Atmosphere: 4.1 Australia's variable climate
ndependent report to the Australian Government Minister for the Environment and Heritage
Beeton RJS (Bob), Buckley Kristal I, Jones Gary J, Morgan Denise, Reichelt Russell E, Trewin Dennis
(2006 Australian State of the Environment Committee), 2006
There is no doubt that Australia has always had a highly variable climate and that Australians are still learning to adapt to it. Paleo-climate data alone show that there have been more extreme wet and dry periods than the relatively few that have been seen in only two centuries of European settlement, and that more can be expected.
Most of Australia was settled before long-term climate data were collected. The result is that agricultural and urban land use patterns were fixed well before there was any understanding that Australia’s climate is highly variable, with a high occurrence of widespread extreme wet and dry years. For example, eastern Australia has experienced several sequences of wet years since the late 1880s (early 1890s, 1916–18, early 1920s, mid-1950s, early 1970s and late 1990s). Dry conditions in various rural regions followed these wet sequences (1896–1902, 1919–20, 1926–31, mid-1960s, early 1980s, 2001 to present) (Power et al 1999, Mantua and Hare 2002). Temperatures have been equally variable.
The direction and magnitude of rainfall trends for the last one hundred years over the continent vary regionally, with increases in north-western Australia and decreases in south-western Australia. Given the influence of decadal and multidecadal variability in the climate system on Australian rainfall, the start and finish year of analysis also affect the reported magnitude and direction of the trends (1900–2005 compared with 1950 to 2005 and with 1970 to 2005)-see Figure 10. The choice of period for analysis has been determined by either scientific need (accuracy of records, behaviour of climate system) or practical application (recent resource management issues).
Source: BoM (2006b)
Rainfall is highly variable across Australia and from year to year (Figure 11). In recent years, rainfall has increased over much of northern Australia, especially in the north-west. South-eastern Australia has been drier than average. Winter rainfall in the south-west of Western Australia has also decreased substantially since the mid-twentieth century, and since the mid-1970s in particular (IOCI 2002). Some areas have experienced eight consecutive years of below-average rainfall (BoM 2005a). Long-term records show that dry sequences are not unusual. For example, Lake George in New South Wales showed a 17-year dry spell in the 1930s and 1940s (Singh and Geissler 1985). More recent data show the trend may be continuing, but a statistical conclusion cannot be reached.
Source: Queensland DNRMW (2006)
For some areas, such as north-east Queensland, reconstructed rainfall records from coral records suggest that the driest and wettest years in the 230 years from 1754 to 1985 occurred in the twentieth century (Lough 2003:33). The year 1902 was the driest and 1974 was the wettest for this period, and the recent drought of 2002–03 was almost as extreme. The coral records show that the driest 10 and 30-year periods occurred at the end of the eighteenth century (1766–75 and 1770–99, respectively), which was well before the period of northern settlement and instrumental record-keeping (post-1860).
It is increasingly clear that the last 50 years of experience with rainfall patterns is not a sufficient time span to plan and design an adequate response to climate variability and change. Planning and adaptation must include the dry periods of 1890–1950 as well as extraordinary events such as the Sydney hailstorms of 1999, 2004 and 2005, the Katherine floods of 1998 and 2006, and numerous storm and cyclonic events, including Cyclone Larry in 2006.
Are extreme events becoming more extreme? The best data are for rainfall, which can be measured in terms of the amount of rain falling in a 24-hour period. Australia’s rainfall is so variable over time that the trends in extreme rainfall during 1910–2005 differ from those during 1970–2005. From 1910 to 2005, for example, the only statistically significant trend in extreme rainfall was in the south-west region, which experienced a decline in the intensity of the most extreme 1 per cent of storms for each year. Apart from the central arid region and the New South Wales tablelands, most regions in Australia show a decline in total and extreme rainfall since the mid-1970s (Figure 10).
Although drought is seen as an extreme event, long periods of low rainfall are common in Australia (Figure 11), as are periods of high rainfall. For example, a series of wet years preceded SoE2001, while the reverse is true for SoE2006. Understanding this aspect of Australia’s climate variability is critical for future environmental policy. The problem is one of scale, both over time and across the landscape. This is illustrated by two stories.
The most recent drought story begins in 2001–02, when drought began in areas of south-western Queensland, western New South Wales, eastern South Australia, north-western Victoria and the Gascoyne region of Western Australia. In 2002–03, an El Niño year (see Glossary), extreme drought occurred across much of eastern Australia and areas of Western Australia, further exacerbating drought conditions in those areas (McKeon et al 2004). Following average conditions in 2003–04, severe drought returned in many regions in 2004–05 (a year with marginal El Niño conditions). For many regions of Australia, the overall five-year period from April 2000 to March 2005 represents extremely low rainfall compared to the historical records commencing in 1890.
A similar but more cautionary story emerges from an analysis of the last 40 years of rainfall records. In much of Australia, this recent drought started after a sequence of above-average years of rainfall from the second half of 1998 to the first half of 2001. Central coastal Queensland and south-west Western Australia had already experienced drier conditions for at least 15 years. For example, eastern Australia received significantly less rainfall during the three years from 2002 to 2005 than during 1961–90 (the current international standard reference period). Coastal areas experienced the greatest rainfall deficits (the difference between actual rainfall in a year and the long-term average) (Figure 12). In contrast, during the same 2002–05 period, rainfall in the north of the Northern Territory and in parts of north-western Western Australia was significantly greater than that experienced from 1961–90 (BoM 2005b). The recent drought may be unusual in that it has been warmer than previous droughts in the last 50 years (the length of temperature records).
