12.1.5.2. Uncertainties and Probabilistic Scenarios

Some of the Australasian scenarios include uncertainty bands, based on the ranges of global warmings resulting from the IPCC IS92 emissions scenarios, the IPCC range of global sensitivity, or ranges of estimates of Australasian temperature or rainfall changes from different GCMs. Unquantified additional sources of uncertainty include changes in emission scenarios, such as to the new SRES scenarios; regional effects of biospheric feedback; and regional effects of global aerosol distributions. Quantification of potential changes in extreme events, tropical cyclones, and ENSO also are major uncertainties (see below), and uncertainty about the strength of the westerly circulation and hence rainfall regimes is a source of uncertainty for New Zealand. The modeled lag in warming in the Southern Ocean in the 20th century appears to be greater than that observed (Whetton et al., 1996a), but this has not yet been thoroughly analyzed. Other conceivable lower probability, high-impact changes (see Chapter 3) such as changes in ocean circulation, ENSO behavior, or tropical cyclones could have important regional impacts.

Probabilistic scenarios for risk and adaptation analyses (see Section 12.8.4), based on the quantifiable range of uncertainties, have been explored by CSIRO (Pittock, 1999; Jones, 2000; Pittock and Jones, 2000).

12.1.5.3 Changes in Extreme Events and Sea Level

Pittock et al. (1999) have summarized the past importance of extreme events for Australia and prospects for the future. Major climatic hazards arise in Australia and New Zealand from tropical cyclones, floods, droughts, windstorm, snowstorm, wildfires, landslides, hail, lightning, heat waves, frost, and storm surges. Events that are directly related to temperature are more predictable (more heat waves, fewer frosts) than those associated with wind and rain; Chapter 3 discusses relevant projections and confidence levels (see Table 3-10). The incidence of wildfire in Australia is expected to increase with global warming (Beer and Williams, 1995; Pittock et al., 1999; Williams et al., 2001), as is that of landslides and storm surges (the latter because of both higher mean sea level and increased storm intensities). Changes in hail and lightning frequencies are uncertain, although there are some arguments for expected increases (Price and Rind, 1994; McMaster, 1999; Pittock et al., 1999).

More intense tropical cyclones in the Australian region (see Table 3-10; Walsh and Ryan, 2000) would have serious implications for storm-surge heights, wind damages, and flooding. If they were to travel further poleward (Walsh and Katzfey, 2000), they would be more likely to impact on coastal regions in the southwest of western Australia, southern Queensland, and the northern NSW coastal region, as well as northern parts of New Zealand. The locations of tropical cyclone genesis in the region are correlated with ENSO (Evans and Allan, 1992; Basher and Zheng, 1995), so any change in the mean state of the tropical Pacific may affect the risk of tropical cyclone occurrence in particular locations.

Mid-latitude storms also may increase in intensity (see Table 3-10), and their frequency and location could change—for example, as a result of changes in the westerlies and ENSO. This would impact return periods for mid-latutude storm surges, high winds, and other phenomena.

Interannual variability in ENSO leads to major floods and droughts in Australia and New Zealand. Such variations are expected to continue under enhanced greenhouse conditions, though possibly with greater hydrological extremes as a result of more intense rainfall in La Niña years and more intense drought resulting from higher rates of evaporation during El Niño years (Walsh et al., 1999). A more El Niño-like mean state of the tropical Pacific Ocean (see Table 3-10; Cai and Whetton, 2000) would imply greater drought frequency (Kothavala, 1999; Walsh et al., 2000), as does the drying trend found over the Murray-Darling Basin in recent AOGCM simulations (Arnell, 1999).

Mean sea level is expected to increase, with local and regional variations as a result of land-sea movements and changes to ocean currents and climatic forcing (see Chapter 3). In addition, local and regional meteorological forcing leads to temporary fluctuations in sea level and extreme events that may cause coastal inundation. In New Zealand, storm surges of as much as about 1 m are possible at open-coast locations (Heath, 1979; Bell et al., 1999). Storm surges in tropical Australia can be several meters as a result of tropical cyclonic forcing and shallow continental shelfs (Hubbert and McInnes, 1999a,b; McInnes et al., 1999).

The actual height reached by a storm surge depends not only on the location and intensity of the storm but on its timing relative to the tides, coastal bathymetry and topography, and slower variations such as those from ENSO. The latter contribute to significant local sea-level variations around the coasts of Australia (Chiera et al., 1997) and New Zealand (Bell et al., 1999). In addition, any changes in storm intensities, frequencies, and locations will change the average time between surges of a given magnitude at particular locations.

12.1.5.4 New SRES Scenarios

Interim characterizations of regional climate changes to 2100 associated with the SRES emissions scenarios have been provided by Hulme and Sheard (1999) and Carter et al. (2000). However, they do not consider aerosol-induced spatial effects, and they use linear scaling of regional patterns of change from seven coupled GCM models, according to a range of global mean warmings generated using MAGICC (Wigley, 1995; Wigley et al., 1997).

Over Australia, these studies show warmings in the 2080s higher than the IS92 scenarios, with similar spatial patterns. In New Zealand, warmings in the 2080s are estimated to be from 0.5 to >2.0°C. Projected precipitation changes are large (>1 standard deviation of the simulated 30-year variability) over much of southern Australia, with a decrease over the mainland in both summer and winter and an increase over Tasmania in winter. Over the South Island of New Zealand, an increase is predicted. For the 2080s, projected decreases in annual rainfall in the southwest of western Australia range from about zero (B1 low scenario) to between 30 and 50% (A2 high scenario). Projected rainfall increases over the South Island of New Zealand of 0-10% (B1) to 10-20% (A2) should be regarded with caution because the AOGCM simulations do not fully incorporate the important influence of the Southern Alps on South Island rainfall patterns.

The SRES scenarios have not yet been applied in any detailed studies of impacts in the region. Unlike parts of the northern hemisphere, high regional concentrations of sulfate aerosols are not expected in the Australasian region under any accepted scenario, so any increase in warming resulting from reduced sulfate aerosols will be less over Australia and New Zealand than in some regions of the northern hemisphere.

To date, impact and vulnerability studies in Australia and New Zealand in general have not taken account of specific socioeconomic scenarios for the future, such as those laid out in the SRES. Thus, vulnerabilities have been based on projected climate change impacts and adaptation, assuming the present socioeconomic situation, in some cases with a qualitative allowance for expected socioeconomic trends (e.g., increased competition for water supplies, increased population and investment in coastal zones).

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