NSW climate
Historical climate variability in NSW
NSW contains four distinct climate zones due to the influence of the Great Dividing Range: the coastal strip, the high country, the Western Slopes of the range, and the flatter country in the west of the State. Temperatures and rainfall vary greatly depending on the region and the time of year. Annual maximum temperatures in NSW range from coldest in the high country (median < 15°C in southern parts) to warmest in the north-west (median > 27°C). Overnight minimum temperatures range from < 6°C in the southern high country to > 12°C in inland northern NSW. Moderated by the warm waters of the Tasman Sea, milder temperatures occur along the coastal strip.
Actual rainfall varies to an even greater degree: any location can record less than half, or more than twice, the average rainfall, and this can happen year after year. These rainfall variations produce a cycle of droughts and floods (CSIRO 2004a).
The El Niño – Southern Oscillation (ENSO) phenomenon plays an important role in this high climatic variability. The Southern Oscillation is a shift in air pressure between two regions of the globe: Australia–New Guinea–South-east Asia, and the South-east Pacific. This shifting air pressure results in climate patterns that are generally referred to as 'El Niño' and 'La Niña' events. In the western Pacific, El Niño events are characterised by dry conditions, La Niña by wet conditions.
El Niño
Normally, strong trade winds blow from east to west across the Pacific, pushing surface water westward so that the western Pacific is about one metre higher than the eastern Pacific. The sun warms the surface water, which accumulates near Indonesia and New Guinea and lies above the cooler water in that region. Water from the warm pool flows into the northern hemisphere as the Kuroshio Current and into the southern hemisphere as the East Australian Current. The heat flux associated with these currents brings above-average rainfall to Japan and the east coast of Australia.
During an El Niño event, the easterly trade winds weaken and the warm pool of water moves eastwards towards Central America. The removal of the warm-water pool from the western Pacific leaves cooler sea-surface temperatures, reducing the strength and heat flux of the Kuroshio and East Australian currents. Rainfall decreases correspondingly and the climate of the western Pacific becomes drier, sometimes leading to drought conditions that can affect the eastern two-thirds of Australia.
El Niño events occur about every four to seven years and typically last for about 12–18 months. They are a natural part of the climate system and have been affecting the Pacific Basin for thousands of years (BoM 2006a).
La Niña
In a La Niña event, air pressure becomes higher over the south-east Pacific region as the trade winds intensify. Tropical rain is then centred over Indonesia and northern Australia, and systems spinning off from this region can produce torrential rain and flooding in eastern Australia.
ENSO and variability
The Southern Oscillation Index (SOI) provides a measure of the strength and phase of the Southern Oscillation. Figure A1.1 shows the annual average SOI values between the years 1876 and 2005. Sustained negative values often indicate El Niño episodes, reducing NSW rainfall and in some instances leading to drought. Before the present dry conditions, droughts occurred in 1978, 1982, 1988, 1991 and 1993, corresponding generally to SOI readings in the range of 0 to –10. The most recent strong El Niño was in 1997–98.
Figure A1.1: Annual average Southern Oscillation Index 1876–2005
Source: Derived from Bureau of Meteorology data 2006
Sustained positive values of the SOI generally indicate a La Niña event. The most recent strong La Niña was in 1988–89; a moderate La Niña event occurred 10 years later which weakened back to neutral conditions before reforming for a shorter period in 1999–2000. This last event finished in autumn 2000 (BoM 2006b).
El Niño and La Niña events may be separated by long periods of less extreme conditions. Alternatively, the climate may swing between the two for several cycles with either or both returning every few years. The Southern Oscillation shifting regional air pressure causes this seesaw effect. In this way the ENSO phenomenon has influenced Australia's high rainfall variability and the frequencies of tropical cyclones, heat waves, bushfires and frosts.
Drought
The term 'drought' is often used to refer to generally dry conditions. However, three different types of drought exist (ESPERE 2006):
Meteorological drought is generally defined by comparing actual rainfall with average rainfall for a particular place and time. Meteorological drought is, therefore, carefully defined for a particular location using specific criteria. Drought defined in this way considers only the reduction in rainfall amounts. It does not take into account the effects of the lack of water on reservoirs, human needs or agriculture.