Source: BoM (2005b)
Both stories acknowledge drought as a regular occurrence; both have been used in the debate about climate change, and both acknowledge the importance of drought as a driver of Australia’s natural systems. Despite this, both urban and rural communities appear to have been as surprised by each succeeding drought as all the others. Lovett (1973) wrote that the notion of “drought” may be meaningless in an environment in which extremes are the norm, as did others some 30 years later (Botterill 2003).
It is likely that the variability in Australia’s climate is at least partly responsible for this national optimism. The wet periods (Figure 11) were likely to have led to unrealistic expectations of long-term agricultural production, livestock-carrying capacity, and water availability. The result has been government and community support for inappropriate land uses, such as cropping of marginal areas and small grazing property sizes (Heathcote 1965, Russell 1988, Condon 2002). The more marginal land uses have suffered during the inevitable dry episodes, and there have been major re-examinations of appropriate land use as well as a public call for major infrastructure development to ‘drought-proof’ regions.
The recent drought was particularly severe because:
- it was accompanied by record high average maximum temperatures (see Temperature)
- it affected virtually the entire continent, with 35 per cent of the continent having rainfall below the tenth percentile
- it was followed by a series of relatively dry years (BoM 2005a).
Extreme events have always had a profound influence on policy in Australia. A feature of the current drought has been a renewal of the debate on drought-proofing agriculture and rural Australia.
Drought-proofing is the changing of management practices and infrastructure to reduce the impact of drought on production and communities. At the core of the debate is the clash between two opposing approaches to managing for drought. On one hand, it is argued that technical advances and financial support are needed to maintain a high level of agricultural production and to protect the stability of rural communities in periods of rainfall deficit. On the other hand, there is a general call for recognition that Australia has a relatively high frequency of drought in many regions and that better individual climate risk management and more appropriate land use would be less environmentally damaging and require less financial support.
The current debate on drought raises two major questions:
- How much of the current variability is due to human-induced effects (climate change, stratospheric ozone depletion, aerosol concentrations, land use change, and land clearing) in contrast to expected variability resulting from natural fluctuations of the global climate system?
- What planning and infrastructure investment should occur?
These questions can be answered only by increasing our ‘climate literacy’ (Botterill 2003:197). It requires our emerging understanding of how the climate systems work, knowledge of historical climate variability, and plausible projections of future climate variation. It also requires longer-term climate data, to better understand that cycles of high and low rainfall should be part of our expectations and built into decision-making and planning.
Australia’s current difficulties may well be because the nation is entering a dry period similar to that experienced in the first half of the last century, and planning has not made adequate provision for this. Further, the dryness may also be influenced by the hotter and more extreme climate that is predicted to result from climate change. The bottom line should surely be that the impact of the demand for water and other resources from increasing population together with a move to drier sequences in southern Australia, will place further stress on already stressed rivers and landscapes. There are indications that, if there were a return to the 1900–50 rainfall patterns, the Murrumbidgee River could not supply the current extraction demand (Khan 2006).
The 1992 image (Figure 13) shows a very poor growing season across most of northern Australia as indicated by the dominance of red values in the north. Conversely, the 2002 image shows a very poor growing season across most of southern Australia, while the 2005 image shows a good growing season in most of the major agricultural areas, as indicated by the purple values.
The analysis is based on the hypothesis that the cover of green vegetation will increase over the growing season, and better seasons show greater greenness. In this analysis, the amount of greenness is estimated from Normalised Difference Vegetation Index data collected by the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer satellite. In the images, season quality is estimated from the relative increase in greenness for each year scaled against the long-term data (1992 to 2005). Zero per cent is the minimum 'increase in greenness' recorded and 100 per cent is the maximum recorded. Each pixel is thus scaled against its own measured ability to become greener following rainfall.
Source ERIN (2006a)
The average temperature across Australia has risen by 0.82°C between 1910 and 2004 (Figure 14), with much of the warming occurring in the second half of the twentieth century (Figure 15). The warmest year on record is now 2005. Until 2004, the warmest year had been 1998, with all of the ten warmest years since 1910 occurring in the last 32 years (1973–2004).
Source: BoM (2005c)
Source: BoM (2005d)
More recently, temperature records show that the year 2005 was 1.09°C warmer than the Australia-wide annual mean temperatures for 1961–90, and 2002 was 0.63°C warmer (BoM 2006a). Australian temperatures have increased slightly more rapidly than the global average (CSIRO 2005a). The warming in Australia since 1950 has been almost 0.2°C per decade.
Although all of Australia has become warmer, the amount of warming has not been uniform across Australia (Figure 14 and BoM 2006a). Greatest temperature increases, of 0.15°C to 0.20°C per decade, have occurred in inland South Australia and in western Queensland. Other regions have warmed by only 0.05°C per decade, including south-eastern New South Wales, western Tasmania, north-western Western Australia and a large swathe inland from south-eastern Queensland to central New South Wales. Increases in annual mean minimum and in annual mean maximum temperatures have resulted in more of Australia experiencing extremely hot temperatures (above the ninetieth percentile) and less of Australia with temperatures below the tenth percentile. Diurnal temperature ranges also show a similar change.
Evaporation affects water availability, and it is determined by humidity, wind, air temperature and radiation. Despite the higher temperatures recorded across much of Australia, the data show a small decline in potential evaporation of around 5 per cent over 30 years, but with some regions experiencing increases or remaining constant (Roderick and Farquhar 2004). The continued monitoring of trends in potential evaporation, and understanding of the contributing factors, will be important in estimating future water availability and demand for rural and urban use.