The hydrological regime of the Arctic is particularly susceptible to predicted climate change because of the dominance of the thermally sensitive cryosphere and its controlling influence on the water cycle. Virtually all major hydrological processes and related aquatic ecosystems are affected by snow and ice processes, including the major Arctic rivers. Although these rivers originate in more temperate southern latitudes, they pass through cold regions before reaching the Arctic Ocean. Changes in precipitation also have great potential for modifying the hydrology and sediment load in rivers, which could affect aquatic life (Michel and van Everdingen, 1994).

16.2.6.1. Changes in Precipitation, Snow Accumulation, and Spring Melt

Almost all climate models forecast increases in precipitation; but estimates vary widely and are complicated by many other factors, such as clouds (Stewart, 2000). Combined changes in temperature and precipitation will produce changes in the pattern of snow storage and, ultimately, Arctic hydrology. Warmer conditions generally will reduce the length of winter. Seasonal snow accumulation could increase in higher elevation zones, especially above the freezing level, where there would be little summer melt. Increased summer storminess will reduce melt at intermediate elevations as a result of increased cloud and summer snowfall, so many existing semi-permanent snowpacks will continue to exist (Woo, 1996). At lower elevations, particularly in the more temperate maritime zones, rainfall and rain-on-snow melt events probably will increase.

An earlier transition from winter to spring will mean that snowmelt will be more protracted, with less intense runoff. This will be accentuated where increased active-layer thickness and loss of permafrost increases the water storage capacity of the ground, leading to a decrease in runoff. Summer base flow will increase. Overall, this means that there will be less seasonal fluctuation in runoff through the year (Rouse et al., 1997).

16.2.6.2. Surface Water Budgets and Wetlands

Predicted climate change will lead to greater evaporation and transpiration. Transpiration should increase as nontranspiring lichens and mosses that currently dominate the tundra regions are replaced by a denser cover of vascular plants (Rouse et al., 1997). These changes are likely to reduce the amount of ponded water and runoff. The unglaciated lowlands of many Arctic islands, where special ecological niches occur, are likely to be especially sensitive. For coastal areas, changes to local micro-climates may occur because longer open-water seasons in the adjacent sea will lead to more frequent fog and low clouds, as well as reduced incident solar radiation. These changes may limit the expected increase in evaporation and transpiration (Rouse et al., 1997).

The ability of Arctic wetlands to act as a source or sink of organic carbon and CH₄ depends on the position of the water. Analysis of subarctic sedge fens to increases in temperature of 4°C suggests reduced water storage of 10-20 cm over the summer in northern peatlands, even with a small increase in precipitation (Rouse, 1998). This is significant, given that Moore and Roulet (1993) suggest that a reduction of 0.1 m in water storage in northern forested peatland is sufficient to convert these areas from a source to a sink of atmospheric CH₄. Wider questions concerning the carbon budget, including the level of uncertainty, are discussed in TAR WGI Chapter 3. Further predictions based on 2xCO₂ changes indicate a 200-300 km retreat of the southern boundary of peatlands in Canada toward the Arctic coast and significant changes in their structure (Gignac and Vitt, 1994). Degradation of permafrost, which currently forms an impermeable layer, will couple many ponds with the groundwater system and lead to their eventual drainage. Areas of special sensitivity include patchy Arctic wetlands on continuous permafrost and those along the southern limit of permafrost (Woo and Young, 1998). In contrast, warming of surface permafrost also could lead to formation of new wetlands, ponds, and drainage networks through the process of thermokarst development, especially in areas with high concentrations of ground ice.

16.2.6.3. Ecological Impact of Changing Runoff Regimes

A warmer climate will create a more pluvial runoff regime as a greater proportion of the annual precipitation is delivered by rain rather than snow and a flattening of the seasonal runoff cycle occurs. Enhancement of winter flow will mean streams that currently freeze to their beds will retain a layer of water beneath the ice. This will be beneficial to invertebrates and fish populations. However, such rivers will then be prone to ice jamming and hence larger annual flood peaks. Warming will lead to a shortened ice season and thinner ice cover. For large northward-flowing rivers, this could reduce the severity of ice jamming in spring, especially if the magnitude of the peak snowmelt that drives breakup also is reduced (Beltaos and Prowse, 2000). Decreased ice-jam flooding will benefit many northern communities located near river floodplains. In contrast, reductions in the frequency and severity of ice-jam flooding would have a serious impact on northern riparian ecosystems, particularly the highly productive river deltas, where periodic flooding has been shown to be critical to the survival of adjacent lakes and ponds (Marsh and Hey, 1989; Prowse and Conly, 1998).

Decreases in winter snowpack and subsequent spring runoff from upstream tributaries has led to reduced frequency and severity of ice jams affecting the very large Peace-Athabasca Delta (Prowse et al., 1996). These changes are analogous to those expected with predicted climate warming. Ice breakup is a major control over aquatic ecology, affecting numerous physical and biochemical processes and the biodiversity and productivity of such northern rivers (Prowse, 1994; Scrimgeour et al., 1994). Major adjustments in their ecology are expected in the future. A similar situation exists for northern ponds and lakes, where ice cover will be thinner, form later, and break up earlier—with consequent limnological changes. Total primary production should increase in all Arctic lakes and ponds with an extended and warmer ice-free season (Douglas et al., 1994). Primary productivity of Arctic aquatic systems also should be boosted by a greater supply of organic matter and nutrients draining from a more biologically productive terrestrial landscape (Schindler, 1997; Hobbie et al., 1999). Thinner ice cover will increase the solar radiation penetrating to the underlying water, thereby increasing photosythnetic production of oxygen and reducing the potential for winter fish kills. However, a longer ice-free season will increase the depth of mixing and lead to lower oxygen concentrations and increased stress on coldwater organisms (Rouse et al., 1997).

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