SOCIOLOGY:

Many scientists, from anthropologists to social scientists, feel that a continually expanding human population is unsustainable. Despite an increasing population, global human population growth rates are slowing down.

The slowdown has led some scientists to predict that human civilization will level off by mid-century to around 9 or 10 billion people. This prediction is based in part on the hypothesis that economic development decreases infant mortality and increases lifespan.

Consequently, fertility is reduced to a point no higher than that required to maintain the population. For this growth hypothesis to come about with respect to the future global human population, it's reasonable to presume that sufficient energy must be available to drive the economic growth which indirectly places a cap on maximum population levels.

John DeLong (Yale University, United States) and coworkers have found that on the scale of individual countries, human population growth does indeed slow down when per-capita energy consumption increases to a certain level. However, they also report that the current global energy supply is nowhere near enough to attain this level of energy consumption, paradoxically suggesting that a lack of energy will drive the expansion of human civilization beyond the commonly accepted 9 or 10 billion limit.

This surprising social finding is complimentary to research demonstrating an increase in water utilization in the Rio Grande River basin due to subsidy-induced water-efficient irrigation, and the finding that the most likely locations for new ski resorts in the United States Rocky Mountains, based on what has been developed previously, are ecologically and/or economically unsuitable.

Data acquisition and model development.

The scientists made use of the World Resources Institute database and previously published research to collect data on birth and death rates, population growth rates, and per-capita energy use on the scale of individual countries, as well as world energy usage from 1950 to 2007. Oil-producing nations in Middle East were excluded, because their energy use is far exceeded by their growth rates.

Their models took into consideration the different energy usages of the developed (85%) and developing (15%) world. These two populations clearly use energy to differing degrees; 1.35 billion people are in the developed world, while 5.32 billion (i.e. far more people yet with far less energy usage) are in the developing world.

No slowdown in the expansion of human civilization is predicted.

The scientists found that human populations stop growing when the per-capita yearly energy usage is approximately 13 kilowatts. Incidentally, this is over 150 times that which is needed to sustain human metabolism.

Growth rates decline with increasing per-capita energy utilization, due to birth rates declining faster than death rates. However, predicted future energy supplies will not be able to provide everyone with the 13 kilowatt per-capita yearly energy requirement to sustain the level of economic activity necessary to in turn keep down the size of human civilization, under any of four predicted scenarios (from optimistic to pessimistic predictions of future energy supplies).

Furthermore, the developing world used energy at 15 times the per-capita rate of the developing world in 2007, even more than the factor of 9 imbalance in 1950. Given that this trend is expected to increase to a factor of 22 imbalance by 2050, this reinforces the scientists' finding that developing world will continue to use an insufficient amount of energy to reduce its population growth, and consequently reduce the growth rate of human civilization as a whole.

Implications.

My personal opinion is that people shouldn't walk away from these findings with some sort of xenophobia ("foreigners are killing us all" etc). I also feel that people shouldn't use this research as an excuse to reinforce a common perception that environmental devastation is OK because it promotes economic growth.

This research should instead further motivate scientists to find alternative sources of energy that are environmentally sustainable, e.g. hydrogen, solar and (on an interim basis) biofuels, to provide the energy (and social equality) we need to slow down planet-scale growth rates. Of course, there are also other ways that human civilization can be reduced in size, e.g. plagues and war, but one hopes that a more pleasant solution can be found, as well as the political will to implement it, before a devastatingly unpleasant reality manifests itself.

NOTE: The scientists' research was funded by the National Science Foundation.

DeLong, J. P., Burger, O., & Hamilton, M. J. (2010). Current Demographics Suggest Future Energy Supplies Will Be Inadequate to Slow Human Population Growth PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013206