Analysing long term trends in climate variables is an important tool for understanding whether the climate is changing and the causes of any change.

For example, examining decades of temperature records can reveal gradual warming, while tracking rainfall patterns over time can highlight shifts in drought frequency or intensity. This knowledge supports informed decision-making in climate-sensitive sectors such as agriculture, water management, and infrastructure planning, helping ensure effective management of emerging risks, such as increased bushfire danger or coastal flooding.

The climate is dynamic. Across Australia there is a wide range of climate zones, ranging from tropical in the north to temperate in the south, and each region experiences significant variability over time. For instance, a single wet year in an otherwise dry decade does not indicate a long-term trend.

This natural variability needs to be considered when analysing trends to avoid drawing misleading conclusions.

To do this:

• consider spatial variation:
  • take a broader regional perspective to detect climate trends, since local variability in climate – especially in rainfall – can mask underlying long-term changes.
• consider changes over time:
  • examine climate trends over long time spans – typically several decades or more – to clearly distinguish them from natural variability from year-to-year variability.
  • check you focus: relying on short-term data alone can lead to decisions that are poorly matched with future climate conditions. See the example in the box below.

Trend analysis is a valuable tool when you need to understand change over time in climate variables such as temperature, rainfall. It helps identify long-term patterns that inform operational decisions, such as adjusting crop calendars, planning water allocations, or designing infrastructure for current conditions.

However, trend analysis only reflects the past and present; it does not predict future changes. For investments with long lifespans or high exposure to climate risk, such as roads, dams, or energy systems, you should complement trend analysis with future climate projections to ensure resilience under changing conditions

Climate change has been a contentious topic, despite the strong scientific consensus around it. Trend analysis is one of the main tools used to support or challenge claims of its existence.

To assess whether the trend has been analysed correctly (whether intentionally or unintentionally) consider the following:

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

Note that climate researchers consider both the time period and spatial variation but also use more complex tools such as a range of statistical analyses to ensure their conclusions are rigorous and robust.

There is a wide range of information available on recent trends in key climate variables from the:

• Bureau of Meteorology website
• State of the Climate reports. This report is a synthesis of the science that underpins our understanding of Australia’s climate and is produced every two years by CSIRO and the BoM
• Climate Change in Australia website, which is managed by CSIRO and the BoM.
• your relevant state government website (see link).

Climate trend information can be presented in a many ways. This section summarises the most common ways of presenting trends in climate information.

Time series graphs generally show the climate variable on the y (vertical) axis with time on the x (horizontal) axis.

• For trends in graphs, it is important that data is considered over a reasonably long time period, preferably several decades or longer, as short-term fluctuations—common in daily or monthly observational data—can obscure long-term patterns.
• Averaging the data helps reveal clearer trends.
• These graphs often show anomalies relative to a fixed baseline, which ensures consistency when combining data from multiple stations.

Bar charts are similar to graphs in that they often present the climate variable on the y (vertical) axis with time on the x (horizontal) axis.

• A bar chart is useful to show trends in the number of occurrences of a particular climate or weather event over time.
• The longer or taller the bar the more frequent the occurrence of the climate event.
• Each bar represents one time period, typically one year.

A climate trend map and a decile map (below) both help visualize climate data, but they serve different purposes and are interpreted differently.

Maps of trends over time show the extent of change in a climate variable from a historic to a more recent point in time.

• A trend map is useful when you are interested in a specific climate variable and its the regional variability.
• It displays the extent of change a specific climate variable (like temperature or rainfall) over a long period, typically decades.
• It illustrates the direction (increase or decrease) and magnitude (how much) of change.

Decile maps show the amount of difference in a region or location from average levels.

• A decile is one tenth of the values in a record. A decile map ranks a climate variable (usually rainfall or temperature) within the historical record, often over a standard period (e.g. 1981–2010).
• A decile map is useful as is allows comparison of one place with another when there is a lot of variation from place to place, as there is with rainfall in Australia.
• It shows how unusual a specific period (like a month, season, or year) was, compared to past years. For example:
  • How wet is the most recent decade compared to the full record?
  • Is it in the highest decile (very wet), the lowest decile (very dry) or somewhere in between?
  • Or described in another way, Decile 1–2: Among the driest or coldest 10–20% of years; Decile 9–10: Among the wettest or hottest 10–20% of years.

Combined approaches may include maps that show trends at specific locations through symbol size.

Combined approaches are useful where understanding regional variability is important and there is also incomplete information. The map must be evaluated as a whole, and not on the basis of a single record.

Good questions to ask are:

• What proportion of stations show an increase/decrease?
• Are they clustered in a particular region?

For many climate variables, there is clear and up-to-date information on recent trends available from the Bureau of Meteorology website.

CSIRO and the Bureau of Meteorology produce a biannual report that captures trends in the key variables. For example, the State of the Climate 2024 summaries the trends for Australia.

To understand local climate trends using maps and graphics, use the following steps:

• Define the area or location of interest.
  • Keep in mind that climate patterns can be quite large – and extend beyond local boundaries – so broader regional trend information can provide important context for local-scale changes.
• Identify the relevant climate variables.
  • Determine which variables – such as temperature, rainfall, or sea level – are most important for your needs.
  • Explore up-to-date maps and visual tools from trusted sources like the Bureau of Meteorology and Climate Change in Australia for data and graphics.
• Consider whether expert guidance is needed.
  • Interpretation of some climate records – especially highly variable ones like rainfall and wind patterns – may require expert input to ensure any trends are accurately understood and applied correctly.
• Consider relevance for local decision-making.
  • Identify whether observed trends in your area of interest are likely to affect local planning, community risk or other decision-making, and incorporate the information in risk assessment if required.