Many activities and ecosystems in Australia and New Zealand are sensitive to climate change, with positive and negative effects. Much climate sensitivity information is still qualitative, and there are substantial uncertainties in predictions of regional- to local-scale climate changes—especially rainfall changes and changes in extreme events. Thus, comprehensive, quantitatively based cross-sectoral estimates of net Australasian costs of climate change impacts are not yet available. Confidence remains very low in the impact estimate for Australia and New Zealand of -1.2 to -3.8% of GDP for an equivalent doubling of CO₂ concentrations (Basher et al., 1998). This estimate is based on climate change scenarios that are now outdated; does not include some potentially important impacts, including changes in weeds, pests and diseases, storm surges, and urban flooding; and does not account for possible adaptations to climate change.

Despite the uncertainties, there is a large body of knowledge and more agreement on changes than often is realized (e.g., between different coupled ocean-atmosphere climate models on the sign of the change in rainfall over large areas of Australia; see Section 12.1.5.1). There also is qualitative agreement on reduced water supplies in large agricultural regions of Australia (see Sections 12.3.1 and 12.5.6).

Potential net impacts on grazing, crops, and forests critically depend on the balance between the competing effects of warmings, positive or negative rainfall changes, direct physiological effects of higher CO₂ concentrations, and spatial and temporal variations in soil fertility. We expect with medium to high confidence that beneficial physiological effects of higher CO₂ concentration will become less dominant with time and effects of warming will become more damaging, especially given the expected tendency toward greater aridity over much of Australia. Impacts (and thus climate change benefits or damages) will not scale linearly with increasing GHG concentrations.

The following subsections draw together material from this chapter that is pertinent to the "policy-relevant questions" identified by the IPCC Bureau, as well as regional policy concerns.

Regional consequences documented in this chapter from floods, droughts, and temperature changes associated with recent ENSO events include losses in the pastoral agriculture sector from droughts (Australia, 1-2% of GDP; New Zealand, NZ$422 million estimated farm-gate costs from 1997 to 1999) and impacts on streamflow, water supply, stream ecology, horticulture, some commercial fisheries, and toxic algal blooms. Parts of the GBR have suffered mass coral bleaching from high SSTs (possibly related to ENSO and recent warming trends) and/or lowered salinity as a result of floods. Storms in the New Zealand region may have been more frequent in past warm epochs, leading to a greater influx of terrigenous material onto the continental shelf. Oceanic productivity east of New Zealand apparently increased immediately prior to and during warm periods 6,000-7,000 years ago and 120,000-125,000 years ago.

Extreme climatic events resulting from natural climatic variability in the past century have caused major damage and loss of life in Australia and New Zealand (see Sections 12.1.5.3 and 12.6.2). These extremes are expected to change in intensity, location-specific frequency, and sequence as a result of climate change (see Table 3-10 and Section 12.1.5.3), with major impacts on infrastructure and society unless strong adaptation measures are adopted.

12.9.3.1. Abrupt or Nonlinear Changes in Impacts

Several potential abrupt or nonlinear responses to climate change are listed in Table 12-1. In some cases, they involve a reversal of the sign of the effect with greater climate change; in others they result from the onset or acceleration of a biophysical or socioeconomic process that occurs or greatly accelerates beyond some threshold of climate change. Such thresholds can be quite location- and system-specific and need to be identified in collaboration between climatologists and stakeholders. The frequency with which thresholds are exceeded often is used in engineering in the form of design criteria to withstand an event with a certain "return period," or average time between occurrences. It also can manifest itself as a change from an average profit to an average loss for a given enterprise such as a farm.

Table 12-1: Nonlinear or rapid climate change responses identified in this chapter.
System	Description of Change	Certainty and Timing	Section
Great Barrier Reef	Reef death or damage from coral bleaching	Medium to high, next 20-50 years	12.4.7
Deep ocean	Chemical, dynamical, and biological changes from reduction in bottomwater formation	Low to medium, century time scale	12.4.7
Southwest of western Australia	Rapid loss of species with narrow annual mean temperature ranges	High, next 20-50 years	12.4.2
Australian alpine ecosystems	Loss of species as a result of warming and reduced snow cover	Medium to high, next 50 years	12.4.4
Insect-borne disease spread	Conditions more favorable to mosquitoes and other disease vectors	High potential, growing vulnerability	12.7.1
Agriculture	Shift from net profit to loss as a result of increased frequency of bad years	High potential in some places, next 20-50 years	12.8.4
Agriculture	Shift from positive to negative balance between benefits of increased CO2 and losses from increasing aridity	High potential in parts of Australia, 50-100 years	12.5.2, 12.5.3
Built infrastructure	Change in magnitude and frequency of extremes to exceed design criteria, leading to rapid increases in potential damages to existing infrastructure	Medium to high in tropical coastal and riverine situations, next 30-50 years	12.6.1

12.9.3.2. Interactions with Other Environmental and Social Factors

Some of the region's ecosystems are extremely vulnerable to invasion by exotic animal and plant species because of relative isolation before European settlement. Land-use changes have left some systems and areas more vulnerable to added stresses from climate changes, as a result of salinization or erosion, and because of ecosystem fragmentation, which lessens adaptation options for movement of species threatened by changing habitats (see Section 12.4.8).

Increasing human populations and development in coastal areas and on floodplains cause increasing vulnerability to tropical cyclones, storm surges, and riverine flooding episodes (see Sections 12.4.7 and 12.6.4), which may become more frequent with climatic change. Development has reduced the area and water quality of many estuaries, increasing the vulnerability of their ecosystems to sea-level rise and climate changes. Pressures on coral reefs including the GBR include increased coastal development, fisheries, tourism, and runoff of nutrients, chemicals, and sediment from land, as well as climate-related stresses such as rising sea level, rising temperatures, changes in tropical storm frequency, and acidification of the ocean from increasing CO₂ concentrations (see Section 12.4.7).

Poorer communities, including many indigenous settlements, are more vulnerable to climate-related natural hazards and stresses on health (see Section 12.7.6) because they often are in exposed areas and have less adequate housing, health, and other resources for adaptation.

Capacity to adapt is a function not only of the magnitude and critical nature of the climatic change but also of the demographics, economy, institutional capacity, and technology of a society. Thus, alternative socioeconomic futures will lead to different capacities to adapt. This has hardly been explored to date in Australia and New Zealand in relation to climate change.

12.9.3.3. Regional-Global Interactions

Reliance on exports of agricultural and forest products makes the region sensitive to changes in commodity prices produced by changes in climate elsewhere (see Section 12.5.9) and to increases in global forests as a result of carbon sink policies. Other extra-regional factors include increased risks of invasion by exotic pests, weeds, and diseases; pressures from immigration from neighboring low-lying Pacific island territories impacted by sea-level rise (see Section 12.7.6); and international agreements that constrain net emissions of GHGs. In an era of increasing globalization, these issues may assume more importance, although this report hardly touches on them.

IPCC Homepage