BIO 599/799
Environmental Science

The Human Population

Human Population History:

Human Population facts:

U.S. at night

As a result of this rapid growth:

When a country darkens on this map, it represents a death caused by hunger.

Factors Contributing to the Population Explosion:

Population growth:

Population Age Structure:

Birth and death rates do not tell the whole story of population growth. A country's TFR can be below replacement level while the population continues to grow, if there are a disproportionately large number of couples in these younger age groups. That is the situation in Kenya and, to a lesser degree, the United States. The age structure of the population, then, is also an important factor in population change (see the population pyramids for Kenya, Sweden, & the U.S. above). When a substantial proportion of a country's population is young, high population growth rates in a country are to be expected, even if the average TFR is modest. The reason is that so many females are of  childbearing age, that even a modest average TFR results in a large number of births.

Fertility Rates:

This map shows the average number of children born to a woman during her lifetime. The darker the color, the greater the
number of children. For example, France at 1.7 children has a lower fertility rate than Egypt at 3.6 children. The total fertility
rate (TFR) is the average number of children a woman would have, assuming that current age-specific birth
rates will remain constant throughout her childbearing years (usually considered to be ages 15-49).

Earth's Carrying Capacity (is the Earth overpopulated?):

     Percentage of Tropical Forest Cleared by Region Between 1960 and 1990

Impacts of continued growth in human populations include:

While grain yields per acre have been increasing steadily until 1990, the rate of  increase has fallen off in the last 10 years. Most of the benefits of irrigation, machinery, fertilizer and plant breeding have already been realized. The production of grain per acre is close to the maximum obtainable through photosynthesis. Geneticists are working on improving the efficiency of photosynthesis, and that may turn out to be possible, but given that nature has been working on the problem for several hundred million years, there is no guarantee that we can improve on the process in the next 10 to 20 years.

See for more information.

Arable land per person

Where are we headed?:

Maximum Global Population Guesses (McCluney 1999):

Everyone at the current U.S. standard of living and with all the health, nutrition, personal dignity and freedom that most Americans currently enjoy.
2 billion
Everyone at the same affluence as the U.S. (current), but with many and onerous restrictions on freedoms relative to behaviors leading to environmental degradation. To accommodate populations greater than 2 billion, restrictions such as the following would have to be instituted:
  • Massive recycling. 
  • Driving restrictions (gasoline rationing, fuel rationing even to mass transit systems).
  • Restrictions on the transport of food (food transported no more than 100 miles for example to its point of retail sales).
  • Prohibitions against cutting trees on one's property. 
  • Limitations on the burning of fossil fuels to save these complex molecules for more valuable or durable uses, such as in the manufacture of plastics and pharmaceuticals.

  • Limitations on the areas of open spaces that can be converted to renewable energy power plants, such as solar thermal, solar photovoltaic, and wind energy systems. This latter results from the need to preserve natural areas for food growing.
4 billion
Only people in the U.S. and Europe at current level of affluence. Everyone else at the current prosperity level of Mexico. 
6 billion
Everyone in the world at Mexico's current prosperity level. 
20 billion
Everyone in the world at the current "prosperity"  level of northwest Africa. 
40 billion

Text of the message of Secretary-General Kofi Annan to mark World Environment Day, 5 June 2000:

We may be at the dawn of a new millennium, but the environmental problems we face are painfully familiar. They may even be
getting worse. Despite the Earth Summit, and despite success stories like the Montreal Protocol to protect the Ozone Layer,
human beings continue to plunder the global environment. Unsustainable practices remain deeply embedded in the fabric of our
daily lives. We are failing to protect resources and ecosystems. We are failing to invest enough in alternative technologies,
especially for energy. We are failing even to keep the debate alive.

These are deeply troubling trends. I recommend four priorities for reversing them.

Technological breakthroughs that are unimaginable today may well solve some of the environmental challenges we face. But it
would be foolish to count on them and to continue with business as usual. The theme for this year's World Environment Day
says it best: the year 2000 begins the environment millennium; the time to act is now. There will be no easy solutions.
Unpleasant ecological surprises lie ahead. But the start of the new century could not be a more opportune time to commit
ourselves -- peoples as well as governments -- to a new ethic of conservation and stewardship.

Useful links:


Why six billion?

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