Regional Warming

Climate model results show that globally average surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 - 2100 (IPCC 2001). Since the ttird Assessment Report (TAR) (IPCC 2001), future climate change projections have been updated regionally (Ruosteenoja et al. 2003). Lai (2004) have applied these to the South Pacific. Models all project increases in temperature: for the 2080s increases of between 1.0 and 3.1°C are indicated with the 2100 surface air temperature to be at least 2.5°C more than in 1990. tte models only simulate a marginal increase or decrease in annual precipitation (10 percent), with a drying in the subtropical South Pacific whilst the equatorial areas become wetter. During summer more precipitation is projected while an increase in daily rainfall intensity causing more heavier rainfall events is likely.

For Australia and New Zealand, scenarios based on more-detailed projections have been developed by CSIRO (2001) and NIWA (Wratt et al. 2004), respectively. Within 800 km of the Australian coast, a mean warming of 0.4 to 6.7°C is likely by the year 2080, relative to 1990. A tendency for decreased rainfall is likely over most of Australia, except Tasmania and New South Wales. A tendency for less run-off is also likely. In New Zealand, a warming of 0.5-3.5°C is likely by 2080s. tte mid-range projection for the 2080s is a 60% increase in annual mean westerly winds (Wratt et al. 2004). Consequently, precipitation is likely to be biased towards increases in the south and west, and decreases in the north and east.

Global warming from anthropogenic forcing is likely to increase extreme events. Extreme temperatures above 30 and 35°C are likely throughout the region. In Australia the number of days over 35°C increasses sihnificantly by 2020 with a 10 to 80% decrease in days below 0°C (Suppiah et al. 2007). In New Zealand there is likely to be a 50-100% decrease in frosts in the lower North Island, and a 50% decrease in the South Island, and a 10-100% increase in the number of days over 30°C (Mullan et al. 2001).

Under 3 x CO2 conditions, there is a 56% increase in the number of simulated tropical cyclones over north-eastern Australia with peak winds greater than 30 ms_1 (Walsh et al. 2004). Maximum tropical cyclone wind intensities could increase 5 to 10 percent by around 2050 (Walsh 2004) with peak precipitation rates likely to increase by 25 percent as a result of increases in wind intensities.

For Australia, increases in extreme daily rainfall are likely where average rainfall increases, or decreases slightly. For example, the intensity of the l-in-20 year daily-rainfall event increases by up to 10% in parts of South Australia by the year 2030 (Mclnnes et al. 2002), 5 to 70% by the year 2050 in Victoria (Whetton et al. 2002), up to 25% in northern Queensland by 2050 (Walsh et al. 2001) and up to 30% by the year 2040 in south-east Queensland (Abbs 2004). In New Zealand the frequency of high intensity rainfall is likely to increase, especially in western areas.

In the South Pacific, projected impacts include extended periods of drought. Projected changes in rainfall and evaporation have been applied to water balance models, indicating that reduced soil moisture and runoff is very likely over most of Australia and eastern New Zealand. Two climate models simulate up to 20% more droughts (defined as soil moisture in lowest 10% from 1974-20 03) over most of Australia by 2030 and up to 80% more droughts by 2070 in south-western Australia (Mpelasoka et al. 2005). By the 2080s in New Zealand, severe droughts (the current one-in-twenty year soil moisture deficit) are likely to occur at least twice to four times as often in the east of both islands, and parts of Bay of Plenty and Northland (Mullan et al. 2005). tte drying of pastures in eastern New Zealand in spring is very likely to be advanced by a month, with an expansion of droughts into spring and autumn.

Finally, an increase in fire danger in Australia is likely to be associated with a reduced interval between fires, increased fire-line intensity, a decrease in fire extinguishments and faster fire spread (Cary 2002;Tapper 2000 Williams et al. 2001). 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% (6-18) more days with veryhigh 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 1-5 days in some western areas. Fire season length is likely to be extended, starting earlier in August and finishing in May in many parts ofNew Zealand, compared with the current October to April season.

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