Continued world population growth is inevitable. The means of controlling it require extensive efforts to influence birth rates, with all the consequent problems of addressing the underlying reasons for large family size. One reason for increasing that effort is to break the vicious circle of population growth and poverty. The influence of population growth on resource availability and environmental quality contributes to the poverty process. At the global level, more people mean a higher consumption of energy, and hence more atmospheric pollution, unless means are found by which to lower per capita consumption of materials and energy without impairing standards of living. More people mean a greater demand for cultivable and residential land, and hence less forest and wetlands, contributing to further pollution and losses of biodiversity.

One way of assessing the ultimate limits of population growth is to look at the carrying capacity of natural resources and land. Simply put, the carrying capacity of a given area is the maximum number of people that can be sustained by the resources on that land. The carrying capacity of a region is categorically not the desirable level of population, unless the level of well-being at which the population is sustained is itself desirable. Usually, however, carrying capacity is defined as relating to the maximum sustainable population at the minimum standard of living necessary for survival. For example, it is possible to compute the maximum food output (measured in calories, say) of a given area of land. Suppose this is represented by Q. Then Q can be divided by the minimum number of calories required for human survival of one individual. Call this M. Carrying capacity is then measured as:

Carrying Capacity = Q/M

The most extensive analysis of the carrying capacity of the world was carried out by the Food and Agriculture Organization (FAO) of the United Nations. The fao approach involved looking at the potential food production of each of 117 countries. Obviously, potential food production depends on the level of technology applied to agriculture. fao categorized these as:

On the basis of these different technological scenarios it was then possible to estimate the potential calorie output for each level of technology (Q in the equation). By dividing this by the per capita calorie intakes recommended by fao and the World Health Organization for each country (M in the equation), a sustainable population can be estimated. These estimates were made for 1975 and the year 2000 Table 1 shows the results in a convenient form.

It shows the ratio of potential sustainable population in 2000 to the expected population in 2000 for various regions of the world, and at the three different levels of technology. For example, for the developing world as a whole, if all cultivable land was devoted to food crops, at the lowest level of technology those lands could support 1.6 times the number of people expected in the year 2000. In South-West Asia the actual expected population will exceed the carrying capacity at both low and intermediate technology levels. As the technological assumptions improve, so, dramatically, does the carrying capacity of the regions.

Box 1 appears to suggest a fairly optimistic picture. Certainly, it highlights the role which technological improvement can play in vastly increasing carrying capacity. However, it is important to understand why the picture is far from an optimistic one. There are several problems:

In fact the situation may be worse even than this. The FAO study was concerned with carrying capacity in terms of food, but other resource scarcities begin to exert an influence before cultivable land. A notable example is the availability of fuelwood.

A study of the Sahelian and Sudanian zones of West Africa computed the carrying capacity of various zones according to the limits set by crops, livestock and fuelwood. The results indicate that the carrying capacity of natural forest cover - the main source of fuelwood - is very much lower than that of crops using traditional technologies. Moreover, in five of the six regions fuelwood carrying capacity is already exceeded, compared to two regions where food and livestock carrying capacity is exceeded. The general picture on world zone carrying capacities may therefore understate the problem of resource carrying capacity generally. What matters is which resource scarcity "bites" first.

Carrying capacity calculations are helpful up to a point. They can be used to indicate the broad- scale seriousness of a problem, but there are considerable dangers in deriving too many conclusions from them. The main drawbacks are as follows:

Despite these drawbacks, a casual glance at the level of population - resource imbalance and the rate of agricultural growth suggests that the greater the pressure on natural resources, the slower agricultural growth becomes. This relationship for four groups of countries in Sub- Saharan Africa is shown in Table 2

Group 1 relates to countries where actual population exceeded the sustainable population in 1982; Group 2 relates to countries where this will occur in 2000; group 3 to countries where it will happen in 2030; and group 4 to the remaining countries - i.e. those whose carrying capacities will not be exceeded by 2030. In all cases the measure of carrying capacity is that of the fao. The data suggest that the closer a country is to its carrying capacity, the slower is its agricultural growth rate. In turn this suggests that the relationship between population growth and food output might reduce to the balance between two forces working in opposite directions: the role of population pressure in inducing technological inducement to higher productivity, and its role in wider resource degradation that reduces agricultural growth.