Living in a variable climate
Dr Greg McKeon, CRC for Greenhouse Accounting, Queensland Department of Natural Resources and Water
prepared for the 2006 Australian State of the Environment Committee, 2006
The imagery of Australia as a 'wide brown land' with stoic Australian farmers and graziers battling drought is rooted deep in the national ethos of an increasingly urban Australia. 'Australia's climate' and its year-to-year variability in rainfall could well claim to be a major determining factor in the development of this national identity and the attitude to the future. As strong as such general national images are, Australia nevertheless has a very wide variety of 'climates' including tropical, monsoonal, temperate, sub-humid, semi-arid, alpine and desert climates, with different patterns of historical rainfall variability. The seasonal distribution of rainfall, which is so important in determining vegetation and land use, varies from summer-dominant rainfall in the northern regions of the continent to winter-dominant rainfall in the southern regions. Despite this large variation in climate types, and associated differences in land use, Australia is overall the driest inhabited continent from the viewpoint of continental rainfall and streamflow (McTainsh and Boughton 1993). Importantly this widespread aridity, in combination with extensive marine sedimentary rock deposits and internally draining river basins, is associated with Australian soils having high proportions of salts (McTainsh 1993) with potential problems for agricultural production.
This 'average' view of a dry continent, however, obscures the fact that reasonable rainfall occurs on coastal fringe of eastern and south-west Australia and across inland south-eastern Australia. This rainfall has supported large urban populations, profitable dryland and irrigated agriculture, and pastoralism. Furthermore, this 'average' view has also contributed to three great myths (Williams 2003, p.40): '(1) water allowed to run to the sea is wasted; (2) we must make the desert bloom; and (3) we must drought-proof Australia.' Williams (2003, p.40) stated that 'each of these persistent ideas holds great danger, both for our landscape and for our sustainable future within it. We need to rid ourselves of them if we are to live like true Australians, in harmony with our land.'
Australia's climates are highly variable as a result of its small landmass in relation to the expanse of ocean that surrounds it and its location across tropical, subtropical and temperate climate zones. Furthermore, year-to-year variations in sea surface temperatures in both the Indian and Pacific Oceans and variations in atmospheric circulation—such as the latitude of the sub-tropical high-pressure belt—exert a significant influence on Australia's rainfall (Drosdowsky 2002, 2005).
Several studies have also noted the larger annual variability of rainfall and streamflow in regions of Australia when compared with other comparable locations in the world (see, for example, McMahon et al. 1992, Nicholls et al. 1997). In most Australian catchments, average annual streamflow is a small proportion of the overall water balance. Small changes in average rainfall due to natural climate variability at longer time scales, or due to climate change, can result in large changes in streamflow. For example, for a catchment in south-west Western Australia, Sadler et al.(1988) estimated that a decrease of 20 per cent in rainfall would result in a 45 per cent decline in streamflow.
The high variability in Australian rainfall is in part the result of the strong control of meteorological mechanisms associated with the phases of the El Niño-Southern Oscillation (ENSO). ENSO is a global phenomenon that involves coupling of anomalies in sea surface temperature and atmospheric pressure (Burroughs 2003, p. 72). The climate science community has several formal definitions of these phases of ENSO. In general terms, El Niño refers to warm sea surface temperature anomalies in the central equatorial and eastern regions of the Pacific Ocean, and it is associated with higher atmospheric pressure over Australia and cold water anomalies in the Coral Sea. As a consequence, in El Niño years, lower rainfall often occurs over much of eastern Australia and some regions of Western Australia (Lindesay 2003). Opposite anomalies of sea surface temperature and pressure occur during La Niña conditions. In La Niña years, high rainfall has occurred over most of Australia and there has been an increased frequency of tropical cyclones, particularly in the number crossing the coast of Queensland (Partridge 1994). Thus, over much of Australia, the phases of the Southern Oscillation (El Niño, La Niña or 'neutral') may be regarded as a second dimension to the traditional seasons.
El Niño and La Niña events generally commence in autumn (March–May) and last for approximately 12 months. About 40 to 50 per cent of years may be classified into these ENSO phases (depending on the definition) and the remainder as 'neutral' ENSO years. Regrettably, the perceived need to simplify the communication of climate science has led to the simplistic association of the words 'drought' with 'El Niño', and 'floods' with 'La Niña' when, in fact, in some years the opposite has occurred. Extremes of rainfall (wet and dry) have also occurred in 'neutral' years, with severe multi-year drought periods occurring during sequences of 'neutral' ENSO years (McKeon et al. 2004).
The understanding of ENSO as a major driver of Australia's climate variability is relatively new. It was not until the late 1970s and early 1980s that the impact of ENSO on Australian rainfall was confidently understood by scientists (Pittock 1975, McBride and Nicholls 1983, Allan 1985, Stone et al. 1996, Nicholls 2005) and communicated to the wider community (see, for example, Coughlan 1988, Clewett et al. 1988, 1991, Coughlan et al.2003). At the same time, analysis by ecologists and climatologists revealed that ENSO has been a driving force, shaping the ecology of Australia (Nicholls 1991). For example, in northern Australia, ENSO events accounted for half of the major ecologically significant extremes of climate, driving biological processes such as population recruitment, distribution and survival (Taylor and Tulloch 1985). ENSO is not the only contributor to temporal climate variability and recent studies have indicated the impact of other components of the climate system (Meinke and Stone, 2005).
The risk of drought has been a major control on agricultural land use and, simply put, 'Drought is a normal feature of the Australian farmer's operating environment' (Botterill and Fisher 2003, p. 1). Yet, despite the physical hardship, social heartbreak, animal suffering, financial and economic consequences—and the environmental damage that is expected to occur—both urban and rural communities appear to be surprised by the next drought. The range of attitudes that Australians have with regard to climate variability (and in particular drought) has recently been reviewed by many leading scientists and commentators (Botterill and Fisher 2003). Botterill (2003, p. 197) concluded the review with a plea for a deeper understanding of the reality of climate variability and recognition that:
The concept of 'average' rainfall is essentially a statistical construct that bears little resemblance to most seasons. This approach suggests that the notion of 'drought' may be meaningless in an environment in which extremes are the norm, particularly as the term is so value laden and evocative of unexpected disaster.
This insight could have been written during the widespread Federation drought (1895–1902) and the extended regional droughts of the 1920s, 1930s, and 1940s, and, in fact it has been stated previously in general terms (for example, Anon 1901, Ratcliffe 1938, 1970). As such, Botterill's conclusion is more of a rebuke to an Australian community that is still failing to acknowledge the variable nature of its climate, and to plan and respond accordingly. The expectation of future climate change (but of somewhat uncertain direction in terms of regional rainfall) highlights the need for the development of improved climate literacy and hence is the theme of this commentary.