12.4.7. Coastal and Marine Systems
Australia has some of the finest examples of coral reefs in the world, stretching
for thousands of kilometers along the northwest and northeast coasts (Ellison,
1996). Coral reefs in the Australian region are subject to greenhouse-related
stresses (see Chapter 6 for a summary), including increasingly
frequent bleaching episodes, changes in sea level, and probable decreases in
calcification rates as a result of changes in ocean chemistry.
Mass bleaching has occurred on several occasions in Australia's Great
Barrier Reef (GBR) and elsewhere since the 1970s (Glynn, 1993; Hoegh-Guldberg
et al., 1997; Jones et al., 1997; Wilkinson, 1998). Particularly
widespread bleaching, leading to death of some corals, occurred globally in
1997-1998 in association with a major El Niño event. Bleaching was
severe on the inner GBR but less severe on the outer reef (Wilkinson, 1998;
Berkelmans and Oliver, 1999). This episode was associated with generally record-high
SSTs over most of the GBR region. This was a result of global warming trends
resulting from the enhanced greenhouse effect and regional summer warming from
the El Niño event, the combined effects of which caused SSTs to exceed
bleaching thresholds (Lough, 1999). Three independent databases support the
view that 1997-1998 SST anomalies were the most extreme in the past 95
years and that average SSTs off the northeast coast of Australia have significantly
increased from 1903 to 1994. Lowered seawater salinity as a result of flooding
of major rivers between Ayr and Cooktown early in 1998 also is believed to have
been a major factor in exacerbating the effects in the inshore GBR (Berkelmans
and Oliver, 1999). Solar radiation, which is affected by changes in cloud cover
and thus by El Niño, also may have been a factor (Brown, 1997; Berkelmans
and Oliver, 1999).
Although warming in Australia's coral reef regions on average is expected
to be slightly less than the global average, according to the SRES global warming
scenarios it may be in the range of 2-5°C by 2100. This suggests that
unless Australian coral reefs can adapt quickly to these higher temperatures,
they will experience temperatures above present bleaching thresholds (Berkelmans
and Willis, 1999) almost every year, well before the end of the 21st century
(Hoegh-Guldberg, 1999). Hoegh-Guldberg (1999) notes that apparent thresholds
for coral bleaching are higher in the northern GBR than further south, suggesting
that some very long-term adaptation has occurred. Coral reef biota may be able
to adapt, at least initially, by selection for the more heat-tolerant host and
symbiont species and genotypes that survived the 1997-1998 summer and by
colonization of damaged sites by more heat-resistant genotypes from higher latitudes
arriving as planktonic larvae. However, it is generally believed that the rate
and extent of adaptation will be much slower than would be necessary for reef
biota to resist the frequency and severity of high SST anomalies projected for
the middle third of the 21st century (medium to high confidence). The most likely
outlook is that mass bleaching, leading to death of corals, will become a more
frequent event on Australian coral reefs in coming decades.
Increasing atmospheric CO2 concentrations will decrease the carbonate
concentration of the ocean, thereby reducing calcification rates of corals (Gattuso
et al., 1998, 1999; Kleypas et al., 1999). This is complicated,
however, by the effects of possible changes in light levels, freshwater discharge,
current patterns, and temperature. For example, Lough and Barnes (2000) report
a historic growth stimulus for the Porites coral that they correlate
with increasing average SSTs. Thus, the net effect on Australian reefs up to
1980 appears to have been positive, but it is unclear whether decreased carbonate
concentration resulting from rapidly increasing CO2 concentration
will outweigh the direct temperature effect later in the 21st century, especially
if regional SSTs reach levels not experienced by the corals of the GBR during
As noted in Chapter 6, expected rates of sea-level rise
to 2100 would not threaten healthy coral reefs (most Australian reefs) but could
invigorate growth on reef flats. However, decreased calcification rates might
reduce the potential ability of the reefs to keep up with rapid sea-level rise.
Possible increases in tropical cyclone intensity with global warming also would
impact coral reefs (high confidence), along with nonclimatic factors such as
overexploitation and increasing pollution and turbidity of coastal waters by
sediment loading, fertilizers, pesticides, and herbicides (Larcombe et al.,
1996). Climate change could affect riverine runoff and associated stresses of
the reefs, including low-salinity episodes. Coupled with predicted rises in
sea level and storminess, bleaching-induced coral death also could weaken the
effectiveness of the reefs in protecting the Queensland coast and adversely
affect the biodiversity of the reef complex.
On the whole, mangrove processes are less understood than those for coral reefs
(Ellison, 1996). Mangroves occur on low-energy, sedimentary shorelines, generally
between mean- and high-tide levels. Australian mangroves cover approximately
11,500 km2 (Galloway, 1982). It is anticipated that they are highly
vulnerable but also highly adaptable to climate change. Studies over glacial/interglacial
cycles show that in the past mangroves have moved landward during periods of
rising sea level (Woodroffe, 1993; Wolanski and Chappell, 1996; Mulrennan and
Woodroffe, 1998). However, in many locations this will be inhibited now by coastal
development. Coastal wetlands are thought to be nursery areas for many commercially
important fish (e.g., barramundi), prawns, and mudcrabs.
