1.    Why climate trend information is important

Analysing trends over time in climate variables is an important tool for understanding whether or not the climate is changing, and the causes of any change. Knowledge of trends can ensure that decision-making reflects current climate in sectors that are climate-sensitive, and that changing climate risks are adequately considered.

The climate is not static. Across Australia there is a wide range of climates, as well as considerable climate variability over time in any region. This needs to be taken into account when looking at trends.

• Thinking about spatial variations: A wider regional scale is generally needed to identify trends, as patterns of climate variability can mask changes in the climate at local scales, particularly for rainfall.
• Thinking about changes over time: You need to search for trends over long time periods - several decades or more. This allows the trend to show clearly against the background of year-to-year natural variability.  Decisions based only on a short period of climate data could turn out to be poorly matched to future climate conditions.

Figure 1 shows clearly how looking at only a short period of climate data can be misleading.  If you take only the period 1980-1990, there looks to be no trend, or even a slight downward trend, in the data.  Yet over the complete period there is a clear upward trend (as shown by the straight line), indicating increasing risk of fire over time.

Climate change is a contentious subject and one of the major tools to prove or disprove its existence is trend analysis. This means there are many articles and web pages that seek to support a position on climate change by demonstrating trends or by seeking to demonstrate that trends don’t exist.  When looking at these, it is worth considering:

• How long is the data set? If only a few years, that should set off alarm bells.
• Is it based on just one station record, or has a regional/global analysis, averaging across a number of stations at each time point, been carried out? If just one station, then again it is worth questioning the results.

This website provides an interesting perspective on how climate data can be manipulated to support a particular point of view: http://www.realclimate.org/index.php/archives/2012/03/data-presentation-a-trend-lesson, (accessed 22 March 2016).

2.    Understanding trend information

There is a wide range of information available on recent trends in key climate variables from the Bureau of Meteorology website, the regular State of the Climate reports on the Bureau’s website, and from the Climate Change in Australia website managed by CSIRO and the Bureau of Meteorology.

Climate trend information can be presented in a number of ways. This section summarises how trends in climate information are typically presented.

3.    What trend information is available for Australia?

3.1    Trends in the oceans and coastal waters

Clear and up-to-date information on recent trends is available for many climate variables, particularly those associated with temperature, from the Bureau of Meteorology website. For example, Figure 7 shows that sea surface temperatures around Australia have warmed since records began. The warming has been consistently identified in both air (see “Observed climate and sea-level change”) and oceanic measurements.

Sea levels reflect temperature trends, since an important cause of sea-level rise is thermal expansion.  Sea levels around Australia show a similar increasing trend to that measured globally. There is some variability in the trend around the Australian coastline, with greater increases observed in the north, north-west and south-east than in the southern and mid-eastern regions. Figure 8 shows the recent trend of sea-level rise around Australia.

3.2    Trends over the land

There are clear trends in increasing temperatures throughout Australia and over time (see Figures 4 and 9).  In contrast, Australian rainfall is highly variable, which makes it difficult to identify significant trends over time. Nevertheless some rainfall changes are discernible, notably declines in winter rainfall in the southwest and southeast of the continent in recent decades.

It is also difficult to draw conclusions regarding trends in the frequency and intensity of tropical cyclones in the Australian region because of the shortness of the satellite record, changes in historical methods of analysis, and the high variability in tropical cyclone numbers.

4.    How to use maps and graphs to understand local climate trends

Typical steps in using maps and graphics to understand local trends are to:

• Consider the area or location of interest, noting that because climate patterns can be quite large, broader-scale trend information can be informative and of value to local-scale considerations.
• Identify the climate variables of interest, and search the Bureau of Meteorology (http://www.bom.gov.au/climate/maps/) and Climate Change in Australia (http://www.climatechangeinaustralia.gov.au/en/climate-campus/australian-climate-change/australian-trends/) websites for relevant maps and graphics (both accessed 22 March 2016).
• Consider whether expert guidance is required to interpret available information and maps on some of the more variable climate records such as rainfall and windstorm. 
• Consider whether your area of interest has any trends that are particularly relevant to local planning and decision-making, and incorporate the information in risk assessment if required.

Finally, while trend information on how the climate has changed over recent decades is important to inform decision-making, for costly decisions with a long-asset life it is important to also consider future climate change (See content on Causes of climate change and Climate change in Australia).