Atmosphere

Theme commentary
Tom Beer, Michael Borgas, Willem Bouma, Paul Fraser, Paul Holper and Simon Torok
CSIRO Atmospheric Research
prepared for the 2006 Australian State of the Environment Committee, 2006

Major issue: climate variability (weather)

Australia’s climate is particularly variable, with a high occurrence of widespread droughts that are frequently, but not always, driven by El Niño events. Since 1980, Australia has experienced more frequent El Niño events than during previous years of the twentieth century.

The Bureau of Meteorology (BoM 2005c) noted that, compared with state-wide averages dating from 1900, the year 2005 was the driest autumn on record in Victoria and South Australia, the second driest in New South Wales and the fourth driest in Tasmania. These rainfall deficits follow average to below-average falls since the severe drought of 2002–03, with no prolonged period of widespread above-average falls to fully remove rainfall deficiencies. Many parts of Australia were still affected by unremitting drought in 2005. Bureau records (BoM 2005e) revealed that the Murray-Darling Basin was in the midst of its worst multi-year period of rainfall deficiencies since the 1940s.

Climatic extremes have a major impact on people, the economy and the environment. Drought in recent years has created great hardship for many regions, and can have major impacts on Australia’s economy, affecting many sectors. Conditions in 2002–03 were not as dry over most of eastern Australia as those associated with the droughts of 1901–02 or 1982–83. The drought during 2002–03 was particularly severe in its impacts for three reasons. Firstly, it was accompanied by record, high average maximum temperatures and consequently increased evaporation. Secondly, the drought affected virtually the entire continent (ABS 2004) with 35 per cent of the continent having rainfall below the tenth percentile . Thirdly, it was not followed by above-average rainfalls.

The 2002–03 drought is estimated to have cost 1.6 per cent of gross domestic product (about $10 billion) and about 70 000 jobs (Adams et al.  2002, p. 239). As well as its economic impacts, drought affects the environment. In addition, overgrazing during droughts can markedly increase the amount of erosion and subsequent runoff of sediment (ASEC 2001, p. 40).

Weather-related events (excluding drought) such as hailstorms, bushfires, storms, cyclones and flooding are responsible for approximately 87 per cent of economic losses  from natural disasters in Australia (Coleman, 2003, p. 3). For example, the 18 January 2003 bushfire in Canberra cost $363.2 million (in 2004 dollars), making it the fifth largest Australian insurance loss since 1967; the Sydney hailstorm of April 1999 was the most costly.

The cost of weather-related disasters is rising. According to the Intergovernmental Panel on Climate Change (IPCC 2001, p. 17) there is now enough momentum in the climate system, from past energy and land use practices, to inflict further warming even if carbon dioxide levels were stabilised at current levels (Meehl et al. 2005). Insurance Australia Group (Coleman 2003) believes that some changes to the future climate are inevitable, noting that the IPCC scenario ‘will lead to more intense and/or more frequent extreme climate and weather events’. To some extent, the losses from extreme weather-related events are due both to climate change and to increased population in vulnerable areas such as the coast.

Rainfall

Despite the increase in Australian average annual rainfall over the twentieth century, south-eastern Australia has been particularly dry in recent years. Some recording stations have experienced an unprecedented eight consecutive years of below average rainfall  (BoM 2005d).

For the three-year period from April 2002 to March 2005, for example, rainfall  over eastern Australia was significantly lower than that experienced from 1961–1990. Deficits were greatest close to the coast. Winter rainfall in the southwest of Western Australia has decreased substantially since the mid 20th century and since the mid-1970s in particular (BoM 2005a).

Winter rainfall in the southwest of Western Australia has decreased substantially since the mid-twentieth century, and the since the mid-1970s in particular (IOCI 2002, p.2).

