Knowledge is not yet adequate to estimate the environmental effects of particular types of migration or to model environmental impacts as a function of household size and composition as distinct from population effects.
A major source of uncertainty in projecting future human contributions to global change and analyzing response costs is the rate at which improved technology will lead to the substitution of abundant natural resources for scarce ones and of reproducible capital for depletable resources. Economists and technologists have typically viewed technical change as widening the possibility of substitution among resources. This has frequently led to a bias in favor of assuming adaptation strategies for response rather than mitigation strategies. Ecologists and other biologists have typically regarded substitutability as being narrowly restricted. The argument about biodiversity is, in part, a reflection of these alternative views. The problem has not yet been modeled satisfactorily, nor has sufficient empirical research been conducted to test the alternative perspectives. However, dialogue between the two theoretical camps is increasing and signs of a conceptual synthesis are beginning to appear, in which the questions are formulated in terms of the relationship between rates of substitution and rates of resource consumption.145
Past research has documented some regularities in the time path of change in environmentally significant technologies, including rates of technology diffusion and secular trends toward so-called dematerialization and decarbonization; it has also documented variations around general time trends.146 There has been a lively empirically based debate about the extent to which scarcity may induce innovations that reduce costs and find substitutes, a debate that may be heading toward synthesis.147 Extensive studies have also been conducted of the conditions favoring adoption of technological innovations. This research is starting to make it possible for modelers of global change effects and builders of integrated assessment models to replace ad hoc coefficients of technological change with numbers based on empirical analysis and sound theory.
Important advances in this field include the following: identification of secular trends toward dematerialization and decarbonization of economies, along with variations around these trends; identification of factors influencing the rates of adoption of technological innovations; and identification of the substitution rate of inexhaustible resources for depletable ones as a key parameter for studies of sustainability.
Knowledge is not yet adequate to model the factors influencing variations in average rates of decline in national energy intensity and related indicators and variations around the average among industries and firms or to model the effects of environmental policies on rates of innovation in environmentally benign technologies.