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Climatic Variability in Western Victoria

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Climatic Variability in Western Victoria DROUGHTS AND FLOODING RAINS: A FINE-RESOLUTION RECONSTRUCTION OF CLIMATIC VARIABILITY IN WESTERN VICTORIA, AUSTRALIA, OVER THE LAST 1500 YEARS. CAMERON BARR Thesis submitted for the degree of Doctor of Philosophy, Discipline of Geographical and Environmental Studies, University of Adelaide, Australia. 2010 CHAPTER 1 – INTRODUCTION 1.1 Introduction This thesis presents the results of a study examining the short-term (approximately 1500 year) history of climatic change from the south-eastern Australian mainland. In order to achieve this, fossil diatom assemblages from sediment cores retrieved from two crater lakes in western Victoria are examined in fine resolution and interpreted through the application of a diatom-conductivity transfer function developed specifically for use in low salinity lake systems. 1.2 Research context In early November 2006, the then Prime Minister of Australia, John Howard, called an emergency ‘water summit’ with the premiers of the south-eastern states to discuss the impact of the prevailing drought on water availability in the Murray Darling Basin, a key agricultural region for the nation. At this meeting, the attendees were informed that the drought that was afflicting the region was “the worst in 1000 years” (Shanahan and Warren, 2006). This claim proved so startling that it was widely disseminated through the local and national media and was even reported as far away as the United Kingdom (Vidal, 2006). The Prime Minister, however, remained unconvinced of the veracity of the claim because “there are no records [to verify it against]” (Shanahan and Warren, 2006). This brief exchange between the Nation’s leaders and the press highlights the problems faced by water, agricultural, environmental and social planners in Australia. That is, a lack of knowledge regarding short-term (decadal to sub-decadal) climatic variability beyond the period covered by even the longest instrumental climate records – around 120 years (Nicholls et al., 2006). There are a multitude of palaeoenvironmental and palaeoclimatological studies that have emanated from Australia, though comparatively few provide data at a resolution that can, for example, identify the last occurrence of a drought equivalent to the current event. Knowledge of past climatic variability is important for several reasons, not least of which is determining natural cycles of variability and locating contemporary events within those cycles. Such knowledge can inform on the potential for an increasing or decreasing frequency of climate anomalies and what form those anomalies may take, while also placing the changing Australian 1 climate of recent decades (Murphy and Timbal, 2008) into a longer-term historical context. The climate of Australia is highly variable both spatially, because of the range of climate zones on the continent, and temporally, due to the variable and inter-connected influences of major oceanic and atmospheric processes emanating from the Pacific, Indian and Southern oceans and the tropics (Turney et al., 2006). Due to this variability, drought is a common occurrence across much of the continent. The most recent period of drought in the south and east of the country commenced in 1996 and continues to the present (Murphy and Timbal, 2008). Sometimes referred to as ‘The Big Dry’ (Ummenhofer et al., 2009) or ‘The Millennium Drought’ (Bond et al., 2008), it is acknowledged as the most severe drought on record (Murphy and Timbal, 2008), with its unprecedented intensity caused by rising temperatures over recent decades which has increased evaporation rates (Nicholls, 2004; Murphy and Timball, 2008). The southeast of the mainland is one of the most severely affected regions (Figure 1.1) and, in terms of agricultural production, one of the most important (Murphy and Timball, 2008). This region includes the southern reaches of the countries’ largest river system, the Murray-Darling Basin, which contains 40% of all Australian farms and produces a third of the nations’ food supply (Sandeman, 2008). The majority of this agriculture is heavily dependant on water from the river system and, in 2005-2006, a period when annual average rainfall across the region was in the lowest three decile ranges (AusBOM; climate statement 2006), the agricultural industry accounted for 83% of all water consumed in the basin (Sandeman, 2008). In the study region of western Victoria, inflows into water storage catchments have been significantly reduced, resulting in water storage levels being measured at only 7% capacity in 2006 (Murphy and Timbal, 2008). The drought has also impacted on wetland resources across western Victoria with numerous lakes drying out. Among these are several Ramsar-listed wetlands, some of which, in 2002, were classified as being ‘permanent saline’ (Lake Bookar, Lake Gnarpurt) or ‘permanent freshwater’ (Lake Terangpom) wetlands (Victorian Government, 2002). The primary cause of the current drought, and other iconic droughts of the 20th century, has been identified as a