Many parts of Australia are likely to show a tendency towards drier seasonal-average conditions under enhanced greenhouse conditions. According to CSIRO (2001), when simulated future increases in potential evaporation are considered in combination with simulated rainfall changes, the overall pattern shows decreases in moisture balance on a national basis. It does not necessarily follow that extreme daily rainfall events will become less frequent or less severe in future, because the pattern varies with the geographic location. In south-western Australia there has been a significant decrease in total rainfall, the frequency of extreme rainfall, the intensity of the extreme rainfall, and the number of rain days (Haylock and Nicholls 2000). In New South Wales and Queensland, the frequency of extreme rainfall has decreased but the number of rain days has increased, indicating an increase in the number of lighter events.

An example of an extreme rainfall event was the one experienced in Melbourne on 2–3 February 2005, with 120.2 millimetres of rain during a 24-hour period; this exceeded the previous record of 108 millimetres that was recorded on 29 January 1963 (BoM 2005b). During 31 hours, Melbourne’s rainfall was equivalent to almost a quarter (23 per cent) of the annual average of 653 millimetres.

Temperature

Australia has warmed  significantly, especially since 1950, with a commensurate increase in the frequency of very warm days and a decrease in the frequency of frosts and very cold days (CSIRO 2004, p. 4). Most of this increase occurred in two periods—from 1910 to 1945 and since 1976.

As for many parts of the world, the increase in mean minimum temperatures over the period is markedly greater than the mean maxima, especially since 1950. Hence the diurnal temperature range has decreased noticeably in most places.

Australasian temperatures have increased slightly faster than the global average (CSIRO 2005). The warming in Australia since 1950 has been almost 0.2 °C per decade.

The past decades have seen Australia’s highest recorded mean annual temperatures (Figure 2). The year 2005 was Australia’s warmest year on record (at the time of writing), with an annual mean temperature that was 1.09 °C above the 1961–90 average (BoM 2006). Previously, more than 150 high-temperature records were set in January–April 2005 (BoM 2005f). The overall Australian annual mean temperature for 2002 was 0.63 °C above the 1961–90 average, making 2002 Australia’s fifth warmest year on record since 1910 when reliable Australia-wide climate records are available. In 2003 Australia experienced its sixth warmest year on record, and in 2004 the tenth warmest year .

Figure 2: Annual mean temperature anomalies for Australia

 Annual mean temperature anomalies for Australia

Source: Bureau of Meteorology (data supplied direct to DEH), January 2006.

While all of Australia has become warmer since 1910, the mean warming  has not been uniform (Figure 3). The least warming, of up to 0.05 °C per decade, has occurred in regions including south-eastern New South Wales, western Tasmania, north-western Australia, and a large swathe inland from south-eastern Queensland to central New South Wales. Greatest warming, of 0.15 °C to 0.20 °C per decade, has occurred in inland South Australia and western Queensland.

Figure 3: Trends in mean temperatures across Australia from 1910 to 2004

 Trends in mean temperatures across Australia from 1910 to 2004

Source: Bureau of Meteorology (data supplied direct to DEH), January 2005

Minimum temperatures have increased by as much as 0.25 °C to 0.30 °C per decade since 1910. The greatest increases in minimum temperatures  have occurred in regions including Queensland and central Australia, inland New South Wales and central Western Australia.

In some regions, maximum temperatures  have fallen slightly. In parts of Queensland and New South Wales, there have been decreases in maximum temperatures of up to 0.05 °C per decade, with a small region of central New South Wales experiencing decreases of 0.05 °C to 0.10 °C per decade. The greatest increases in maximum temperature occurred in parts of central Australia, where maxima have risen by 0.15°C to 0.20°C per decade.

Increases in both annual mean minimum and in annual mean maximum temperatures have resulted in an increase in the area of Australia experiencing extreme temperatures ; that is, above the ninetieth percentile, and a decrease in the area with temperatures below the tenth percentile.

While variability from year to year will ensure that not every year will be a record, the variability is expected to occur on top of a warming trend into the future. This trend will affect many sectors, including agriculture, energy supply, infrastructure, and the environment. McMichael et al. (2002, 3) predict that, in the absence of adaptive measures, the projected rise in temperature over the next 50 years will result in a substantial increase in heat-related deaths in the Australian and New Zealand cities studied. Projections for higher temperatures in future highlight the need for adaptation strategies, which can reduce the risk of damage and increase any benefits of a warmer world.