In New Zealand, estuaries are the most heavily impacted of all coastal waters.
Most are situated close to or within urban areas (Burns et al., 1990).
Most have been modified by reclamation or flood control works and have water-quality
problems resulting from surrounding land use. Increasing coastal sedimentation
is having a marked effect on many estuaries. This may increase with increased
rainfall variability. In the South Island, increased coastal sedimentation has
disrupted fish nursery grounds and destroyed weed beds, reef sponges, and kelp
forests; in the North Island it has been linked to loss of seagrasses through
worsening water clarity (RSNZ, 1993; Turner, 1995).
Over a long period, warming of the sea surface is expected (on average) to
be associated with shoaling (thinning) of the mixing layer, lowering of phytoplankton
growth-limiting dissolved inorganic nutrients in surface waters (Harris et
al., 1987; Hadfield and Sharples 1996), and biasing of the ecosystem toward
microbial processes and lowered downward flux of organic carbon (Bradford-Grieve
et al., 1999). However, this would be modified regionally by any change
in the Pacific Ocean to a more El Niño-like mean state. Warming also
may lead to decreased storage of carbon in coastal ecosystems (Alongi et
There is now palaeo-oceanographic evidence documenting environmental responses
east of New Zealand to climatic warming, especially the Holocene "optimum"
(~6-7 ka) and interglacial optimum (~120-125 ka), when SSTs were 1-2°C
warmer than present. Immediately prior to and during those two periods, oceanic
production appears to have increased, as manifested by greater amounts of calcareous
nanoplankton and foraminifers (e.g., Lean and McCave, 1998; Weaver et al.,
1998). Other evidence suggests that storms in the New Zealand region may have
been more frequent in warmer epochs (Eden and Page, 1998), affecting the influx
of terrigenous material into the continental shelf (Foster and Carter, 1997).
There also may be a relationship between strong El Niño events and the
occurrence of toxic algal blooms in New Zealand waters (Chang et al.,
1998). Nevertheless, we do not know, over the longer term, how the oceanic biological
system in the southwest Pacific will be influenced by the interaction of ENSO
events with the overall warming trend.
South of the subtropical front, primary production is limited by iron availability
(Boyd et al., 1999), which has varied in the past. It is not known how
or whether aeolian iron supply to the Southern Ocean in the southwest Pacific
(Duce and Tindale, 1991) may be altered by climate change, although it could
be affected by changes in aridity and thus vegetation cover over Australia as
well as by strengthening of the westerlies. In any case, Harris et al.
(1988) demonstrate that the strength of the zonal westerly winds is linked to
recruitment of stocks of spiny lobsters over a wide area.
If reduction or cessation of North Atlantic or Antarctic bottomwater formation
were to occur (Manabe and Stouffer, 1994; Hirst, 1999), this could lead to significant
changes in deep ocean chemistry and dynamics, with wide ramifications for marine
life. The common southern hemisphere copepod Neocalanus tonsus could
be affected because it spends part of the year at depths between 500 and 1,300
m but migrates seasonally to surface waters, becoming the focus of feeding of
animals such as sei whales and birds (Bradford-Grieve and Jillett, 1998).
The northern part of New Zealand is at the southern extension of the distribution
of marine subtropical flora and fauna (Francis and Evans, 1993). With a warming
climate, it is possible that many species would become a more permanent feature
of the New Zealand flora and fauna and extend further south.
12.4.8. Landscape Management as a Goal for Conservation and Adaptation
Ecosystems that are used for food and fiber production form a mosaic in a landscape
in which natural ecosystems also are represented. Aquatic systems, notably rivers
and groundwater, often play a crucial role. Given the issues of fragmentation
and salinization in many parts of the region, especially Australia, landscape
management as an integrated approach (PMSEIC, 1999) may be one of the best ways
of achieving conservation goals and human needs for food and fiber in the face
of multiple stressesof which climate change is only one.
This complex interconnection of issues in land management is evident in most
parts of Australia and New Zealandnotably in the tropical coastal zone
of Queensland, where rapid population and economic growth has to be managed
alongside agricultural land use that impacts soil and riverine discharge into
the waters of the GBR, a growing tourist industry, fisheries, indigenous people's
rights, as well as the climatic hazards of tropical cyclones, floods, and droughts.
Climate change and associated sea-level rise are just one of several major issues
in this context that may be significant in adding stress to a complex system.
Similar complexities arise in managing other major areas such as the Murray-Darling
basin, where control of land degradation through farm and plantation forestry
is being considered as a major option, partly for its benefits in controlling
salinization and waterlogging and possibly as a new economic option with the
advent of incentives for carbon storage as a greenhouse mitigation measure.
Similar problems and processes apply in New Zealand, where plantation forestry
is regarded as a major option in land use and GHG mitigation.