Many human actions affect what people value. One way in which the actions that cause global change are different from most of these is that the effects take decades to centuries to be realized. This fact causes many concerned people to consider taking action now to protect the values of those who might be affected by global environmental change in years to come. But because of uncertainty about how global environmental systems work, and because the people affected will probably live in circumstances very much different from those of today and may have different values, it is hard to know how present-day actions will affect them. To project or forecast the human consequences of global change at some point in the relatively distant future, one would need to know at least the following:
the future state of the natural environment,
the future of social and economic organization,
the values held by the members of future social groups,
the proximate effects of global change on those values, and
the responses that humans will have made in anticipation of global change or in response to ongoing global change.
These elements form a dynamic, interactive system (Kates, 1971, 1985b; Riebsame et al., 1986). Over decades or centuries, human societies adapt to their environments as well as influence them; human values tend to promote behavior consistent with adaptation; and values and social organization affect the way humans respond to global change, which may be by changing social organizations, values, or the environment itself.
This complex causal structure makes projecting the human consequences of global change a trickier task than is sometimes imagined. It is misleading to picture human impacts as if global change were like a meteorite striking an inert planet, because social systems are always changing and are capable of anticipation. So, for example, an estimate of the number of homes that would be inundated by a one-meter rise in sea level and the associated loss of life and property may be useful for alerting decision makers to potentially important issues, but it should not be taken as a prediction, because humans always react. Before the sea level rises, people may migrate, build dikes, or buy insurance, and the society and economy may have changed so that people's immediate responses—and therefore the costs of