What is the Earth's Human Carrying-Capacity?

This page is still under construction, right now it just contains the basic concepts.

Issues surrounding human population are complex, not just because they involve emotional and religious issues, but also because humans control their environment and make demands far beyond just staying alive.  But whenever you are faced with evaluating a complex issue, it helps to to look at the limits of the problem - the extremes.  For humans, we can consider two extremes - the minimum, namely none, in which humans disappear from the Earth, and the maximum, some number of humans just kept alive by the "ultimate" carrying capacity of the Earth.  Once we know the limits of the issue, we know wherein the actual answer will lie.  Such an approach helps to put the addition of complexity in perspective, whatever the issue that is being considered.

Earth's Basic Life-Support System

In order to estimate the carrying capacity of any ecosystem, it is necessary to understand the fundamental factors that control a given population.  If we ignore for the moment the issue of population growth, i.e. we're not worried about how long it takes a population to grow, only how big it can get, then we'll find that the primary control of population is the supply of energy for organisms to live.  That energy comes from food and the ultimate source of the energy contained in almost all food is the Sun.

With the Sun as our major source of energy, we need to consider how that sunlight is transformed into food, and finally how much of that food we need to support the average human.

How much Sunlight?

Our Sun is a star that derives its energy from thermo-nuclear (fusion) reactions occurring within it.  Although the Sun's output has varied over Geologic time (since the Earth and the solar system began more than 4.5 billion years ago), it has not varied very much, and, over the short period of human existence on Earth, we can consider it a constant, the solar constant.  The value of the solar constant is about 2 cal/cm²/min (2.88 x 10⁷ cal/m²/day).   This is how much sunlight reaches the Earth as a whole.  Of course, there are many factors that control how much of this energy is available at the surface of the Earth to support the growth of plants - for example, half of the Earth is in shadow (day and night); some sunlight is reflected back into space; the Earth is a sphere and sunlight is spread out over a wider area toward the poles, etc.  These are physical factors, however, and we can calculate them accurately.

Agricultural Productivity

The process of photosynthesis converts incident sunlight into plant material's that will eventually, in part, be our source of food.

Several factors come into play here when we consider how the Sun's energy can be converted into food energy:

1. The total area of the Earth that can intercept this energy is finite - about 510 x 10⁶ km².

2. Not all the Earth is land, and most agriculture occurs on land.  Land occupies about 29% of the Earth (145 x 10⁶ km²).  Even if we used the ocean's at their maximum capacity to support aquaculture, we could only double the agricultural capacity of the land.

3. Not all the land is arable (able to grow crops) - we currently use about 11% (15.9 x 106 km², or about 1.59 x 10⁹ hectares) for agriculture.  If we destroyed all forests and converted all rangeland (where cattle roam and feed) to growing crops, we could increase the total amount of arable land to about 88 x 10⁶ km² (about 9 x 10⁹ hectares).

4. Crops vary in their yield.  To estimate the maximum possible yield we would have to consider the best yielding crops - like cereals, grains, and legumes.  Currently maximum farm yields of grains are about 160 gC/m²/year on land and about 50 gC/m²/year in the ocean.

5. No crop fixes all of the Sun's energy - no matter what we plant, some most incident sunlight will be lost as heat or reflected back into the atmosphere.

Human Food Needs

Human's need about 2000 cal/day just to stay alive, and about 2500 cal/day to function properly.  Of course, this does not include vitamins and essential minerals in our diet, just an energy source, but, remember, we're looking now just for an extreme value, not a realistic one.

The Bottom Line

If we do the arithmetic that is implied in all the above discussion, we will find that the ultimate carrying-capacity of the Earth for humans is about 24 billion people. The world would consist of one producing plant species and one consumer - humans.  There would be no other plants on land or in the ocean, and no other consuming animals

Energy Source - Sunlight

Right now, the daisies to the left don't do anything, but eventually they will take the reader to other pages and web sites that have more information about each aspect of the topic.

Transforming sunlight into food

What humans need to live