FIGURE 1.2 Number of dams constructed in the United States over the past 200 years (by decade) categorized by dam height. SOURCE: Doyle et al. (2003).

FIGURE 1.2 Number of dams constructed in the United States over the past 200 years (by decade) categorized by dam height. SOURCE: Doyle et al. (2003).

and sustained changes resulting from flow regulation, including changes in channel properties, sediment transport, and reduced ecological habitat (Chin et al. 2002; Phillips et al., 2005).

Because of their concern with spatiotemporal dynamics, geographical scientists have been at the forefront of efforts to use paleoenvironmental data to provide long proxy records of climatic and environmental change. Through techniques such as fossil pollen analysis, fossil charcoal analysis, tree-ring analysis, diatom analysis, chironomid analysis, and various sedimentological and geochemical techniques, geographical scientists have been able to reconstruct changes in terrestrial and aquatic environments on timescales ranging from decades to millennia. Such reconstructions can identify the specific nature of human impacts in the past, provide insight into the natural variability in environmental systems prior to human alteration, and show how environments have responded to past episodes of climate change. They can also be used to validate climate models used for estimating future climate change scenarios (Figure 1.3). In addition to providing qualitative and quantitative information on past environments, paleontological approaches are increasingly being refined and used to provide quantitative records of past temperature, precipitation, drought severity, and river flow (Cook et al., 2007). These records provide the only means of identifying the processes creating climatic variability and determining when anthropogenic climate changes have exceeded natural variability (Diffenbaugh et al., 2006; Herweijer et al., 2006; MacDonald et al., 2008b).

The integrated and synthetic research that is a hallmark of the geographical sciences is essential to address one of the major challenges in climate-change research: determining the natural (as opposed to human) contribution to climatic variability. Many paleoclimatic records and long instrumental data series provide evidence of variations in temperature that persist for decades to centuries. This natural variability in the climate system has two important implications for anticipating the impacts of global warming from increased greenhouse gases. First, if we do not understand their causes and properties, natural variations in climate make it difficult to detect or attribute current and future changes in climate to anthropogenic factors such as increased greenhouse gases. Second, such natural variations are likely to persist even in the face of greenhouse gas–induced climate changes and should be taken into account when planning for climate change. Often the relationships between the ultimate climatic forcing factors are mediated by complex relationships between the atmosphere, oceans, and land surface that play out differently from place to place (Feddema et al., 2005).



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