combination of the influences of El Niño and the positive mode of the Indian Ocean Dipole (IOD) (Ummenhofer et al., 2009). The converse also 2 appears to be the case with the co-occurrence of La Niña and the negative mode of the IOD identified as a driver of positive rainfall anomaly in the southeast of the country (Meyers et al., 2007). The relatively recent discovery of the IOD (Saji et al., 1999), and the recognition of its contribution to rainfall anomalies in various regions in Australia (e.g., Smith et al., 2000; Ashok et al., 2003; Meyers et al., 2007; Ummenhofer et al., 2009), demonstrates the growing body of knowledge about sub-decadal scale climate variability in Australia. However, knowledge of similar scale variability of past climates is, by and large, limited to the period of instrumental records or restricted geographic regions such as western Tasmania (Cook et al., 1991, 1992, 2000). In order to gain an understanding of climate in its entirety, there is a need to increase the focus on documenting short-term, high-amplitude, climatic variability beyond what is known from instrumental records. NOTE: This figure is included on page 3 of the print copy of the thesis held in the University of Adelaide Library. Figure 1.1: Observed national mean rainfall anomalies for the period December 2003 – November 2006. Source; Australian Bureau of Meteorology website. Redrawn from Hunt (2009) Two decades ago, Wasson (1990) concluded that “[in terms of palaeoclimate studies], the last 2000 years has been largely ignored in the Australian context.” In the years since, relatively little has changed. With the notable exception of the Tasmanian 3 temperature reconstructions by Cook et al. (1991; 1992; 2000), the studies that do exist are generally either of a low temporal resolution (e.g., Holdaway et al., 2002; Pollack et al., 2006 and, to a lesser extent, Mooney, 1997) or cover only a brief window of this time period (e.g., Hendy et al., 2002a). In fact, at present, there are no sub-decadal resolution, millennia-length studies from the Australian mainland. The lack of finely-resolved data from the Southern Hemisphere has been noted in reviews of climate studies (e.g., Mayewski et al., 2004; Ljungqvist, 2009), hemispheric temperature reconstructions (e.g., Mann and Jones, 2003; Mann et al., 2008) and, recently, by the Intergovernmental Panel on Climate Change (Jansen et al., 2007). Partly as a consequence of this ‘research gap’, there exists a large discrepancy in the knowledge and understanding of climatic variability of the last two millennia in the Northern Hemisphere and that which exists in the Southern Hemisphere and, more specifically, in Australia. In particular, the two most notable periods of climatic anomalies during the last 1500 years – the so called ‘Little Ice Age’ (LIA – ca. 1350- 1850 AD) and the ‘Medieval Warm Period’ (MWP – ca. 800-1200 AD) – are widely documented in Northern Hemisphere studies, though the impacts of these periods, if any, and the timing of their onset and duration, have not been identified in mainland Australian studies. Generally characterised as being a period of elevated temperatures (Lamb, 1965), the MWP occurred when global boundary conditions (e.g., insolation, ice-cover, sea-levels) were similar to present and may provide, therefore, a potential analogue (Bryson, 1985) to future climates under the influence of global warming. Hence, knowledge of climatic variability during such times may provide a possible insight into future climate regimes. 1.3 Aims of the study In recognition of the paucity of, and need for, highly-resolved palaeoclimate studies from Australia, the aim of this project is to reconstruct, at sub-decadal resolution, climatic change from south-eastern mainland Australia over the last millennium or more. In order to achieve this, sediments from two volcanic crater lakes in western Victoria were examined at high-resolution for changes in the species composition of diatoms. The importance of these changes, and their relationship to climate, are determined through the application of a diatom-conductivity transfer function. More specific aims of the study are: 4 1. To identify high frequency climate variability and determine if and how this has changed over time. 2. To identify low frequency climate variability in order to determine the presence or absence of long-term cycles and, if these cycles are evident: a. contextualise contemporary climates within these and, 3. To identify the frequency, intensity and duration of previous drought episodes in order to: a. Compare the current drought to previous episodes b. Provide evidence of long-term regional drought history 4. To compare and contrast results with other relevant studies in order to provide a regional synthesis and determine possible driving mechanisms behind identified changes. 5. To characterise the climate in western Victoria during the periods commonly associated with the LIA and MWP and assess this in terms of the debate around the spatial extent of climate phenomena observed during these periods. The research plan has been specifically structured towards achieving these aims.
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