ables, some of which might have to be constructed for the purpose. Detailed case studies using qualitative methods are also important, as the case summaries in this chapter illustrate. Qualitative methods can offer a depth of understanding not available from quantitative analyses, which by their nature are limited to those variables already quantified. Moreover, each method acts as a check on conclusions drawn from the other.

Driving forces can cause each other. For example, new technologies can promote economic growth, which in turn allows for further technological development; materialistic ideologies contributed to the rise of capitalism, which promotes materialistic ideas. More complex mutual causal links also exist among several driving forces. Such relationships are difficult to disentangle and further complicate analyses of the human causes of global change. To understand the nature of these interactive relationships, it is important to compare different places and to follow the relationships over time.

The forces that cause environmental change can also be affected by it. Population growth is a good example of feedbacks between human actions and the global environment. Population growth increases the demand for food, which creates pressure to make agriculture both more intensive and extensive. These changes eventually bring diminishing returns, reducing food production per capita and creating downward pressure on population. The diminishing returns can be postponed by improved technology, but technology also interacts with the environment. Humans can increase food production by using tractor power, chemical fertilizers, pesticides, and herbicides, but these technologies rely on fossil energy and therefore eventually reach limits imposed by scarcity, price, or environmental consequences.

Relationships among the driving forces depend on place, time, and level of analysis. It is easy to illustrate the principle. For places: economic growth has been more dependent on fossil-fuel energy in China than in other countries, even other developing countries; the causes of deforestation in Brazil are distinct from its causes in other countries. For times: fossil-fuel energy use increased almost in lockstep with economic activity in industrialized countries for many years; since the 1970s, the correlation has been nearly zero (see Chapter 4). Also, the long-term effects on the global environment of a technology such as refrigeration with CFCs have been much different from the effects over a shorter time span—not only because of increasing use of the technology, but also because of the secondary effects of migrations made

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