• are they and at what cost? How do we value and decide among the range of options?
• What are the underlying social processes or driving forces behind the human relationship to the global environment, such as human attitudes and behavior, population dynamics, and economic transformation? How do they function to alter the global environment?

Research on the human dimensions of global change has value both as basic science and for informing environmental decisions. It increases basic understanding of how past human activities have created present environmental conditions, how past environmental changes and variations have affected human well-being, and how people have responded to these variations and changes. By developing understanding of human-environment dynamics, human dimensions research improves the knowledge base for anticipating future environmental changes and for informing policies aimed at reshaping the environmental future. Studies of the human consequences of and responses to global change help inform judgments about what kinds of responses would be most desirable (e.g., mitigation, adaptation options) and about how to organize those responses to achieve the desired effects. Below we describe the major science issues, review progress that has been made in understanding them, and identify some lessons that have been learned from previous research.

What has been learned in recent years about human causes of global environmental change? One major focus of research has been the explanation of changes in the composition of the Earth's atmosphere. Looking at the atmosphere through human history, one finds that the concentrations of several gases (carbon dioxide, methane, nitrous oxide) changed only a little for more than a thousand years and then started to increase rapidly around 1800. The obvious hypothesis to explain these data is that prior to industrialization in the nineteenth century the related basic cycles of the Earth's environment were in approximate equilibrium and aggregate human activity was too small to be detectable in globally averaged data; then, increasingly since the Industrial Revolution, aggregate human activity has changed the composition of the atmosphere, in particular adding measurably to the concentrations of certain gases. Similarly, looking at the history of land use and land cover, one finds significant changes occurring, although over longer time periods. The obvious hypothesis to explain these observations again is that human beings altered the land and used resources to meet the needs of a rapidly growing population and an expanding industrial economy. Research into the direct human causes of global change has thus focused on changes in land and