11.7.3.5 Temperature and Precipitation Extremes

Where the analysis has been done for Australia (e.g., Whetton et al., 2002), the effect on changes in extreme temperature due to simulated changes in variability is small relative to the effect of the change in the mean. Therefore, most regional assessments of changes in extreme temperatures have been based on adding a projected mean temperature change to each day of a station-observed data set. Based on the CSIRO (2001) projected mean temperature change scenarios, the average number of days over 35°C each summer in Melbourne would increase from 8 at present to 9 to 12 by 2030 and 10 to 20 by 2070 (CSIRO, 2001). In Perth, such hot days would rise from 15 at present to 16 to 22 by 2030 and 18 to 39 by 2070 (CSIRO, 2001). On the other hand, cold days become much less frequent. For example, Canberra’s current 44 winter days of minimum temperature below 0°C is projected to be 30 to 42 by 2030 and 6 to 38 by 2070 (CSIRO, 2001).

Changes in extremes in New Zealand have been assessed using a similar methodology and simulations (Mullan et al., 2001b). Decreases in the annual frequency of days below 0°C of 5 to 30 days by 2100 are projected for New Zealand, particularly for the lower North Island and the South Island. Increases in the annual number of days above 25°C of 10 to 50 days by 2100 are projected.

A range of GCM and regional modelling studies in recent years have identified a tendency for daily rainfall extremes to increase under enhanced greenhouse conditions in the Australian region (e.g., Hennessy et al., 1997; Whetton et al., 2002; McInnes et al., 2003; Watterson and Dix, 2003; Hennessy et al., 2004b; Suppiah et al., 2004; Kharin and Zwiers, 2005). Commonly, return periods of extreme rainfall events halve in late 21st-century simulations. This tendency can apply even when average rainfall is simulated to decrease, but not necessarily when this decrease is marked (see Timbal, 2004). Recently, Abbs (2004) dynamically downscaled to a resolution of 7 km current and enhanced greenhouse cases of extreme daily rainfall occurrence in northern New South Wales and southern Queensland as simulated by the CSIRO GCM. The downscaled extreme events for a range of return periods compared well with observations and the enhanced greenhouse simulations for 2040 showed increases of around 30% in magnitude, with the 1-in-40 year event becoming the 1-in-15 year event. Less work has been done on projected changes in rainfall extremes in New Zealand, although the recent analysis of Ministry for the Environment (2004) based on Semenov and Bengtsson (2002) indicates the potential for extreme winter rainfall (95th percentile) to change by between –6% and +40%.

Where GCMs simulate a decrease in average rainfall, it may be expected that there would be an increase in the frequency of dry extremes (droughts). Whetton and Suppiah (2003) examine simulated monthly frequencies of serious rainfall deficiency for Victoria, which show strong average rainfall decreases in most simulations considered. There is a marked increase in the frequency of rainfall deficiencies in most simulations, with doubling in some cases by 2050. Using a slightly different approach, likely increases in the frequency of drought have also been established for the states of South Australia, New South Wales and Queensland (Walsh et al., 2002; McInnes et al., 2003; Hennessy et al., 2004c). Mullan et al. (2005) show that by the 2080s in New Zealand there may be significant increases in drought frequency in the east of both islands.