The human behavior in question is both complex and poorly understood. To illustrate this point and to provide a backdrop for the analysis to come, we begin with three vignettes that convey a sense of the range of concerns addressed in this book. These three examples reappear later on to illustrate issues in the human dimensions of global change.
Human activities threaten to alter the global climate by releasing so-called greenhouse gases, principally carbon dioxide, methane, CFCs, and nitrous oxide, that have the effect of increasing the proportion of heat from the sun that is retained at the top of the atmosphere. While it is easy to establish a connection between the onset of the Industrial Revolution and increases of carbon dioxide in the earth's atmosphere, the story of greenhouse gases is far more complicated. An examination of emissions of carbon dioxide, a by-product of the combustion of fossil fuels to produce energy and the most important of the anthropogenic greenhouse gases, suggests a clear link between economic development and a global increase in greenhouse gases. Yet there are striking disparities between otherwise similar economies in how much energy they use to produce a unit of economic output. Japan uses less than half the energy the United States does; Bangladesh uses about half what India or Pakistan does; the range in sub-Saharan Africa includes the least energy-intensive economy in the world (Lesotho) and one of the most intensive (Zambia), which differ by a factor of 56 (World Bank, 1989). Other human activities, such as the destruction of tropical moist forests, are major contributors to carbon dioxide emissions worldwide. In the case of methane, population pressure appears to be a key factor, since the expansion of rice paddies to feed the growing human populations of East and South Asia is a major source of methane emissions. The case of CFCs points to the role of technology as a source of the greenhouse effect; CFCs were developed initially to eliminate problems with existing refrigerants, like ammonia, rather than as a response to population pressure or the forces of economic development (although, of course, population growth and economic production add to the demand for CFCs). Nitrous oxide tells yet another tale. The largest single source of this greenhouse gas is the use of nitrogenous fertilizers to increase crop yields, mainly in advanced agricultural systems.