15.4.1.2 Impacts on physical regime
Changes in Arctic freshwater systems will have numerous impacts on the physical regime of the Arctic, particularly affecting hydrological extremes, global feedbacks and contaminant pathways.
Hydrological models based on atmosphere-ocean general circulation models (AOGCMs) have consistently also predicted increases in flow for the major Arctic river systems, with the largest increases during the cold season (Miller and Russell, 2000; Arora and Boer, 2001; Mokhov et al., 2003; Georgievsky et al., 2005). Less clear is what may occur during the summer months, with some results suggesting that flow may actually decrease because of evaporation exceeding precipitation (e.g., Walsh et al., 2005). Reductions in summer flow could be enhanced for many watersheds because of increases in evapotranspiration as dominant terrestrial vegetation shifts from non-transpiring tundra lichens to various woody species (e.g., Callaghan et al., 2005). CO2-induced reductions in transpiration might offset this, and have been suggested as being responsible for some 20th-century changes in global runoff (Gedney et al., 2006).
Since Arctic river flow is the major component of the freshwater budget of the Arctic Ocean (Lewis et al., 2000), it is important to the supply of freshwater to the North Atlantic and related effects on the thermohaline circulation (Bindoff et al., 2007). The sum of various freshwater inputs to the Arctic Ocean has matched the amount and rate at which freshwater has accumulated in the North Atlantic over the period 1965 to 1995 (Peterson et al., 2006). Under greenhouse gas scenarios, the total annual river inflow to the Arctic Ocean is expected to increase by approximately 10 to 30% by the late 21st century (Walsh et al., 2005). An additional source of future freshwater input will be from melting of large glaciers and ice caps, most notably from Greenland (Gregory et al., 2004; Dowdeswell, 2006). The cumulative effect of these increasing freshwater supplies on thermohaline circulation remains unclear but is a critical area of concern (Loeng et al., 2005; Bindoff et al., 2007).
Warming is also forecast to cause reductions in river- and lake-ice cover, which will lead to changes in lake thermal structures, quality/quantity of under-ice habitat and effects on ice jamming and related flooding (Prowse et al., 2006a). Specific to the latter, forecasts of earlier snowmelt freshets could create conditions more conducive to severe break-up events (Prowse and Beltaos, 2002), although a longer period of warming could also reduce severity (Smith, 2000). This effect, however, is likely to be offset on some large northward-flowing rivers because of reduced regional contrasts in south-to-north temperatures and related hydrological and physical gradients (Prowse et al., 2006a).
Projected changes of permafrost, vegetation and river-runoff may have noticeable impacts on river morphology, acting through destabilisation of banks and slopes, increased erosion and sediment supply, and ultimately leading to the transformation between multi- and single-channel types. Geological reconstructions and numerical simulations indicate that such transformations and also erosion events and flood risks occur especially at times of permafrost degradation (Bogaart and van Balen, 2000; Vandenberghe, 2002). Such changes are largely controlled by thresholds in sediment supply to the river and discharge (Vandenberghe, 2001). However, historical examples have shown that variability in flow regime is less important than variability in sediment supply, which is especially determined by the vegetation cover (Huisink et al., 2002; Vandenberghe and Huisink, 2003). Thus an increasingly denser vegetation cover may counter increased sediment discharge, which has been modelled to rise in Arctic rivers with both increases in air temperature and water discharge (Syvitski, 2002).
Various changes in Arctic hydrology have the potential to effect large changes in the proportion of pollutants (e.g., persistent organic pollutants and mercury) that enter Arctic aquatic systems, either by solvent-switching or solvent-depleting processes (e.g., MacDonald et al., 2003). Given that the Arctic is predicted to be generally ‘wetter’, the increase in loadings of particulates and contaminants that partition strongly into water might more than offset the reductions expected to accrue from reductions in global emissions (e.g., MacDonald et al., 2003). Shifts in other hydrological regime components such as vegetation, runoff patterns and thermokarst drainage (Hinzman et al., 2005) all have the capacity to increase contaminant capture. Changes in aquatic trophic structure and related rate functions (see Section 15.4.1.3) have further potential to alter the accumulation of bio-magnifying chemicals within food webs.