Methodology

Climate models used

Global climate model simulations used for these projections come from the international Coupled Model Intercomparison Project Phase 5 (CMIP5 ). A list of  the global climate models used in these projections can be found here . 

Use of downscaling

Model results from the CMIP5 ensemble are complemented here with some fine resolution downscaled data. Two new sets of downscaling (one statistical and one dynamical) results are used, and downscaled results from some previous regional studies are also consulted. The downscaled data do not play a major role in these projections, with their role confined to the assessment of confidence in the global climate model-based projections (Technical Report Section 7.2), and as sources for application ready data (projected climate change data applied to observations) in some circumstances where the added value was clearly established (Technical Report Section 6.3.2). This site contains more information on the use of downscaling in these projections and on downscaling in general.

Model evaluation and model selection

Climate models were assessed as to how well they simulated aspects of current climate (Technical Report Chapter 5). Although models varied considerably in their skill, no subset of climate models performed well across all metrics and there was not a strong link between model evaluation results and projected changes (Technical Report Chapter 6). Consequently, a skill-based weighting was not applied and all available simulations were considered in preparing quantitative ranges of projected change. However, model evaluation results were used in informing selection of individual models for the development of application-ready data sets.

Eight models used for provision of 'application-ready' data

A sample of eight CMIP5 models, from a total of 40, has been selected for use in climate change impact assessments, ensuring that the range of change in the smaller sample is representative of the broader range of results for Australia, while also considering model skill and model genealogy.The method used for selection of these models is described in Chapter 9 of the Technical Report and here .

Representative Concentration Pathways

Projected changes are given for a range of emission scenarios as defined by the Representative Concentration Pathways (RCPs) used by the IPCC. These scenarios range from RCP2.6 with low emissions and ambitious and sustained global emissions reduction, to RCP8.5, a high-emission scenario with ongoing increase in emissions beyond the 21st century. 

Calculating projected ranges of change relative to a reference baseline

Projected changes are calculated based on area-averaged data for each of four super-clusters, eight clusters, and fifteen sub-clusters covering the entire nation. These projections are presented as ranges of change for different 20-year time periods in the future with respect to the reference period (1986-2005). Generally, the 10th to 90th percentile range of the distribution of CMIP5 global climate model results is used to characterise the projections (Technical Report Section 6.2.2). In addition to model ranges, changes for individual models are also available on the website. Time series charts of simulated climate change (1901 to 2099) against a longer baseline (1950-2005) are also presented within this work to illustrate the interplay between the slowly emerging climate change signal and natural internal variability.

The approach used here differs from that used in the 2007 Climate Change in Australia projections (CCIA 2007) in a number of technical aspects mainly related to how natural climatic variability is represented in the projections. Projected ranges of change presented here include differences from the baseline due to natural variability as well as changes due to enhanced greenhouse conditions, whereas this was not the case in CCIA 2007. 

Key messages and confidence

Projections presented on this site include summary key messages and associated confidence statements. These are based on the projected ranges of changes from the new generation climate models, but also include additional lines of evidence such as physical theory, understanding of processes driving the projected change, climate model agreement, model evaluation and consistency of global climate model results with downscaled (or high/fine resolution) simulations. Our approach to assigning confidence is similar to that of Intergovernmental Panel on Climate Change (IPCC) and is explained further in Section 6.4 of the Technical Report. Examples of the synthesis of different lines of evidence in assigning confidence may be found in the projections sections of the Technical and Cluster Reports.

Variables

A list of climate variables available from this website can be found here . For some variables, selected measures of frequency or intensity of extremes are included as well as mean conditions. A range of products is available for most of the variables. However, in some cases only qualitative information (e.g. direction of change) is highlighted, as confidence in the reliability of quantitative information from the climate models was low.