|Working Group II: Impacts, Adaptation and Vulnerability|
|Other reports in this collection|
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 the Holocene.
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 al., 1998).
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.
Other reports in this collection