4.3.4. Coastal Systems
Summary: The impacts on coastal zones of sea-level rise and climate change
include inundation, riverine flooding, saline intrusion, erosion, and wave damage.
The impacts of changes in weather conditions (winds, waves, storms, and storm
surge) may be comparable to those of sea-level rise alone. Beaches, estuaries
and coastal wetlands, and reefs-including the Great Barrier Reef-have adapted
naturally to past changes in climate and sea level. However, in the future they
are likely to face faster rates of change, and in many cases landward migration
will be blocked by human land uses and infrastructure. Impacts will be complex,
both physically and socioeconomically, and will vary greatly from place to place.
There is potential for considerable damage to the region's low-lying coastal
settlements and infrastructure where populations, tourism, and capital investment
are large and growing. New Zealand also is exposed to impacts on its Pacific
island territories, including the eventual possibility of environmental refugee
fluxes. Some parts of Australasia's coastline, including a number of coastal
cities, and indigenous communities in low-lying coastal settlements and islands
are highly vulnerable.
Adaptation options include integrated coastal zone management; redesign, rebuilding,
or relocation of capital assets; protection of beaches and dunes; development
zone control; and retreat plans.
220.127.116.11. Coastal Zones and Responses
The Australasian region contains diverse coastal zones spanning tropical to
subantarctic latitudes and including continental coasts and small islands. The
habitats of coastal zones and their adjacent small islands support a great range
of ecosystems and complex food networks. A variety of economic activities are
located in the coastal zone, which also provides important landscape and recreation
values. Direct modification of the coastline through structural works is confined
to limited areas of industrial, urban, and resort development. However, changes
in sediment processes as a result of upstream land-use change and modification
of areas adjacent to the shoreline and of coastal dune systems have resulted
in some significant morphological changes in coastal areas. In addition, discharge
of nutrients, sediments, and pollutants from agricultural and urban runoff and
domestic and industrial wastes affects the composition and quality of some coastal
and estuarine waters (Zann, 1995; Larcombe et al., 1996).
The Australian coastline is about 70,000 km in length (at the 0.1 km scale);
it includes 12,000 islands and 783 major estuaries, 415 of which are in the
tropics. About 24% of the Australian coast consists of dunes and beaches, and
there are large areas of intertidal and supratidal mud, alluvium, mangroves,
and seagrass, as well as thousands of kilometers of coral reefs (Zann, 1995).
The New Zealand coastline is approximately 15,000 km-of which 25% is eroding,
19% accreting, and 56% (mostly hard-rock cliffs) static (Gibb, 1980); it is
more tectonically active than the Australian coast.
The potential impacts of climate change on the coastal zone include the direct
effects of rising sea level; rising temperatures; and possible changes in weather,
storminess, and wave characteristics, together with the indirect impacts of
changes on adjacent lands and rivers-resulting in altered inputs of freshwater,
floodwaters, sediments, nutrients, and biota to the coastal seas. Global sea
level is projected to rise 15-110 cm above current levels by the year 2100,
with a central estimate of about 50 cm (IPCC 1996, WG I, Section 7.5.2). The
estimated rate of rise is about two to five times faster than that experienced
over the past 100 years.
Actual rates of rise at specific coastal locations can vary substantially due
to local rise or fall of the land, different rates of oceanic warming and expansion
between regions, and changes in oceanic and atmospheric circulation under climate
change (Pittock et al., 1995; IPCC 1996, WG I, Section 7.2; IPCC 1996, WG II,
Section 9.3.1). It is likely that storminess will increase in some regions and
decrease in others, with parallel increases and decreases in impacts.
The coasts of Australia and New Zealand are not simple, passive systems but
respond dynamically to sea level and climate in a range of ways depending on
local circumstances. Weather elements and related processes such as wind speed
and direction, rainfall intensities, ocean wave energies, and storm surge have
major roles in coastal geomorphology with respect to the mean state and extreme
events. Even small changes in weather patterns may cause impacts commensurate
with those of sea-level rise alone (Cowell et al., 1996; IPCC 1996, WG II, Box
9.4; McInnes and Hubbert, 1996). Recent reviews of climate change impacts and
adaptations for the region may be found in Chappell et al. (1996) and Kay et
Global research indicates that estuaries and coastal wetlands have coped with
historical sea-level rise-for example, by migration landward. Salt marshes and
mangroves have survived where the rate of sedimentation approximates the rate
of local sea-level rise; beaches have grown or decayed according to changes
in prevailing winds and seas; and coral reefs have demonstrated the capacity
to grow vertically in response to past sea-level rise (IPCC 1996, WG II, Section
9.4). However, these past rates of natural adaptation may be insufficient for
higher rates of future sea-level rise; in many cases, landward migration will
be blocked by human infrastructure such as causeways, flood protection levees,
and urban development, leading to a reduction in the area of the wetland or
Sea-level rise will increase the penetration of the saltwater wedge in the
many tidal estuaries, leading to salinity changes and consequent effects on
estuarine ecosystems (see Section 18.104.22.168). Some coastal
aquifers also may be affected by saline intrusion (Ghassemi et al., 1991).
Natural responses may be complex. For example, in macrotidal estuaries in northern
Australia, channel widening from sea-level rise may contribute sediment to adjacent
estuarine plains, enhancing vertical accretion. Backwater swamps and freshwater
ecosystems on the estuarine plains may be endangered (IPCC 1996, WG II, Section
9.4.2). In these sediment-rich areas, where strong tidal currents redistribute
sediments, mangrove communities have a better chance of survival (IPCC 1996,
WG II, Section 9.4.4; see also Section 22.214.171.124 below).
However, losses of coastal wetlands for Australia are expected to be greater
than the global average (IPCC 1996, WG II, Section 126.96.36.199).
The potential impacts of climate change on Australia's coral reefs, including
the Great Barrier Reef, have been discussed in Section 188.8.131.52.
Coral reefs face the types of coastal impacts described above, questions about
whether reef growth can keep pace with sea-level rise, and the problems of coral
bleaching and algal invasions. For small coral islands, the higher sea level
will erode inhabited coral cays and contaminate water tables to an extent that
is likely to be prohibitively expensive to combat or rectify (Wilkinson and
Buddemeier, 1994). On many of Australia's coral reef islands, the impacts are
likely to be highly visible and would have significant consequences for tourism
Climate change impacts on the coasts will be compounded by rapid growth in
environmental stresses arising from existing population growth and increasing