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 (>35°C) of 0.10 days/yr, an increase in hot nights (>20°C) of 0.18 nights/yr, a decrease in cold days (<15°C) of 0.14 days/yr and a decrease in cold nights (<5°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 warmed0.3°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).
11.2.2 Human systems: sensitivity/vulnerability to climate and weather
Extreme events have severe impacts in both countries (Box 11.1). In Australia, around 87% of economic damage due to natural disasters (storms, floods, cyclones, earthquakes, fires and landslides) is caused by weather-related events (BTE, 2001). From 1967 to 1999, these costs averaged US$719 million/yr, mostly due to floods, severe storms and tropical cyclones. In New Zealand, floods are the most costly natural disasters apart from earthquakes and droughts, and total flood damage costs averaged about US$85 million/yr from 1968 to 1998 (NZIER, 2004).
11.2.3 Natural systems: sensitivity/vulnerability to climate and weather
Some species and natural systems in Australia and New Zealand are already showing evidence of recent climate-associated change (Table 11.1). In many cases, the relative contributions of other factors such as changes in fire regimes and land use are not well understood.
11.2.4 Sensitivity/vulnerability to other stresses
Human and natural systems are sensitive to a variety of stresses independent of those produced by climate change. Growing populations and energy demands have placed stress on
Box 11.1. Examples of extreme weather events in Australia and New Zealand*
Droughts: In Australia, the droughts of 1982-1983,19911995 and 2002-2003 cost US$2.3 billion, US$3.8 billion and US$7.6 billion, respectively (Adams et al., 2002; BoM, 2006a). In New Zealand, the 1997-1998 and 1998-1999 droughts had agricultural losses of US$800 million (MAF, 1999).
Sydney hailstorm, 14 April 1999: With the exception of the droughts listed above, this is the most expensive natural disaster in Australian history, costing US$1.7 billion, of which US$1.3 billion was insured (Schuster et al., 2005).
Eastern Australian heatwave, 1 to 22 February 2004:
About two-thirds of continental Australia recorded maximum temperatures over 39°C. Temperatures reached 48.5°C in western New South Wales. The Queensland ambulance service recorded a 53% increase in ambulance call-outs (Steffen et al., 2006).
Canberra fire, 19 January 2003: Wildfires caused US$261 million damage (Lavorel and Steffen, 2004; ICA, 2007). About 500 houses were destroyed, four people were killed and hundreds injured. Three of the city's four dams were contaminated for several months by sedimentladen runoff.
South-east Australian storm, 2 February 2005: Strong winds and heavy rain led to insurance claims of almost US$152 million (ICA, 2007). Transport was severely disrupted and beaches were eroded.
Tropical cyclone Larry, 20 March 2006: Significant damage or disruption to houses, businesses, industry, utilities, infrastructure (including road, rail and air transport systems, schools, hospitals and communications), crops and state forests, costing US$263 million. Fortunately, the 1.75 m storm surge occurred at low tide (BoM, 2006b; Queensland Government, 2006).
New Zealand floods: The 10 April 1968 Wahine storm cost US$188 million, the 26 January 1984 Southland floods cost US$80 million, and the February 2004 North Island floods cost US$78 million (Insurance Council of New Zealand, 2005).
* All costs are adjusted to 2002-2006 values.
energy supply infrastructure. In Australia, energy consumption has increased 2.5%/yr over the past 20 years (PB Associates, 2007). Increases in water demand have placed stress on supply capacity for irrigation, cities, industry and environmental flows. Increased water demand in New Zealand has been due to agricultural intensification (Woods and Howard-Williams, 2004)
and has seen the irrigated area of New Zealand increase by around 55% each decade since the 1960s (Lincoln Environmental, 2000). Per capita daily water consumption is 180-300 litres in New Zealand and 270 litres for Australia (Robb and Bright, 2004). In Australia, dryland salinity, alteration of river flows, over-allocation and inefficient use of water resources, land clearing, intensification of agriculture, and fragmentation of ecosystems still represent major stresses (SOE, 2001; Cullen, 2002). From 1985 to 1996, Australian water demand increased by 65% (NLWRA, 2001). Invasive plant and animal species pose significant environmental problems in both countries, particularly for agriculture and forestry (MfE, 2001; SOE, 2001); for example, Cryptostegia grandiflora (Kriticos et al., 2003a, b).
Taxa or system
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