This chapter has discussed the impacts of climate change on the North American region largely in the context of sector-by-sector assessments of plausible impacts. Several common characteristics among sectors can be identified, however. Also, viewed collectively, interactions between sectors and subregions can be assessed, and insights about the integrated nature of the effects of climate change can emerge.
Most impact studies have assessed how systems would respond to climate change resulting from an arbitrary doubling of equivalent CO2 concentrations. These so-called 2xCO2 scenarios are limited for regional-scale analyses to the extent that they inadequately correspond to the spatial scales of variability in North American natural and human systems. They also do not permit an examination of the effects of climate variability on physical, biological, and socioeconomic systems. Very few studies have considered dynamic responses to steadily increasing concentrations of greenhouse gases. Consequently, important insights about the ability of systems to respond to changing climate over time are lost. This lack of information is of particular concern because the ability of natural ecological systems to migrate often may be much slower than the predicted rate of climate change. Even fewer studies have examined the consequences of increases beyond a doubling of equivalent atmospheric concentrations.
All of the potential impacts of climate change exhibit a regional texture. Variations in the regional distribution of impacts need to be clearly articulated for policymakers. Failure to do so can lead to misleading impressions about the potential changes in social welfare as a result of climate change and alternative policy responses. A simple look at aggregate impacts on U.S. agriculture, for example, might suggest that climate change is not likely to harm agriculture enough to significantly affect the overall U.S. economy; policymakers might be left with the erroneous impression that no policy-relevant problems exist. Distributional differences emerge, however, upon examination of the regional texture of agricultural impacts.
Different adaptation strategies and options will be necessary to deal with these regional and sectoral differences. In areas where production significantly increases-such as the northern edge of agricultural production in North America-additional adaptation may be necessary in the development of infrastructure to support expanded population and transportation requirements associated with growth. The texture of the distribution of sectors and their biological, physical, and social components across the North American landscape cannot adequately be captured at a fine enough scale to be relevant to long-range planning at the present time; these are essential elements of future assessment needs.
It is also recognized-but poorly understood because of limited research-that climate change may have some benefits (e.g., it may reduce stress or provide opportunities) for certain areas or sectors within North America (e.g., expanded agriculture, reduced heating costs) or have a neutral effect on climate-insensitive sectors. If one examines any one particular climate impact, it is likely that there will be "winners" and "losers" either across subregions or within a subregion (e.g., across demographic groups). Nevertheless, the weight of evidence suggests that when all potential impacts are considered collectively, every subregion will incur some negative impacts of climate change.
Some future climate change is inevitable. Strategies for technological and behavioral adaptation offer an opportunity to reduce the vulnerability of sensitive systems to the effects of climate change and variability. Some adaptive strategies can be undertaken in anticipation of future climate change; others are reactive and can be undertaken as the effects of climate change are realized.
Four points must be kept in mind when considering the extent to which adaptive strategies should be relied upon. First, adaptation is not without cost. Scarce natural and financial resources must be diverted away from other productive activities into adaptive practices. These costs must be carefully weighed when considering the tradeoffs among adapting to the change, reducing the cause of the change, and living with the residual impacts. Second, the economic and social costs of adaptation will increase the more rapidly climate change occurs. Third, although many opportunities exist for technological and behavioral adaptation, uncertainties exist about potential barriers and limitations to their implementation. Fourth, uncertainties exist about the efficacy and possible secondary effects of particular adaptive strategies.
Water is a linchpin that integrates many subregions and sectors. Available water supplies will be directly affected by climate change, but they also are affected by changes in demand from the many sectors that rely upon the water. Water is a scarce resource used in the agriculture, forest, and energy sectors. It is used in urban areas and in recreational activities. It also is essential for the survival of wetlands, nonforest ecosystems, wildlife, and other ecological systems.
Assessments of the potential impacts of climate change and variability on any of these systems and sectors must account for the inherent competition for water supplies and the need for water of varying qualities in various activities. For example, in an assessment of the potential impacts of climate change on agriculture, an assumption that farmers will be able to adapt to changing climatic conditions through a reliance on irrigation is valid only to the extent that water is available under future climate scenarios. In many cases, the scarcity of available water supplies will increase because of the direct effects of climate change on water, as well as increased demands for available water supplies.
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