Climate change trends

Information on greenhouse gas emissions is based on scientific observations by thousands of scientists around the world for the International Panel on Climate Change (IPCC).

The IPCC now considers that greenhouse gas emissions are very likely to be contributing to the trends in climate we have been experiencing. How much is part of natural variability and how much is attributable to greenhouse emissions and human activity is discussed below.

Analysis of these trends helps us to identify the risks to which we are exposed and the frequency of occurrence of adverse conditions. These frequencies can be expressed as a probability, such as one year in two, or 50% of the time. Analysis of the trends will also allow us to predict the probability of adverse conditions resulting from climate change.

Climate change trends

The basic parameters of weather — temperature, precipitation and wind are already changing:

• Higher maximum, minimum and average temperatures are being recorded in most (but not all) areas. Restricted, local geographic variations may cause small localised reductions in temperatures.
  
• Changes in rainfall patterns, encompassing timing and distribution of rainfall, have already been observed. For example, in the southwest of Western Australia stream flows fell to 75% of historic levels after 1975 and to about 60% of those levels after 1995 (19).
  
• As atmospheric temperatures rise inconsistently and heat is redistributed through greater movement of the air, which is, wind. Even a small difference in sea surface temperature is believed to be responsible for more destructive cyclone and hurricane activity.

Figure 4: Spatial patterns of changes in (a) maximum, (b) minimum and (c) mean temperatures in Australia from 1950 to 2006. Source: Australian Bureau of Meteorology (6).

Temperature trends

Figure 4 shows the temperature trends in Australia from 1950 to 2006. It shows that for most of South Australia, temperatures have increased by about 0.1°C per decade. Details of this are also shown in Table 1.

Table 1: Observed temperature trend increases for Australia and South Australia (6).

The Bureau of Meteorology climate change pages at www.bom.gov.au gives temperature trends for summer, autumn, winter and spring. For all seasons, temperature has increased, but temperature has increased more in spring since 1950 than in other seasons in South Australia. Spring temperature trends are shown in Figure 5.

It is of concern that the peaks in mean temperature keep getting higher. The Spring 2006 temperature anomaly was 1.09 degrees higher than normal and Spring 2006 was 2.1 degrees, the highest since 1950.

Figure 5: Mean temperature trends for South Australia in spring since 1951 (Source: Australian Bureau of Meteorology).

Rainfall trends

Australian Bureau of Meteorology (BOM) rainfall records from 1900 to 2006 (Figure 6) show that some parts of Australia have experienced a trend towards increased rainfall over that period. BOM suggest this is largely due to a number of dry years in the early part of the centuries and wet periods in the 1950s and 1970s.

However, in South Australia the changes in rainfall have been least, with a slight increase observed in the pastoral regions and a slight decrease over the agricultural areas. This decrease appears more pronounced and extends further north and north east if we limit the analysis to the last 50 years of rainfall.

Decadal fluctuations in annual rainfall are dominated by summer and spring rainfall fluctuations (9).

Figure 6: Rainfall trends in Australia for (a) 1900–2006 and (b) 1950–2006 (27). Trends are shown as mm change per 10 years. Source: Australian Bureau of Meteorology (6).

Figure 7 shows the rainfall trends for summer, autumn, winter and spring in South Australia between 1950 and 2006. The autumn rainfall has decreased for all arable areas in South Australia and winter rainfall has decreased in the western half of South Australia. Spring rainfall has increased slightly for most of the state, between 1950 and 2006, except for the south east corner. However it is not clear how much of these changes are due to natural variability and how much is due to greenhouse induced climate change.

General projections resulting from climate change are also for rainfall to occur in less frequent and more intense events resulting in increased runoff, erosion and flooding risks.

Figure 7: Rainfall trends in South Australia for 1950–2006 for summer, autumn, winter and spring (27). Trends are shown as mm change per 10 years. Source: Australian Bureau of Meteorology.

Evaporation trends

Figure 8 shows the general increase in overall pan evaporation for South Australia. This is partially due to the fact that the 1970s was an unusually wet decade, thereby reducing evaporation. However, evaporation is also impacted by temperature, solar radiation, humidity, wind speed and exposure. All these factors need to be taken into consideration when analysing evaporation trends.

Figure 8: South Australian Mean Annual Pan evaporation and 11-year running averages are shown by black curve. Source: Australian Bureau of Meteorology.

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