11.3 Assumptions about future trends

11.3.1 Climate

Regional climate change projections are provided in Chapter 11 of the Working Group I Fourth Assessment Report (Christensen et al., 2007). For Australia and New Zealand, these projections are limited to averages over two very broad regions: northern Australia and southern Australia (including New Zealand). More detailed regional projections are required to assess local impacts and are described below. Developed over the past five years, these are similar to those presented in the TAR, and include the full range of emissions scenarios from the IPCC Special Report on Emissions Scenarios (SRES: Naki?enovi? and Swart, 2000) (see Chapter 2.4.6). Some SRES scenarios have been suggested as surrogates for CO₂ concentration stabilisation scenarios: the SRES B1, B2 and A1B emissions scenarios are similar to the CO₂ stabilisation scenarios for 550 ppm by 2150, 650 ppm by 2200, and 750 ppm by 2250, respectively. Projected changes will be superimposed on continued natural variability including ENSO and the IPO. There is uncertainty about projected changes in ENSO as discussed in Chapter 10 of the Working Group I Fourth Assessment Report (Meehl et al., 2007).

In New Zealand, a warming of 0.1 to 1.4°C is likely by the 2030s and 0.2 to 4.0°C by the 2080s (Table 11.3). The mid-range projection for the 2080s is a 60% increase in the annual mean westerly component of wind speed (Wratt et al., 2004). Consequently, a tendency for increased precipitation is likely except in the eastern North Island and the northern South Island. Due to the projected increased winter precipitation over the Southern Alps, it is less clear whether snow will be reduced (MfE, 2004a), although snowlines are likely to be higher (Fitzharris, 2004). By 2100, there is likely to be a 5 to 20 day decrease in frosts in the lower North Island, 10 to 30 fewer frost days in the South Island, and a 5 to 70 day increase in the number of days with temperatures over 30°C (Mullan et al., 2001). The frequency of heavy rainfall is likely to increase, especially in western areas (MfE, 2004a).

In Australia, within 800 km of the coast, a mean warming of 0.1 to 1.3°C is likely by the year 2020, relative to 1990, 0.3 to 3.4°C by 2050, and 0.4 to 6.7°C by 2080 (Table 11.4). In temperate areas, this translates to 1 to 32 more days/yr over 35°C by 2020 and 3 to 84 more by 2050, with 1 to 16 fewer days/yr below 0°C by 2020 and 2 to 32 fewer by 2050 (Suppiah et al., 2007). A tendency for decreased annual rainfall is likely over most of southern and sub-tropical Australia, with a tendency for increases in Tasmania, central Northern Territory and northern NSW (Table 11.4). The 15-model average shows decreasing rainfall over the whole continent (Suppiah et al., 2007). A decline in runoff in southern and eastern Australia is also likely (see Section 11.4.1).

The area of mainland Australia with at least one day of snow cover per year is likely to shrink by 10 to 40% by 2020 and by 22 to 85% by 2050 (Hennessy et al., 2003). Increases in extreme daily rainfall are likely where average rainfall either increases or decreases slightly. For example, the intensity of the 1-in-20 year daily rainfall event is likely to increase by up to 10% in parts of South Australia by the year 2030 (McInnes et al., 2002), by 5 to 70% by the year 2050 in Victoria (Whetton et al., 2002), by up to 25% in northern Queensland by 2050 (Walsh et al., 2001) and by up to 30% by 2040 in south-east Queensland (Abbs, 2004). In NSW, the intensity of the 1-in-40 year event increases by 5 to 15% by 2070 (Hennessy et al., 2004). The frequency of severe tropical cyclones (Categories 3, 4 and 5) on the east Australian coast increases 22% for the IS92a scenario (IPCC, 1992) from 2000 to 2050, with a 200 km southward shift in the cyclone genesis region, leading to greater exposure in south-east Queensland and north-east NSW (Leslie and Karoly, 2007). For tripled pre-industrial CO₂ conditions, there is a 56% increase in the number of simulated tropical cyclones over north-eastern Australia with peak winds greater than 30 m/s (Walsh et al., 2004). Decreases in hail frequency are simulated for Melbourne and Mt. Gambier (Niall and Walsh, 2005).

Potential evaporation (or evaporative demand) is likely to increase (Jones, 2004a). Projected changes in rainfall and evaporation have been applied to water-balance models, indicating that reduced soil moisture and runoff are very likely over most of Australia and eastern New Zealand (see Section 11.4.1 and Meehl et al., 2007). Up to 20% more droughts (defined as the 1-in-10 year soil moisture deficit from 1974 to 2003) are simulated over most of Australia by 2030 and up to 80% more droughts by 2070 in south-western Australia (Mpelasoka et al., 2007). Projected increases in the Palmer Drought Severity Index for the SRES A2 scenario are indicated over much of eastern Australia between 2000 and 2046 (Burke et al., 2006). In New Zealand, severe droughts (the current 1-in-20 year soil moisture deficit) are likely to occur every 7 to 15 years by the 2030s, and every 5 to 10 years by the 2080s, in the east of both islands, and parts of Bay of Plenty and Northland (Mullan et al., 2005a). The drying of pastures in eastern New Zealand in spring is very likely to be advanced by one month, with an expansion of droughts into both spring and autumn.

An increase in fire danger in Australia is likely to be associated with a reduced interval between fires, increased fire intensity, a decrease in fire extinguishments and faster fire spread (Tapper, 2000; Williams et al., 2001; Cary, 2002). In south-east Australia, the frequency of very high and extreme fire danger days is likely to rise 4-25% by 2020 and 15-70% by 2050 (Hennessy et al., 2006). By the 2080s, 10-50% more days with very high and extreme fire danger are likely in eastern areas of New Zealand, the Bay of Plenty, Wellington and Nelson regions (Pearce et al., 2005), with increases of up to 60% in some western areas. In both Australia and New Zealand, the fire season length is likely to be extended, with the window of opportunity for controlled burning shifting toward winter.

Relative to the year 2000, the global-mean projection of sea-level rise by 2100 is 0.18 to 0.59 m, excluding uncertainties in carbon cycle feedbacks and the possibility of faster ice loss from Greenland and Antarctica (Meehl et al., 2007). These values would apply to Australia and New Zealand, but would be further modified by as much as ±25% due to regional differences in thermal expansion rates, oceanic circulation changes (as derived from atmosphere-ocean general circulation model experiments; Gregory et al., 2001) and by local differences in relative sea-level changes due to vertical land movements. An increase in westerly winds is probable south of latitude 45°S, with a strengthening of the East Australian Current and southern mid-latitude ocean circulation (Cai et al., 2005).