We can begin to answer our question with a little abstract thought. It is a reasonable hypothesis or conjecture that humans are in some way primarily responsible for the environmental change we see around us. The more people there are, the more change, particularly in landcover, we might expect. So population size is probably most important in environmental change.

A second hypothesis is that it is not just the number of people that determines the total impact on the environment. The size of the share of the resources that each person demands also has an influence. As an example, the average Australian uses approximately 100 times more energy in one year than one person from a developing country. Therefore it is reasonable to conclude that for equal population sizes, the environmental impact of Australians will probably dramatically exceed that of the people of any developing country. So affluence as well as population size is important in determining the level of environmental impact.

The last idea is that the means by which we extract the resources to support the level of affluence of the population also has an influence on environment impact. Some technologies are cleaner than others and cause less environmental change.

Again using an energy example, it is possible to burn coal to generate electricity and to remove the sulphur compounds from the gases before they are emitted to the atmosphere. If this is not done, widespread pollution from acid rain results. Thus for populations of the same size and affluence, the types of technology used can induce very different levels of environmental impact.

So we have three factors that are likely to determine the level of environmental change, particularly of landcover: population size; the level of affluence and technological capability. These factors interact in a multiplicative manner. If population increases but affluence falls, the level of general environmental impact may not change. More commonly, if population and affluence both increase, then the level of general environmental impact will increase exponentially. These three factors and their interaction comprise a powerful and fundamental idea that we will return to several times in this book.