Ultimately, it is desirable to understand how choices, and the factors that shape them, lead to specific environmental outcomes (Dietz et al., 2009c; Vayda, 1988). A variety of hypotheses have been offered and tested about the key societal factors that shape environmental change—what are often called the drivers of change (NRC, 1992a). Growth in population and consumption, technological change, land and resource use, and the social, institutional, and cultural factors shaping the behavior of individuals and organizations have all been proposed as critical drivers, and some empirical work has elucidated the influence of each of them (NRC, 1997b, 1999c, 2005a, 2008b). However, much of this research has focused on only one or a few factors at a time and has used highly aggregated data (Dietz et al., 2009a). To understand the many human drivers of climate change as a basis for better-informed decision making, it will be necessary to develop and test integrative models that examine multiple driving forces together, examine how they interact with each other at different scales of human activity and over time, and consider how their effects vary across different contexts.
To evaluate the effectiveness of policies or other actions designed to limit the magnitude of climate change, increased understanding is needed about both the elasticity of climate drivers—the extent to which changes in drivers produce changes in climate impacts—and the plasticity of drivers, or the ease with which the driver can be changed by policy interventions (York et al., 2002). For example, analyses of the effects of population growth on GHG emissions suggest an elasticity of about 1 to 1.5; that is, for every 1 percent increase in human population, there is roughly a 1 to 1.5 percent increase in environmental impact (Clark et al., 2010; Dietz et al., 2007; Jorgenson, 2007, 2009; Shi, 2003; York et al., 2003). On the other hand, recent research suggests that environmental impact is more directly related to the number of households than to the number of people (Cole and Neumayer, 2004; Liu et al., 2003). Thus, a shift to smaller average household size could offset or even overwhelm the reduction in climate drivers resulting from reduced population growth. Similarly, it has been argued that increasing affluence leads at first to increased environmental impact but, once a threshold level of affluence has been reached, environmental impact declines (Grossman and Krueger, 1995; Selden and Song, 1994). In the case of GHG emissions, however, emissions apparently continue to increase with increasing affluence (Carson,
2010; Cavlovic et al., 2000; Dasgupta et al., 2002; Dietz et al., 2007; Stern, 2004), suggesting that economic growth alone will not reduce emissions.
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