Agricultural drought occurs when the available soil moisture is inadequate to sustain agricultural production. This type of drought is influenced not only by the amount of rainfall, but also by the inefficient use of that water. Agricultural drought is typically seen after meteorological drought but before a hydrological drought.
Hydrological drought is associated with the effect of low rainfall on water levels in rivers, reservoirs, lakes and groundwater aquifers. Hydrological droughts are usually noticed some time after meteorological droughts. First, precipitation decreases and, some time after that, water levels in rivers and lakes drop. Changes in water levels can affect ecosystem health, hydro-electrical power generation, and recreational, industrial and urban water use.
The relationships between the three types of drought are shown in Figure A1.2.
Figure A1.2: Relationship between various types of drought and duration of drought events
Source: BoM 2006c
Drought is not simply a period of low rainfall – if it was, much of inland Australia would be in almost perpetual drought. Rather, it is the interaction between climatic events (precipitation, winds, temperature and humidity), land-management practices and various water uses that creates economic, social and environmental impacts (BoM 2006c). The effects of a persistent lack of rainfall can be exacerbated by extreme weather (such as high temperatures and winds) and certain types of land- and water-management practices. For example, a lack of rainfall, high temperatures and evaporation can combine with low availability of water storages to create difficult circumstances for agriculture. Increased irrigation may then be used to forestall agricultural drought or allow production to occur even when the water available from rainfall is inadequate. This can create or exacerbate hydrological drought, reducing flows to water storages (for human use) and lakes, ponds, and wetlands (for environmental needs). Alternatively, hydrological drought may precede or coincide with agricultural drought.
During the current SoE reporting cycle (2003–06), much of NSW has experienced drier than average conditions, with regions throughout the State suffering record meteorological drought. Beginning with dry conditions from a weak to moderate El Niño event in 2002–03, a major Australian drought developed, ranking in severity and extent with the extreme droughts of 1895–1903 and 1982–83. In some areas of NSW this was exacerbated by several preceding years of dry conditions. Severe bushfires in eastern NSW and widespread hydrological drought were some of the main effects. Despite rainfall increases compared to 2002 levels, 2003 rainfall totals remained below normal. Annual rainfall totals were between 80% and 100% of the 1961–90 mean over much of the State (BoM 2003).
There were two main episodes of short-term meteorological drought in 2004 in different parts of the State. First, the effects of a hot and dry summer across inland south-eastern Australia were compounded by a lack of autumn rains. By the end of May, five-month rainfall deficiencies were widespread in southern NSW, with several small patches of lowest-on-record precipitation. This marked the nadir of the deficiencies, with rainfall thereafter increasing slowly in intensity and extent with near-average falls across the area in both winter and spring. One exception was near the ACT where conditions continued to worsen during winter. Rainfall deficiencies subsided by the end of the year in this region.
The second event began in April and affected the east coast and ranges from the NSW–Victoria border through NSW to central Queensland. By the end of September, record low six-month rainfall affected two areas of the NSW coast: the far north approaching the Queensland border and the district around Gosford. Rain came to NSW during October 2004, but annual rainfall totals for the State were mostly below normal (BoM 2004).
The lack of rainfall in the first five months of 2005 followed a partial recovery in 2004 from the severe drought of 2002–03. As a result the agricultural areas of central and northern NSW were hit particularly hard. However, in June and July widespread heavy rain fell, and by the end of the year the only deficiencies remaining from January were between Bourke and Charleville (Qld) (BoM 2005).
Observed climate change
While long-term historical records show the influence of ENSO on NSW climate, more recent records show other changes. Since 1950, NSW mean maximum and minimum temperatures have increased. Trends calculated over 1950 to 2003 show an increase of 0.15°C per decade for the NSW annual mean maximum temperature and 0.19°C per decade for annual mean minimum temperature. This temperature increase has been attributed to anthropogenic causes, primarily the enhanced greenhouse effect (Karoly 2001; Stott 2003; CSIRO 2004a). The enhanced greenhouse effect and global climate change are explained in Atmosphere 3.1.
