This region is defined here as Australia, New Zealand, and their outlying islands. Australia is a large, flat continent spanning the tropics to mid-latitudes, with relatively nutrient-poor soils and a very arid interior, whereas New Zealand is much smaller, mountainous, and well watered. Both have "Gondwanan" ecosystems and unique flora and fauna. They have been subject to significant human influences-particularly from fire, agriculture, deforestation, and introduced exotic plants and animals. The total land area is 8 million square kilometers, and the population is 22 million. In contrast to other Organisation for Economic Cooperation and Development (OECD) countries, commodity-based industries of agriculture and mining dominate the economies and exports. Tourism is a major and rapidly growing industry.
Climate: Australasia's climate is strongly influenced by the surrounding oceans. Key climatic features include tropical cyclones and monsoons in northern Australia; migratory mid-latitude storm systems in the south, including New Zealand; and the El Niño-Southern Oscillation (ENSO) phenomenon, which causes floods and prolonged droughts, especially in eastern Australia. Unfortunately, climate models at present cannot provide reliable predictions for these features under climate change.
Climate trends: The region's climatic trends are consistent with those of other parts of the world. Mean temperatures have risen by up to 0.1°C per decade over the past century; nighttime temperatures have risen faster than daytime temperatures; and the past decade has seen the highest mean annual temperatures ever recorded. Increases in the frequency of heavy rainfalls and average rainfall have been reported for large areas of Australia. Sea levels have risen on average by about 20 mm per decade over the past 50 years.
Climate scenarios: Climate modeling and climate change scenarios for the region are relatively well developed. For Australia, the recently revised (1996) Commonwealth Scientific and Industrial Research Organization (CSIRO) scenarios for 2030 indicate temperature increases of 0.3-1.4°C and rainfall changes of up to 10% in magnitude (decreases in winter, increases or decreases in summer, and overall a tendency for decreases). The projected changes for 2070 are about twice those of the 2030 changes. Increases in the intensity of heavy rainfall events are indicated. For New Zealand, the temperature increases are expected to be similar to those for Australia, but the recent revision indicates the possibility of an increase in westerly winds (unlike the decreases of previous scenarios)-and hence precipitation increases in the west and precipitation decreases in the east. The changes in scenarios serve to caution against overinterpretation of impact studies based on any single scenario.
Ecosystems: In responding to climate change, Australasia's biota may face a greater rate of long-term change than ever before. They also must respond in a highly altered landscape fragmented by urban and agricultural development. There is ample evidence for significant potential impacts. Alterations in soil characteristics, water and nutrient cycling, plant productivity, species interactions (competition, predation, parasitism, etc.), and composition and function of ecosystems are highly likely responses to increases in atmospheric carbon dioxide (CO2) concentration and temperature and to shifts in rainfall regimes. These changes would be exacerbated by any increases in fire occurrence and insect outbreaks.
Aquatic systems will be affected by the disproportionately large responses in runoff, riverflow and associated nutrients, wastes, and sediments that are likely from changes in rainfall and rainfall intensity and by sea-level rise in estuaries, mangroves, and other low-lying coastal areas. Australia's Great Barrier Reef and other coral reefs are vulnerable to temperature-induced bleaching and death of corals, in addition to sea-level rise and weather changes. However, there is evidence that the growth of coral reef biota may be sufficient to adapt to sea-level rise. Our knowledge of climate change impacts on aquatic and marine ecosystems is relatively limited.
Prediction of climate change effects is very difficult because of the complexity of ecosystem dynamics. Although Australasia's biota and ecosystems are adapted to the region's high climate variability (exemplified in arid and ENSO-affected areas), it is unclear whether this will provide any natural adaptation advantage. Many species will be able to adapt through altered ecosystem relationships or migration, but such possibilities may not exist in some cases, and reduction of species diversity is highly likely. Climate change will add to existing problems such as land degradation, weed infestations, and pest animals and generally will increase the difficulties and uncertainty involved in managing these problems.
The primary human adaptation option is land-use management-for example, by modification of animal stocking rates in rangelands, control of pests and weeds, changed forestry practices, and plantings along waterways. Research, monitoring, and prediction, both climatic and ecological, will be necessary foundations to human adaptive responses. Active manipulation of species generally will not be feasible in the region's extensive natural or lightly managed ecosystems, except for rare and endangered species or commercially valuable species. In summary, it must be concluded that some of the region's ecosystems are very vulnerable to climate change.
Hydrology and water resources: The four hydrological situations of most concern to the region are drought-prone areas, flood-prone urban areas, low-lying islands, and alpine snowfields. Model simulations suggest changes of as much as ±20% in soil moisture and runoff in Australia by 2030, with considerable variation from place to place and season to season and with the possibility of an overall reduction in average runoff. Water shortages would sharpen competition among various uses of water, especially where large diversions are made for economic purposes. One study shows Australia's major Murray-Darling River system facing constraints on existing irrigation uses and/or harm to the riverine environment. More frequent high-rainfall events would enhance groundwater recharge and dam-filling events but also would increase the impacts of flooding, landslides, and erosion. A preliminary study for an urban area near Sydney showed a tenfold increase in the potential damage of the "100-year" flood under a doubled CO2 scenario. Water supplies on atolls and low-lying islands are vulnerable to saltwater intrusion of groundwater from rising sea levels and to possible rainfall reductions. Reduced snow amounts and a shorter snow season appear likely and would decrease the amenity value of the mountains and the viability of the ski industry. The glaciers of New Zealand's Southern Alps are likely to shrink further.
Adaptation options include integrated catchment management, changes to water pricing systems, water efficiency initiatives, building or rebuilding engineering structures, relocation of buildings, urban planning and management, and improved water supply measures in remote areas and low-lying islands. The financial exposure, and cost involved in potential adaptations, indicate a high vulnerability with respect to hydrology.
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