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In the 20th century, there has been increasing human pressure on mountainous regions, initially through trapping, forestry, and reservoir construction and now through development of ski areas and other resorts and construction of residences, as well as forestry and continuing reservoir construction in northern Canada. At the same time, however, the national park systems in the United States and Canada and wilderness preserves have expanded to include many mountainous areas that are essentially pristine with respect to human development, especially in the Rocky Mountains. It is now recognized that these protected mountain ecosystems are still vulnerable to anthropogenic change through transport of atmospheric contaminants, such as nitrate and sulfate in acid rain, and through climate change. Warming of the climate eventually will cause two major changes—retreat of mountain glaciers and upward movement of treeline—and the response times for reaching new equilibrium conditions are on the order of 100 years or more, so these responses will lag continuing climate change.
Retreat of glaciers is driven by the rate of ablation, which includes melt,
exceeding the rate of advance driven by snow accumulation over the glacier,
and corresponds to a change in the shape of the glacier to a new equilibrium.
Retreat of mountain glaciers already has begun in North America (Brugman et
al., 1997) and in other regions of the world, and this retreat will contribute
to sea-level rise in an amount comparable in magnitude to expansion of ocean
waters as a result of warming. On a regional scale, the retreat of glaciers
will affect water resources by changing (probably decreasing) water supply from
glacial melt during summer or changing the spatial location of the melt source
[summer flows initially may increase but eventually will decline as glacier
reservoir capacity declines (Pelto, 1993)]. Furthermore, glacial retreat will
expose terrain that gradually will evolve with soil development and revegetation,
and new lakes will form in exposed basins. These changes eventually will influence
the water quality of drainage from these lakes. These sequences of glacial advance
and retreat have occurred through the quaternary; the effect of climate change
is to induce these changes. In terms of human vulnerability to climate change,
retreat of mountain glaciers also is significant because it is an observable
change that can be directly comprehended by the public as an indicator of warming—more
so than warming of the open ocean or an increase in extreme hydrological events.
For example, a recent article in a travel magazine (Conde Nast) outlined vacations
to view retreating glaciers in North America, Europe, and Africa while they
were still there.
From paleolimnological studies of alpine and subalpine lakes, the rise in treeline in response to past warming of climate is well-documented. The boundary between alpine tundra and subalpine forest is controlled by extremes of temperature, moisture, and wind. Vegetation in both ecosystems is long-lived, and changes will proceed slowly and in a manner that depends on whether total annual snowpack decreases or increases and whether melt occurs earlier; both factors control the growth of alpine and subalpine species. Movement of treeline could have a minor feedback on climate change by sequestering more carbon in subalpine forests. The eventual effect of upward movement of the treeline will be to shrink the extent of alpine tundra in North America, possibly causing species loss and ecosystem degradation through greater fragmentation (see Section 15.2.6. and Chapter 5).
Wetlands represent a variety of shallow water and upland water environments
that are characterized by hydric soils and plant and animal species that are
adapted to life in saturated conditions (NRC, 1995). These ecosystems are considered
to be of great importance in a variety of functional contexts, including waterfowl
habitat, carbon sequestration, CH4 production, flood regulation,
pollutant removal, and fish and shellfish propagation (Mitsch and Gosselink,
1993). About 14% of Canada's surface area is covered by wetlands, which
is 24% of the global total (NWWG, 1988). Approximately 6% of the United States
is wetland (Kusler et al., 1999).
Mid-latitude wetlands have been greatly affected by a variety of human activities
over the past 200 years. More than 50% of the original wetlands in the United
States have been destroyed for agriculture, impoundment, road building, and
other activities (Dahl, 1990). Most of the remaining wetlands have been altered
by harvest, grazing, pollution, hydrological changes, and invasion by exotic
species (Kusler et al., 1999). High-latitude wetlands have experienced
much lower levels of human disturbance (Schindler, 1998).
Climate change can have significant impacts on wetland structure and function,
primarily through alterations in hydrology, especially water-table level (Clair
et al., 1998; Clair and Ehrman, 1998). Wetland flora and fauna respond
very dynamically to small changes in water-table levels (Poiani et al.,
1996; Schindler, 1998). Moreover, climate change can exacerbate other stresses
(e.g., pollution), especially in fragmented landscapes where wetlands have been
cut off from other wetlands by a variety of landscape-level alterations (Mortsch,
1998; Kusler et al., 1999). With rising sea levels, shoreline development
and efforts to protect private property from coastal erosion could lead to loss
of public tidelands and coastal marshes, particularly along bayshores where
preservation of natural shorelines has received less policy attention than is
the case for most ocean beaches (Titus, 1998).
Specific changes predicted to occur in North American wetlands are wide ranging.
Sea-level rise will result in loss of coastal wetlands in many areas, with potentially
important effects on ocean fisheries (Michener et al., 1997; Turner,
1997). Increased drought conditions in the Prairie Pothole Region of the northern
Great Plains, which are forecast to occur under nearly all GCM scenarios, will
significantly reduce U.S. breeding duck populations (Sorenson et al.,
1998). Tourism may benefit from extended seasons but will suffer if key processes
(e.g., hunting, birding) are disrupted (Wall, 1998a). Alteration of water-table
levels could affect the carbon sequestration function of the vast northern wetlands
of Canada, but there is great uncertainty about the nature and extent of this
effect (Moore et al., 1998; Waddington et al., 1998).
Wetlands have been the target of numerous protection and restoration efforts (NRC, 1995), which suggests that there is high potential for adaptive management in response to climate change, at least in mid-latitudes. Kusler et al. (1999) recommend a series of strategies for reducing the impacts of climate change on wetlands. These strategies include better control of filling and draining of wetlands, prevention of additional stresses, prevention of additional fragmentation, creation of upland buffers, control of exotic species, protection of low flows and residual water, enhanced efforts to restore and create wetlands, and aggressive efforts in stocking and captive breeding of critical wetland species. Five states in the United States have adopted rolling easement policies, which ensure that wetlands and/or beaches can migrate inland as sea level rises, instead of being squeezed between coastal development and the advancing sea (Titus, 1998). These efforts would be greatly enhanced by creation of regional inventories and management plans for wetlands at greatest risk from climate change.
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