NSW annual total rainfall has decreased by 14.3 millimetres per decade since 1950. However, rainfall is highly variable, and since 1900 the annual mean NSW rainfall has increased by 10.6 millimetres per decade (CSIRO 2004a). Due to the high variability of NSW rainfall, and the numerous influences on it, it is difficult to attribute rainfall changes to any particular cause. Karoly 2003 suggests that rainfall changes could be due to some combination of natural and anthropogenic factors, such as decreasing stratospheric ozone (see Atmosphere 3.2) and the enhanced greenhouse effect.
The projected impacts of climate change in NSW
Projections of climate change in NSW undertaken by CSIRO and the Bureau of Meteorology (CSIRO 2004b) conclude that, without global action to limit greenhouse gas emissions, NSW can expect:
- a warming of between 0.2ºC and 2.1ºC over the next three decades (with the greatest rise in spring and summer) and a warming of 0.7ºC to 6.4ºC by 2070
- a general tendency for decreasing annual average rainfall, particularly in spring and particularly in south-western NSW.
Expected changes in annual average rainfall, however, are predicted with less certainty and accuracy than expected annual average temperature changes.
While much of NSW shows a tendency for drier conditions, heavy rainstorms may become more intense and more frequent and other extreme weather events will become more frequent (see Atmosphere 3.1). Future changes in NSW rainfall are dependent on how ENSO responds to the enhanced greenhouse effect. The projected response includes increased frequency and intensity of El Niño and La Niña events (CSIRO 2004a). El Niño events have already become more frequent, persistent and intense during the last 20 to 30 years compared to the previous 100 years (IPCC 2001). Many El Niño years (such as 1965, 1982, 1994 and 2002) were associated with very low rainfall.
The atmospheric, land, water and living systems of natural habitats and human settlements are intricately linked. The impacts of climate change on these systems are complex, difficult to predict, and extensive, because changes to one element of a system will likely influence other parts. The impact of climate variability and change, and the international, national, and NSW Government responses are summarised in Atmosphere 3.1. Greater detail on specific issues can be found throughout this report according to each relevant theme.
Human settlement and atmosphere
Emissions to air from transport, electricity generation and other industrial processes influence the degree to which our climate is projected to change (see Atmosphere 3.1). The Atmosphere and Human Settlement themes discuss the human activities that result in greenhouse gas emissions and contribute to the enhanced greenhouse effect (see Human Settlement 2.3, Human Settlement 2.4, Atmosphere 3.2 and Atmosphere 3.3). Human Settlement 2.1 notes the threat to coastal development from a predicted rise in sea level.
Land and water
Climate variability and land and water management practices are key factors in the healthy functioning of Australia's ecosystems and human settlements. Efficient resource management is needed to mitigate agricultural and hydrological drought and adapt to climate change. This is highlighted in the Human Settlement, Land and Water chapters. Related issues in which climate change may further intensify existing challenges for resource management include:
- inappropriate land management changes leading to land degradation (see Land 4.1)
- impacts of accelerated soil erosion on agriculture and biodiversity (see Land 4.2)
- population growth and water use (see Human Settlement 2.2)
- reduced river flows for environmental needs (see Water 5.2)
- deteriorating surface water quality (see Water 5.3) and freshwater riverine ecosystem health (see Water 5.1)
- increased groundwater extraction (see Water 5.4)
- the influence of fluctuating groundwater on dryland salinity (see Land 4.3).
Biodiversity
The land and water management issues discussed above combine with climate changes to create especially challenging conditions for ecosystem health. Climate change is one of seven categories of threat recognised worldwide that are driving biodiversity loss. The others are habitat loss, fragmentation and degradation; land degradation; invasive species; unsustainable levels of natural resource harvesting/extraction; nutrient loading; and pollution (Millennium Ecosystem Assessment 2005). Long-term changes in temperature and precipitation affect the physiology, population distribution and timing of life-cycle events for plants, animals and other organisms inhabiting terrestrial and aquatic ecosystems.
Climate change is expected to exacerbate other processes threatening biodiversity, such as fragmentation and degradation of habitat (see Biodiversity 6.1 and Biodiversity 6.6), changed fire regimes (see Biodiversity 6.5) and competition from invasive species (see Biodiversity 6.4 and Biodiversity 6.9). Evidence exists that this effect is placing some types of birds, mammals and other species under further pressure (see Biodiversity 6.3 and Biodiversity 6.8).
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