there will be fewer people on Earth at any one time. These steps would first slow population growth and then lead to a long-term decline.

Reducing demand is a critical component of the solution, but in itself is not sufficient, given the magnitude of the problem. Technological progress, particularly in the energy field, is essential, but we also think it unwise to bet too heavily on unspecified future breakthroughs. Reducing and eventually reversing population growth needs to be a large part of the solution. Eventually, difficult decisions will have to be made about the size of an optimum population and how to achieve it.


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