Geoscience Reference
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(e.g. Nicholls 2004) and increase rates of coastal
cliff failure ( Jones 1993b). In large part, this
will occur because increasing sea-levels raise the
plane of activity over which wave influence is
exerted. Along temperate sediment-dominated
coasts, this is likely to result in a landward
migration of beaches, dunes and barrier islands,
although the potential for landward retreat
will depend upon the nature of the backshore
environment and on the presence or absence of
coastal infrastructure. Where roads and urban
conurbations exist, landward migration may be
prevented by coastal defence structures and
thus, in such cases, progressive loss of coastal
sedimentary environments ('coastal squeeze')
may occur (Chapters 7 and 8). Rates of change
will be exacerbated along low-lying coasts and
in areas that are undergoing active subsidence
(see Pirazzoli 1996, fig. 117).
Predicting the response of individual coastal
systems is further complicated by local sediment
dynamics and rates of sediment supply. Coastal
dune systems represent an interesting example
where these factors are highly site-specific and
thus shoreline response modes highly variable.
Dunes are sites of temporary sediment storage
on the coast and significant sediment exchange
occurs with adjacent environments. This may be
a response to changes in energy levels during
storm events, but will also occur during periods
of rising sea-level as the plane of wave influence
is reset (see Chapter 8). Hence dune responses to
sea-level change, as with most coastal systems,
can be considered realistically only on a site-by-
site basis. In the tropics, sea-level rise may bring
the benefit of renewed phases of growth on coral
reefs that have reached sea-level and ceased to
accrete during the late Holocene. The potential
for such expansion will, however, depend upon
reef community status and in severely degraded
coral reefs partial submergence of the reef struc-
tures may occur. This, in turn, may facilitate
increased wave-overtopping and adjacent shore-
line erosion (see Chapter 9). Along mangrove-
dominated coasts, shoreline response to sea-level
rise will depend, in part, upon whether sediment
supply is sufficient to maintain the seaward
fringe. As along temperate coastlines, landward
migration of mangroves may occur if the back-
shore area is unimpeded. Hence environmental
responses are likely to be highly site-specific.
Predicting future responses to sea-level rise along
many coastal fringes is further complicated by
the fact that sea-level is rising from a position of
pre-existing sea-level highstand. Hence many
coastal sedimentary environments will be migrat-
ing across areas of relatively low-lying land and
thus the conditions under which the environ-
ments transgress will be very different to those
that were submerged during the Holocene sea-
level rise. This will complicate attempts to model
future landform migration on the basis of changes
that occurred during the early Holocene.
Climate change models also predict an increase
in the frequency of severe weather conditions
(McCarthy et al. 2001). These include not only
storms and cyclones, but also shifts in large
scale, ocean-driven climate oscillations such as
the El Niño-Southern Oscillation (ENSO) and
the North Atlantic Oscillation (NAO). There
is, for example, some evidence to suggest that
El Niño events have become more frequent and
intense during the past 20 -30 years and that
these events may increase in frequency and inten-
sity through to 2100 in tropical Pacific areas.
Such changes may result in modified rainfall
patterns, in rainfall intensity and the frequency
of drought conditions. Predicted increases in
high-intensity precipitation events may result in
increased flooding, landslides and mudslides,
increased rates of soil erosion and increased
flood runoff, with obvious implications both for
upland and fluvial sedimentary systems. Past
records of increased slope failures in Central and
South America have, for example, been linked
to interannual variations in the magnitude and
intensity of rainfall events between El Niño
and La Niña years (Trauth et al. 2000). Any
change in storm frequency and intensity will
also have significant implications for many
coastal sedimentary systems due, in large part,
to likely increases in storm-wave surges. Under
these conditions, increased rates of coastal ero-
sion (Chapter 8) and, in the tropics, increased
damage to coral reefs and mangroves can be
expected (Chapter 9).
