15.4 Key future impacts and vulnerabilities
15.4.1 Freshwater systems and their management
15.4.1.1 Arctic freshwater systems and historical changes
Some freshwater systems exist wholly within the Arctic but many others are fed by river and lake systems further south. The latter includes five of the world’s largest river catchments, which act as major conduits transporting water, heat, sediment, nutrients, contaminants and biota into the Arctic. For these systems, it will be the basin-wide changes that will determine the Arctic impacts.
Historically, the largest changes to northern river systems have been produced by flow regulation, much of it occurring in the headwaters of Arctic rivers. For Canada and Russia, it is these northward-flowing rivers that hold the greatest remaining potential for large-scale hydroelectric development (e.g., Shiklomanov et al., 2000; Prowse et al., 2004). Similar to some expected effects of climate change, the typical effect of hydroelectric flow regulation is to increase winter flow but also to decrease summer flow and thereby change overall inter-seasonal variability. In the case of the largest Arctic-flowing river in North America, the Mackenzie River, separating the effects of climate change from regulation has proven difficult because of the additional dampening effects on flow produced by natural storage-release effects of major lake systems (e.g., Gibson et al., 2006; Peters et al., 2006). For some major Russian rivers (Ob and Yenisei), seasonal effects of hydroelectric regulation have been noted as being primarily responsible for observed trends in winter discharge that were previously thought to be a result of climatic effects (Yang et al., 2004a, b). By contrast, winter flow increases on the Lena River have resulted primarily from increased winter precipitation and warming (Yang et al., 2002; Berezovskaya et al., 2005). Spatial patterns in timing of flows, however, have not been consistent, with adjacent major Siberian rivers showing both earlier (Lena – Yang et al., 2002) and later (Yenisei – Yang et al., 2004b) spring flows over the last 60 years. Although precipitation changes are often suspected of causing many changes in river runoff, a sparse precipitation monitoring network in the Arctic, makes such linkages very difficult (Walsh et al., 2005). Seasonal precipitation-runoff responses could be further obscured by the effects of permafrost thaw and related alterations to flow pathways and transfer times (Serreze et al., 2003; Berezovskaya et al., 2005; Zhang et al., 2005).
Over the last half-century, the combined flow from the six largest Eurasian rivers has increased by approximately 7% or an average of 2 km3/yr (Peterson et al., 2002). The precise controlling factors remain to be identified, but effects of ice-melt from permafrost, forest fires and dam storage have been eliminated as being responsible (McClelland et al., 2004). Increased runoff to the Arctic Ocean from circumpolar glaciers, ice caps and ice sheets has also been noted to have occurred in the late 20th century and to be comparable to the increase in combined river inflow from the largest pan-Arctic rivers (Dyurgerov and Carter, 2004).
The Arctic contains numerous types of lentic (still-water) systems, ranging from shallow tundra ponds to large lakes. Seasonal shifts in flow, ice cover, precipitation/evapotranspiration and inputs of sediment and nutrients have all been identified as climate-related factors controlling their biodiversity, storage regime and carbon-methane source-sink status (Wrona et al., 2005). A significant number of palaeolimnological records from lakes in the circumpolar Arctic have shown synchronous changes in biological community composition and sedimentological parameters associated with climate-driven regime shifts in increasing mean annual and summer temperatures and corresponding changes in thermal stratification/stability and ice-cover duration (e.g., Korhola et al., 2002; Ruhland et al., 2003; Pienitz et al., 2004; Smol et al., 2005; Prowse et al., 2006b).
Permafrost plays a large role in the hydrology of lentic systems, primarily through its influence on substrate permeability and surface ponding of water (Hinzman et al., 2005). Appreciable changes have been observed in lake abundance and area over a 500,000 km2 zone of Siberia during an approximate three-decade period at the end of the last century (see Figure 15.4; Smith et al., 2005). The spatial pattern of lake disappearance strongly suggests that permafrost thawing is driving the changes.