11.2 Current sensitivity/vulnerability
11.2.1 Climate variability and 20th-century trends
In this section, climate change is taken to be due to both natural variability and human activities. The relative proportions are unknown unless otherwise stated. The strongest regional driver of climate variability is the El Niño-Southern Oscillation (ENSO). In New Zealand, El Niño brings stronger and cooler south-westerly airflow, with drier conditions in the north-east of the country and wetter conditions in the south-west (Gordon, 1986; Mullan, 1995). The converse occurs during La Niña. In Australia, El Niño tends to bring warmer and drier conditions to eastern and south-western regions, and the converse during La Niña (Power et al., 1998). The positive phase of the Inter-decadal Pacific Oscillation (IPO) strengthens the ENSO-rainfall links in New Zealand and weakens links in Australia (Power et al., 1999; Salinger et al., 2004; Folland et al., 2005).
In New Zealand, mean air temperatures have increased by 1.0°C over the period 1855 to 2004, and by 0.4°C since 1950 (NIWA, 2005). Local sea surface temperatures have risen by 0.7°C since 1871 (Folland et al., 2003). From 1951 to 1996, the number of cold nights and frosts declined by 10-20 days/yr (Salinger and Griffiths, 2001). From 1971 to 2004, tropical cyclones in the south-west Pacific averaged nine/year, with no trend in frequency (Burgess, 2005) or intensity (Diamond, 2006). The frequency and strength of extreme westerly winds have increased significantly in the south. Extreme easterly winds have decreased over land but have increased in the south (Salinger et al., 2005a). Relative sea-level rise has averaged 1.6 ± 0.2 mm/yr since 1900 (Hannah, 2004). Rainfall has increased in the south-west and decreased in the north-east (Salinger and Mullan, 1999) due to changes in circulation linked to the IPO, with extremes showing similar trends (Griffiths, 2007). Pan evaporation has declined significantly at six out of nineteen sites since the 1970s, with no significant change at the other thirteen sites (Roderick and Farquhar, 2005). Snow accumulation in the Southern Alps shows considerable interannual variability but no trend since 1930 (Owens and Fitzharris, 2004).
In Australia, from 1910 to 2004, the average maximum temperature rose 0.6°C and the minimum temperature rose 1.2°C, mostly since 1950 (Nicholls and Collins, 2006). It is very likely that increases in greenhouse gases have significantly contributed to the warming since 1950 (Karoly and Braganza, 2005a, b). From 1957 to 2004, the Australian average shows an increase in hot days (³35°C) of 0.10 days/yr, an increase in hot nights (³20°C) of 0.18 nights/yr, a decrease in cold days (²15°C) of 0.14 days/yr and a decrease in cold nights (²5°C) of 0.15 nights/yr (Nicholls and Collins, 2006). Due to a shift in climate around 1950, the north-western two-thirds of Australia has seen an increase in summer monsoon rainfall, while southern and eastern Australia have become drier (Smith, 2004b). While the causes of decreased rainfall in the east are unknown, the decrease in the south-west is probably due to a combination of increased greenhouse gas concentrations, natural climate variability and land-use change, whilst the increased rainfall in the north-west may be due to increased aerosols resulting from human activity, especially in Asia (Nicholls, 2006). Droughts have become hotter since about 1973 because temperatures are higher for a given rainfall deficiency (Nicholls, 2004). From 1950 to 2005, extreme daily rainfall has increased in north-western and central Australia and over the western tablelands of New South Wales (NSW), but has decreased in the south-east, south-west and central east coast (Gallant et al., 2007). Trends in the frequency and intensity of most extreme temperature and rainfall events are rising faster than the means (Alexander et al., 2007). South-east Australian snow depths at the start of October have declined 40% in the past 40 years (Nicholls, 2005). Pan evaporation averaged over Australia from 1970 to 2005 showed large interannual variability but no significant trend (Roderick and Farquhar, 2004; Jovanovic et al., 2007; Kirono and Jones, 2007). There is no trend in the frequency of tropical cyclones in the Australian region from 1981 to 2003, but there has been an increase in intense systems (very low central pressure) (Kuleshov, 2003; Hennessy, 2004). Relative sea-level rise around Australia averaged 1.2 mm/yr from 1920 to 2000 (Church et al., 2004).
The offshore islands of Australia and New Zealand have recorded significant warming. The Chatham Islands (44°S, 177°W) have warmed 1°C over the past 100 years (Mullan et al., 2005b). Macquarie Island (55°S, 159°E) has warmed 0.3°C from 1948 to 1998 (Tweedie and Bergstrom, 2000), along with increases in wind speed, precipitation and evapotranspiration, and decreases in air moisture content and sunshine hours since 1950 (Frenot et al., 2005). Campbell Island (53°S, 169°E) has warmed by 0.6°C in summer and 0.4°C in winter since the late 1960s. Heard Island (53°S, 73°E) shows rapid glacial retreat and a reduced area of annual snow cover from 1948 to 2001 (Bergstrom, 2003).