Last week’s announcement that the world’s seven billionth person was born has prompted a barrage of news coverage, most of it lacking in any cogent analysis. In today’s blog you will get the Common Science view on this landmark starting with a story from a Petri dish.

If you took Biology in high school or college you probably have seen a graph of the population of a colony of bacteria like the one shown at the top of the page. If you put bacteria in a closed system like a Petri dish, meaning that no new resources can be added and no wastes can be removed, the population of the colony follows a predictable four-stage pattern. The four stages are:
1.      Lag Phase: The population is very small compared to the available space and resources. Population growth is slow since there are a limited number of individuals available to reproduce.
2.      Exponential Growth Phase: The population grows very quickly since there are many individuals reproducing, resources are plentiful, and plenty of space exists to absorb waste products. The end of the exponential growth phase can be detected when, although the population is still growing, the rate of population growth begins to stop increasing with time.
3.      Stationary Phase:  The population is stable as limitations in resources and the challenges stemming from the accumulation of waste products results in the rates of reproduction and death becoming equal for a time.
4.      Die Off: A tipping point is reached as resources are nearly exhausted and the remaining bacteria are literally swimming in their own waste. The population collapses.
The question that I am addressing in this blog is “how similar is the behavior human population on earth compared to that for bacteria in a Petri dish?” It’s tempting to think that with our rational minds and our opposable thumbs that we would be better stewards of our living space than a single-celled organism. As yet, the data do not support that assertion.

The lag phase for humans lasted from the dawn of time until about the year 1800 when global population was approximately one billion. Then we entered our exponential growth phase hitting populations of 2, 3, 4, 5, 6, and 7 billion in 1927, 1960, 1974, 1987, 1999, and 2011, respectively. So far our population graph looks exactly like the one from our little bacteria friends. Currently there are approximately 131 million births and 55 million deaths per year, for net increase of approximately 76 million people per year, approximately the population of Turkey.
Although it is hard to see in the graph above, the rate of growth of world population has started to slow down, from around 1.3% growth per year in 2000 to 1.1% now. For a bacteria colony this type of behavior would herald the impending onset of the stationary phase, implying that the maximum human population on earth will be no more than seven or eight billion. There are two important distinctions between bacteria in a Petri dish and humans on Earth that could result in different demographic behavior; humans can think and the earth is not a closed system.
Humans are making some conscious and serious attempts to limit the rate of population growth. China, the most populous nation on earth, has implemented its “One Child Policy” program to slow population growth for several decades. In other high-population-growth parts of the world, many governments and non-governmental organizations have worked to improve economic conditions and enhance the empowerment of women, two factors which strongly correlate with a reduction in number of births per woman. Directionally speaking, these efforts should slow the rate of population growth, but I don’t have the data to determine if they are sufficient to explain the 0.2% reduction in per annum growth rate from 1.3 to 1.1%. 
There are also strong arguments that rate of population growth is beginning to slow for the same reasons as bacteria in a Petri dish, scare resources and lack of space. Below is a very short list of the many resources which, through their growing scarcity, could be limiting population growth:
  • World petroleum production seems to have topped out at approximately 87 million barrels per day. As I explained in “Everything Comes from Oil, Everything” oil is essential for nearly everything in our modern world including, in particular, agriculture, food distribution, and pharmaceutical production.
  • Production of phosphorus, an essential element for all forms of life and a limiting factor in   agricultural production, has been flat over the last several decades. 
  • Large portions of the world’s populations lack access to sufficient fresh water. This results in poor sanitation and health and is clearly limiting population growth in many areas of the world.
  • Since humans tend to congregate in high concentrations along the edge of bodies of water, I also perceive lack of space as an issue as well. As we cluster together we facilitate the spread of deadly infectious diseases and tend to kill one another through violence, car accidents, and other unfortunate interactions.
The other reason that human population could behave differently than a bacteria colony is the Sun. The Sun, by pouring in energy, keeps the Earth from being closed system. As I explained in “Petroleum: 300 Million Years of Sunlight”, we are using solar energy, as stored in fossil fuels, at rate much, much faster than was is being delivered by the Sun. This drawn down of stored energy can only happen once.
If you look on the web for projections of human population you will find that nearly all models predict that the population will reach a maximum at some point in this century. I suggest that you stop and consider that for a moment. Human population has been slowing growing for the last 5 million years and our children are expected to see it reach its peak, amazing really. Most of the estimates predict peak populations ranging from 7 to 12 billion. Some models assume that populations will reach a maximum and remain at steady state, like a never-ending stationary phase. Most models assume that after the peak there will be a decline, either slow or precipitous, to reach an eventual steady state at a lower population.
My opinion is that seven billion human beings is well beyond the long-term carrying capacity of the earth and is being temporarily sustained by the energy stored in fossil fuels. When the oil and coal is gone, human population will need to decline to what can be sustained by the energy delivered by the sun. Most estimates I’ve seen for this range from 2 to 5 billion people. You could fill a library with the books which opine on possible path to this lower population, ranging from the utopian to the apocalyptic.   One thing is for sure, the future is going to be vastly different from the past, just ask the bacteria.
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