The Regional Impacts of Climate Change

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Although large increases in annual runoff will affect flooding and flood management, large reductions may pose more serious threats to uses such as potable drinking water, irrigation, assimilation of wastes, recreation, habitat, and navigation. The greatest impact of declines in supply are projected to be in arid and semi-arid regions and areas with a high ratio of use relative to available renewable supply, as well as in basins with multiple competing uses. For example, reductions in outflow of 23-51% from Lake Ontario from assessments using four GCM scenarios suggest impacts on commercial navigation in the St. Lawrence River and the port of Montreal, as well as hydropower generation (Slivitzky, 1993). Lower flows also may affect the ecosystem of the river by allowing the saltwater wedge to intrude further upstream.

Seasonal patterns in the hydrology of mid- and high-latitude regions could be altered substantially, with runoff and streamflows generally increasing in winter and declining in summer.

Higher air temperatures could strongly influence the processes of evapotranspiration, precipitation as rain or snow, snow and ice accumulation, and melt-which, in turn, could affect soil moisture and groundwater conditions and the amount and timing of runoff in the mid- and high-latitude regions of North America. Higher winter temperatures in snow-covered regions of North America could shorten the duration of the snow-cover season. For example, one climate change scenario (CCC, Annex B) indicates up to a 40% decrease in the duration of snow cover in the Canadian prairies and a 70% decrease in the Great Plains (Boer et al., 1992; Brown et al., 1994). Warmer winters could lead to less winter precipitation as snowfall and more as rainfall, although increases in winter precipitation also could lead to greater snowfall and snow accumulation, particularly at the higher latitudes. Warmer winter and spring temperatures could lead to earlier and more rapid snowmelt and earlier ice break-up, as well as more rain-on-snow events that produce severe flooding, such as occurred in 1996-97 (Yarnal et al., 1997).

Damages to structures, hydropower operations, and navigation and flooding caused by late-winter and spring ice-jam events are estimated to cost CAN$60 million annually in Canada and US$100 million in the United States. About 35% of flooding in Canada is caused by ice jams-principally in the Atlantic Provinces, around the Great Lakes, in British Columbia, and in northern regions (Beltaos, 1995). Northern deltas and wetlands, however, depend upon flooding for periodic recharge and ecological sustainability (Prowse, 1997). The 2xCO₂ GCM simulations (IPCC 1996, WG I, Summary for Policymakers) suggest milder winters in higher latitudes and a general pattern of increased precipitation, with high regional variability. Where warmer winters result in reduced ice thickness, less severe breakups and reduced ice-jam flooding can be expected. However, major changes in precipitation patterns also are predicted. In some regions, there is an increased likelihood of winter or early spring rains. These climatic factors trigger sudden winter thaws and premature breakups that have the greatest potential for damage. Thus, although average conditions may be improved, the severity of extreme events in some regions appears likely to increase. In the more southern latitudes commonly affected by spring ice jams-such as the lower Great Lakes and central Great Plains areas, parts of New England, Nova Scotia, and British Columbia-there may be a reduction in the duration and thickness of the ice cover on rivers, as well as in the severity of ice jamming. In the north, similar effects are expected. In the intermediate latitudes-such as the prairies; much of Ontario and Quebec; and parts of Maine, New Brunswick, Newfoundland, and Labrador-spring jamming may become more common and/or severe. Such events are presently rare or completely unknown in some of these areas (Van Der Vinne et al., 1991; Beltaos, 1995).

In mountainous regions, particularly at mid-elevations, warming could lead to a long-term reduction in peak snow-water equivalent, with the snowpack building later and melting sooner (Cooley, 1990). Glacial meltwater also is a significant source of water for streams and rivers in some mountainous regions, with the highest flows occurring in early or midsummer (depending on latitude). For example, glacial meltwater contributes an average of 85% of the August flow in the Mistaya River near Banff, Alberta (Prowse, 1997). Accelerated glacier melt caused by temperature increases means more runoff in the short term, but loss of glaciers could result in streams without significant summer flow in the future (IPCC 1996, WG II, Sections 7.4.2 and 10.3.7). Late-summer stream discharge could decrease suddenly within only a few years. A steady pattern of glacial retreat is apparent in the southern Rocky Mountains below central British Columbia and Alberta. Water supplies in small communities, irrigation, hydroelectric generation, tourism, and fish habitat could be negatively impacted (IPCC 1996, WG II, Chapter 7; Brugman et al., 1997; Prowse, 1997).

In Arctic regions, permafrost maintains lakes and wetlands above an impermeable frost table and limits subsurface water storage. As described in Section 8.3.1, discontinuous and continuous permafrost boundaries are expected to move poleward as a result of projected changes in climate. Thawing of permafrost increases active-layer storage capacity and alters peatland hydrology. Although climatic warming could have a large effect on Arctic hydrology, the changes are highly uncertain at this time.

Altered precipitation and temperature regimes will affect the seasonal pattern and variability of water levels of wetlands, thereby affecting their functioning-including flood protection, carbon storage, water cleansing, and waterfowl/wildlife habitat.

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