6.4.6. High-Latitude Coasts
High-latitude coasts are highly susceptible to a combination of climate change
impacts in addition to sea-level rise, particularly where developed in ice-bonded
but otherwise unlithified sediments. In this context, atmospheric warming affects
ground-surface temperatures and thaw, as well as SST and sea ice.
Ground temperatures determine the presence of perennially frozen ground (permafrost),
which often contains large volumes of excess ice that may occur in the form
of massive ice. The seasonal cycle of ground and nearshore seawater temperatures
determines the depth of the seasonally active thaw layer in high-latitude beaches
and the nearshore, with implications for limiting beach scour during storms
(Nairn et al., 1998). Deepening of the active layer (Vyalov et al., 1998) also
can lead to melting of near-surface massive ice and may trigger additional coastal
slope failure (Dallimore et al., 1996; Shaw et al., 1998b).
Rapid coastal retreat already is common along ice-rich coasts of the Beaufort
Sea in northwestern Canada (e.g., Dallimore et al., 1996), the United States,
and the Russian Arctic (e.g., Are, 1998). Where communities are located in ice-rich
terrain along the shore, warmer temperatures combined with increased shoreline
erosion can have a very severe impact. For example, at Tuktoyaktukthe
main community, port, and offshore supply base on the Canadian Beaufort sea
coastmany structures are located over massive ice along eroding shore
(Wolfe et al., 1998).
Coastal recession rates along the Arctic coast also are controlled by wave
energy during the short open-water season. An early study based on historical
records of shoreline recession, combined with hindcast waves derived from measured
wind and observed ice distributions, showed that coastal recession rates at
several sites are correlated with open-water fetch, storminess, and wave energy.
Using estimates of less extensive ice distribution under a doubled-CO2 atmosphere,
Solomon et al. (1994) were able to demonstrate an increase in coastal erosion
rates to a mean value comparable to the maximum observed rates under present
climate conditions. In the Canadian Arctic Archipelago region, where many fine-grained
(mud and sand) shorelines and deltas now experience almost zero wave energy
(e.g., Forbes and Taylor, 1994), any increase in open water will lead to rapid
reworking and potentially substantial shoreline retreat.
Sea ice may erode the seabed in the nearshore zone, but it also may supply
shoreface sediments to the nearshore and beach (Reimnitz et al., 1990; Héquette
et al., 1995). Thinner ice or later freeze-up resulting from climate warming
may lead to changes in nearshore ice dynamics and associated sediment transport.
Warmer temperatures and associated changes in winter sea-ice distribution at
mid-latitudes are expected to have a negative impact on coastal stability. Rapidly
eroding sandy coasts in the southern Gulf of St. Lawrence are partially protected
in winter by development of an icefoot and nearshore ice complex. The strongest
storms and most persistent onshore winds occur in winter, partially overlapping
the ice season. Severe erosion in recent years has been linked to warmer winters
with late freeze-upan anticipated outcome of greenhouse warming (Forbes
et al., 1997b).