Variability and extremes of weather are key characteristics of Australia’s climate. Australian scientists are increasingly coming to understand the complex interplay of atmospheric and oceanic processes that shape our climate in both the short and the long term, and the processes that are responsible for cycles of drought and wet years in different parts of the country. These include natural events, such as fluctuations between El Niño and La Niña conditions in the Pacific, variations in the Indian Ocean Dipole, the southern annular mode and longer term features such as the Interdecadal Pacific Oscillation.
Against this background of natural variability, identifying a climate change ‘signal’ is often challenging. The Commonwealth Scientific and Industrial Research Organisation (CSIRO), in the phase 1 report on the South Eastern Australian Climate Initiative, contrasts the recently ended 13-year drought in the southern Murray–Darling Basin and Victoria with other droughts since 1900.14 The report notes that:
- the most recent drought was generally limited to southern Australia (in comparison with previous droughts, including the severe Federation and World War 2 droughts)
- the recent period has experienced lower annual variability in rainfall, characterised by a complete absence of wet years
- a characteristic of recent rainfall across the southern Murray–Darling Basin, and the south-east of the continent in general, has been severe deficiencies (occurring as a step-change) in autumn and early winter rainfall
- the loss of early-season rainfall across both south-eastern and south-western parts of the continent has led to dramatically reduced streamflow and water storage in the past two decades.
The report concludes that, although natural variability is likely to have contributed to episodic drought (i.e. sequences of very dry years), the major and prolonged decline in rainfall is at least partly due to the effects of global warming on large-scale atmospheric circulation (specifically, the intensification of high-pressure cells across southern Australia—a phenomenon referred to as the ‘subtropical ridge’). The report further suggests that the changes in rainfall and streamflow data during the drought may be indicative of a climatic shift in south-eastern Australia similar to that experienced in south-west Western Australia, where one study has suggested that half of the 10–15% decrease in winter rainfall experienced since around 1975 is attributable to climate change.24
Despite the uncertainties, there is an increasingly robust scientific consensus on the effects of climate change at a continental scale on temperature and on likely future impacts under the various climate change scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). With respect to temperature, the IPCC’s fourth assessment concluded that Australian temperatures have increased and that most of the rise can be attributed to human-induced increases in emissions of GHGs.25 In the case of rainfall, a longstanding result from climate modelling over the past two decades demonstrates that global warming leads to increased rainfall in the tropics and decreased rainfall in mid-latitude regions, including more prolonged drought. Rainfall intensity (periods of heavy rain) is expected to increase, including in regions where an overall decline in rainfall is projected.25 However, determining the extent (if any) of the contribution of climate change to particular climatic events such as drought remains problematic and is an ongoing area of research.14