Living in a variable climate

Integrative commentary
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

Historical variability in temperature and potential evaporation

The human-induced climate forcings listed above are expected to influence, and are already affecting components of Australia's climate (see, for example, Cai 2003, Nicholls 2003, Whetton and Suppiah 2003, Karoly and Braganza 2005, Syktus 2005, Watkins 2005, Cai 2006). Night-time minimum temperatures have increased by 1.08°C since 1910, with most of the increase since 1950. The increase in daytime maximum temperatures averaged across Australia (0.58°C) has not been as great, which has resulted in a reduction in diurnal temperature range. The average temperature (that is, average of maximum and minimum) across Australia  (Figure 3) has risen by 0.82°C between 1910 and 2004, with much of the warming occurring in the second half of the twentieth century.The warmest year on record is now 2005, which highlights the current nature of climate change. Up to 2004, the warmest year had been 1998 with all of the ten warmest years since 1910 occurring in the last 32 years (1973–2004). Important regional differences in a warming trend of maximum and minimum temperature are described in Beer 2006.

Rainfall is not the sole climatic determinant of land use and water availability. Potential evaporation, the rate of evaporation from a wet surface, such as an irrigated crop or open water surface, affects water availability. It also determines the water use efficiency of photosynthesis (how much photosynthesis is completed for a given amount of water). The term 'potential' is used because it is an upper limit on the actual evaporation rate, which is negligible when there is little water available in the soil (as in drought). Potential evaporation depends on several climatic factors—humidity, wind, air temperature and (mostly solar) radiation. With the increasing daytime temperatures that are projected as part of global warming, there has long been an expectation that potential evaporation will increase, effectively reducing water availability. However, as evident from the above list of climate factors, this is not necessarily the case, as potential evaporation depends on more than just air temperature.

Although temperatures averaged across Australia have increased, the average continental trends in potential evaporation over the last 30 years surprisingly indicate a decline, albeit small—for example, about three millimetres per year for each year at 30 measuring sites from 1970 to 2002 (Roderick and Farquhar 2004). This represents a decline of approximately five per cent over about 30 years. A similar pattern of declining potential evaporation has been reported in many regions of the world (the United States of America, China, the former Soviet Union, Turkey, India, Thailand and New Zealand). In Australia, there are large regional differences in the trends: south-eastern and western Australia show a decrease in potential evaporation; and some sites in north-eastern Australia show an increase or little change, depending on the starting year (Roderick and Farquhar 2004). The observed decline in potential evaporation across important agricultural regions of south-eastern Australia may have enhanced plant growth and plant water use efficiency. These trends have occurred while average air temperature has increased. It can be concluded that something else must have changed, such as a decline in wind speed or solar radiation, or an increase in relative humidity, or some combination of these variables. The continued monitoring of trends in potential evaporation, and understanding of the contributing factors, will be important in estimating future water availability and demand for rural and urban use.