3 Atmosphere | 2 Climate | 2.1 State and trends of Australia's climate

State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.

3 Atmosphere

2.1 State and trends of Australia’s climate

Perhaps the salient feature of the Australian environment over the past decade, at least for the majority of Australians, was an extended drought. This drought was characterised not only by low rainfall but also by higher than average temperatures. For many places, the severity and duration of drought were unprecedented, with profound environmental, social and economic implications. In southern Australia, the drought (sometimes known as the millennium drought) lasted from 2000 to 2010, although in some areas it began as early as 1997. For parts of the country, the drought broke in 2010 (in some cases, with extreme flooding); in other places, like the south-west of Western Australia, the extended drought deepened further.

2.1.1 Temperature

From 1970 to 2010, Australia’s mean daily temperature rose in almost all parts of the country (Figure 3.2).

Figure 3.2

© Commonwealth of Australia, 2011, Australian Bureau of Meteorology

Source: Bureau of Meteorology15

Figure 3.2 Trend in mean temperature, 1970–2010 (°C per 10 years)

The increase in terrestrial temperatures over this period was consistent with a general warming of ocean surface temperatures around Australia (Figure 3.3). To the south-east of the continent, the southward extension of the East Australian Current has continued, with consequences for marine ecosystems and biodiversity in the ocean off the coast of eastern Tasmania.

Figure 3.3

Source: Bureau of Meteorology16

Figure 3.3 Trend in sea surface temperature for the Australian region (°C per 10 years), 1970–2010

2.1.2 Rainfall

From 1970 to 2010, total annual rainfall declined over much of eastern Australia and south-west Western Australia (Figure 3.4). This decline in rainfall affected all capital cities except Darwin. In contrast, rainfall increased over north-west and central Western Australia.

Figure 3.4

© Commonwealth of Australia, 2011, Australian Bureau of Meteorology

Source: Bureau of Meteorology17

Figure 3.4 Trend in annual total rainfall, 1970–2010 (millimetres per 10 years)

The 13-year period from April 1997 to March 2010 (156 months) shows rainfall deficiencies for much of south-western and south-eastern Australia and south-eastern Queensland (Figure 3.5). Most notable are the large areas of lowest rainfall on record for this period: large parts of Western Australia’s south-western coast, western Tasmania and large areas in Victoria received the lowest rainfall on record for the 13-year period.

Figure 3.5

© Commonwealth of Australia, 2011, Australian Bureau of Meteorology

Source: Bureau of Meteorology18

Figure 3.5 Rainfall deficiencies over 156 months, 1 April 1997 – 31 March 2010

‘Serious deficiency’ refers to rainfalls in the lowest 10% of historical totals, but not in the lowest 5%; ‘severe deficiency’ refers to rainfalls in the lowest 5% of historical totals; and ‘lowest on record’ refers to the lowest rainfalls since at least 1900, when the data analysed begin.

For the more recent eight-year period from April 2002 to March 2010 (96 months), much of south-eastern Australia still experienced severe, long-term rainfall deficiencies. Approximately 95% of Victoria received rainfall in the lowest 10% of historical totals when considered over such a period. The south-eastern corner of Queensland also had serious to severe rainfall deficiencies over this period. Serious to severe deficiencies also remained in central to eastern coastal districts of South Australia, large areas of Tasmania (especially in the north), and a large area covering the south-west coast and adjacent inland regions of Western Australia. Rainfall deficiencies across the south-western and south-eastern corners of the continent have been most severe in autumn and winter.

For the 12 months from April 2009 to March 2010 (Figure 3.6), serious to severe rainfall deficiencies remained evident over much of the central Western Australian coast, reaching inland to cover much of the Pilbara and Gascoyne districts, where they intensified to some extent. Serious to severe rainfall deficiencies also remain over the south-east coastal and Great Southern districts of Western Australia, with a small area near Esperance reporting the lowest rainfall on record for the period.

Figure 3.6

Source: Bureau of Meteorology19

Figure 3.6 Rainfall deficiencies over 12 months, 1 April 2009 - 31 March 2010

‘Serious deficiency’ refers to rainfalls in the lowest 10% of historical totals, but not in the lowest 5%; ‘severe deficiency’ refers to rainfalls in the lowest 5% of historical totals; and ‘lowest on record’ refers to the lowest rainfalls since at least 1900, when the data analysed begin.

However, from March 2010, large parts of the continent experienced above-average rainfall, associated with an extremely strong La Niña event (Figure 3.7). (A La Niña event refers to a periodic cooling of ocean surface waters off the western coast of South America. This leads to low rainfall in countries along that coast and in the south-west of the United States, and above-average rainfall in countries of the western Pacific, including the Philippines and northern and eastern Australia—outcomes that are the opposite of those associated with an El Niño event.) Most notably, eastern Australia received widespread record-breaking rains, with associated loss of life and massive damage to agriculture, homes and infrastructure. For the Murray–Darling Basin averaged as a whole, 2010 was the seventh wettest start to the year since records began in 1900. This rainfall has effectively ended a prolonged (decade or longer) sequence of very low rainfall years across parts of eastern Australia, most notably in the central and lower Murray–Darling Basin and south-east Queensland. Although Victoria experienced its wettest summer since 1974 in 2010–11, long-term rainfall deficiencies remained during autumn and winter.

South-west Western Australia missed out on La Niña–driven rainfall for most of 2010, experiencing its lowest winter rainfall on record.20-21 However, this situation changed markedly over the summer of 2010–11. Averaged across the state, summer rainfall was the second highest on record. In the north, this mainly reflected the influence of the monsoon that was active throughout the summer. Across the state more generally, it reflected the impact of the particularly strong La Niña. In much of the lower south-west, the Bureau of Meteorology characterised summer rainfall as ‘very much above average’, due mainly to rainfall in January.22 Despite this, during the 12 months from March 2010, rainfall in the lower south-west ranged from below average to the lowest on record.

Figure 3.7

Source: Bureau of Meteorology23

Figure 3.7 Australian rainfall deciles, 1 March 2010 – 28 February 2011

‘Highest on record’ refers to the highest rainfalls since at least 1900 when the data analysed begin. ‘Very much above average’ refers to rainfalls in the highest 10% of historical totals; ‘above average’ to rainfalls in the highest 30% of historical totals, but not in the highest 10%; ‘average’ to rainfalls in the middle 40% of historical totals; ‘below average’ to rainfalls in the lowest 30% of historical totals, but not in the lowest 10%; ‘very much below average’ to rainfalls in the lowest 10% of historical totals; and ‘lowest on record’ to the lowest rainfalls since at least 1900, when the data analysed begin.

2.1.3 Climate research

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