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Increases in hydrological variability (larger floods and longer droughts) are likely to result in increased sediment loading and erosion, degraded shorelines, reductions in water quality, reduced water supply for dilution of point-source water pollutants and assimilation of waste heat loads, and reduced stability of aquatic ecosystems. Projected changes in snowfall and snowmelt-as well as suggested increases in warm-period rainfall intensity-could shift the periodicity of the flood regime in North America, possibly stressing the adequacy of dams, culverts, levees, storm drains, and other flood prevention infrastructures. The impacts of flooding are likely to be largest in arid regions, where riparian vegetation is sparse; in agricultural areas during winter, when soils are more exposed; and in urban areas with more impervious surfaces. Increases in hydrological variability may reduce productivity and biodiversity in streams and rivers and have large impacts on water resources management in North America, with increased expenditures for flood management. Increases in water temperature and reduced flows in streams and rivers may result in lower dissolved oxygen concentrations, particularly in summer low-flow periods in low- and mid-latitude areas.
Projected increases in human demand for water would exacerbate problems associated with the management of water supply and quality. Managing increased water demands will be particularly problematic in regions experiencing increases in variability and declines in runoff. Improved management of water infrastructure, pricing policies, and demand-side management of supply have the potential to mitigate some of the impacts of increasing water demand.
Food and Fiber: Agriculture (Section 8.3.4). As the climate warms, crop patterns will shift northward. Most studies of these shifts have focused on changes in average climate and assume farmers effectively adapt. They have not fully accounted for changes in climate variability, water availability, and imperfect responses by farmers to changing climate. Future consideration of these factors could either increase or decrease the magnitude of changes projected by these earlier studies.
Climate modifications that lead to changes in daily and interannual variability in temperatures and, in particular, precipitation will impact crop yields. Although changes in average temperature and precipitation can be expected to impact agriculture, few studies have considered the effects of increased climate variability on crop and livestock production. Increased variability in daily and interannual temperature and precipitation are likely to be as important or more important than the effects of mean changes in climate. Droughts, floods, and increased risks of winter injury will contribute to a greater frequency and severity of crop failure. An increased reliance on precision farming has increased vulnerability to climate variability outside a narrow range of change. These impacts are projected to be both site- and crop-specific; reliable forecasts for such occurrences, however, are not yet regionally available.
The direct effects of a doubling of CO2 on crop yields are largely beneficial. Food and fiber production for crops like cotton, soybean, and wheat are expected to increase an average of 30% (range -10% to +80%) in response to a doubling of CO2 concentration. The magnitude of this response will be highly variable and will depend on the availability of plant nutrients, temperature, and precipitation.
Crop losses due to weeds, insects, and diseases are likely to increase and may provide additional challenges for agricultural sector adaptation to climate change. Less severe winters due to climate change may increase the range and severity of insect and disease infestations. Increasing pressure to reduce chemical inputs (i.e., pesticides) in agriculture will necessitate a greater emphasis on concepts of integrated pest management and targeted application of agricultural chemicals through precision agricultural technologies.
Recent analyses of issues of long-run sustainability associated with agricultural adaptation to climate change from an arbitrary doubling of equivalent CO2 concentrations have concluded that there is considerably more sectoral flexibility and adaptation potential than was found in earlier analyses. Much of this reassessment arises from a realization that the costs and benefits of climate change cannot be adequately evaluated independently of behavioral, economic, and institutional adjustments required by changing climate. Although scientific controversy over the nature and rate of climate change remains, most existing scenarios suggest gradual changes in mean climate over decades-providing ample opportunities for adaptation measures to be implemented within vulnerable subregions of North America. However, uncertainties remain about the implications of changes in climate variability, as well as crop responses to increases beyond a doubling of equivalent atmospheric CO2 concentrations.
Existing studies that have looked at changes in mean temperature and precipitation suggest that climate change is not likely to harm agriculture enough to significantly affect the overall economy of North America. The economic consequences of climate change to U.S. agriculture are expected to be both positive and negative, depending on the nature of temperature and precipitation changes that occur in specific subregions. Subregions of North America that are dependent on agriculture may be more vulnerable than areas offering economic diversity. The Great Plains area, for example, relies heavily on crop and livestock production and, as a result, is potentially vulnerable to climate change, with negative consequences projected for southern extremes and potential positive impacts in northern areas as temperatures rise. Warmer temperatures at northern latitudes may lessen the adverse effects of frost damage, but the risk of early- and late-season frost will remain a barrier to the introduction of new crops.
Consumers and producers could gain or lose; the long-term stability of the forest-products market is uncertain. Consumer prices could increase by 100-250% with severe forest dieback, producing losses of 4-20% of the net value of commercial forests. Alternatively, consumer prices could decrease with increased forest growth and harvest in Canada, and producers could sustain economic losses. With exports from Canada to the United States, however, the net changes (consumers plus producers) could be negative for Canadians and positive for the U.S. market.
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