SW WA Projections


Southern and South-Western Flatlands: Cluster Report

As part of CSIRO’s report Climate Change in Australia. Information for Australia’s Natural Resource Management Regions: Technical Report (2015),  CSIRO and the Australian Bureau of Meteorology have prepared tailored climate change projections reports for each of the NRM regions across Australia grouped into ‘clusters’. Our South West NRM Region is a part of the Southern and South-western Flatlands cluster. This cluster covers all the NRM regions of Western Australia except the Rangelands, and South Australia’s Eyre Peninsula, Northern and Yorke, Kangaroo Island, Adelaide and Mt Lofty Ranges NRM Regions. The technical cluster report can be found here and on the website. For easy communication of their findings an 8 page brochure has also been prepared for the cluster, found here.

Information below on the South-Western Flatlands West sub-cluster has been sourced from this technical report.

Current Climate

The Southern – South-Western Flatlands West (being the South West of Western Australia including our SW NRM Region) area has a predominantly Mediterranean climate, with high winter rainfall and little summer rainfall. Strong gradients in rainfall and temperature exist within the sub-cluster.

Maps of daily mean temperature show the largest spatial variability in summer, with a strong gradient from south to north. Mean summer maximum temperatures range from 24˚C in the south up to more than 33˚C at the northern extent of the sub-cluster (being just north of Geraldton). In winter, there are slightly warmer mean temperatures along the coast, but a small range across the cluster. The average annual temperature for the sub-cluster is 17.7˚C.

Geographically, there are strong gradients in mean rainfall across the sub-cluster. The far south-west of the sub-cluster receives annual rainfall totals of 1200 mm while only 300 mm falls further inland.

Past Temperature Trends

Temperatures have increased over the past century, with the rate of warming higher since 1960. Mean temperature increased between 1910 and 2013 by around 1.1˚C in the south-west sub-cluster.


For the sub-cluster report, climate change projections are based on historical and future climate simulations from the CMIP5 model archive. In addition to the CMIP5 model results, downscaling has been used to derive finer spatial information in the regional projections. While down-scaling can provide added value on finer scale processes, it does increase the uncertainty in the projections since there is no single best downscaling method. For the regional projections, CSIRO considered downscaled projections from two techniques: outputs from a dynamical downscaling model, the Conformal Cubic Atmospheric Model (CCAM)(McGregor and Dix, 2008) and the Bureau of Meteorology analogue-based statistical downscaling model. The downscaled results are only emphasised by the authors if there are strong reasons for giving the downscaled data more credibility than the GCM data (the primary source of climate change projections in the report).

 Projected changes have been prepared for four Representative Concentration Pathways (RCPs) used by the latest IPCC assessment (CMIP5).

RCP2.6 – requiring very strong emission reductions from a peak at around 2020 to reach a carbon dioxide (CO2) concentration at about 420 parts per million (PPM) by 2100
RCP4.5 – slower emission reductions that stabilise the CO2 concentration at about 540 ppm by 2100
RCP8.5 – assumes increases in emissions leading to a CO2 concentration of about 940 ppm by 2100.

Confidence ratings for the projections are based on the judgement of the authors derived from multiple lines of evidence.

Summary of Climate Change Projections for the sub-cluster

Average temperatures will continue to increase in all seasons.

  • Very high confidence – continued substantial increases in projected mean, maximum and minimum temperatures.
  • High confidence – For the near future (2030), the annually averaged warming across all emission scenarios is projected to be around 0.5 to 1.1˚C above the climate of 1986-2005. By late in the century (2090), for a high emission scenario (RCP8.5) the projected range of warming is 2.6 to 4.0˚C. Each individual season is projected to warm by the same amount as the annual mean.


Hotter and more frequent hot days, less frost.

  • very high confidence – substantial increases in the maximum temperature on the hottest days, the frequency of hot days and the duration of warm spells are projected.
  • high confidence – a decrease in the frequency of frost days is projected.
  • For example, in Perth, the numbers of days above 35˚ and above 40˚ C are 150% greater by 2090 under RCP4.5 and median warming.


Less rainfall in winter and spring, changes in other seasons unclear.

  • high confidence – decreases in winter and spring (and annual) rainfall.
  • By 2030 winter rainfall may change by -15 to +5%. By 2090, these ranges are around -30 to -5% under RCP4.5 and -45 to -15% under RCP8.5. Changes in autumn and summer are unclear.


Increased intensity of heavy rainfall events, drought duration to increase.

  • medium confidence – intensity of heavy rainfall events will increase in the south west sub-cluster.
  • This increase is projected despite projected decreases in mean rainfall. Projections of the magnitude of change have low confidence and therefore the time when any change may be evident against natural variability cannot be reliably projected.
  • high confidence – time spent in meteorological drought is projected to increase compared to the present climate.
  • Under all RCPs, and at all time periods into the future, the time spent in meteorological drought is projected to increase compared to the present climate. These increases become quite marked by 2090 under RCP8.5. The projected changes are strongly driven by the projected decline in annual rainfall.


Decrease in winter mean wind speed.

  • high confidence – small changes in mean surface wind speeds under all RCPs by 2030
  • high confidence – decreases in winter mean surface winds for 2090 under RCP4.5 and RCP8.5 based on model results and physical understanding (relating to decreased storminess)
  • medium confidence – decreases are suggested for extreme wind speeds, particularly for the rare extremes under both RCP4.5 and RCP8.5


Increased solar radiation and reduced relative humidity in winter and spring.

  • high confidence – little change to solar radiation by 2030
  • high confidence – an increase in winter radiation by 2090, under RCP4.5 and RCP8.5 (5 and 10% increase respectively)
  • medium confidence – an increase in spring radiation (about half of that expected in winter) by 2090, under RCP4.5 and RCP8.5 and little change in summer and autumn.
  • high confidence – little change in relative humidity by 2030
  • high confidence – decrease in relative humidity for winter and spring by 2090 (up to 5% under RCP8.5)


Increased evaporation rates, and reduced soil moisture and runoff.

  • high confidence – increases in potential evapotranspiration in all seasons with the largest absolute rates projected in summer by 2090.
  • high confidence – decreases in rainfall and increases in potential evapotranspiration are projected to lead to a decrease in soil moisture and runoff by 2090 under both RCP4.5 and RCP8.5


Harsher fire-weather climate in the future.

  • high confidence – climate change will result in a harsher fire-weather climate in the future
  • low confidence – in the magnitude of the change as this is strongly dependent on the summer rainfall projection


Higher sea levels and more frequent sea level extremes.

  • very high confidence – sea level will continue to rise during the 21st century.
  • Relative sea level has risen around Australia at the average rate of 1.4 mm/year between 1966 and 2009 and 1.6mm/year after the influence of the El Nino Southern Oscillation (ENSO) on sea level is removed. By 2030, the projected range of sea level rise at Fremantle is 0.07 to 0.17 m above the 1986-2005 level, with only minor differences between emission scenarios.  As the century progresses, projections are sensitive to emissions pathways, By 2090, RCP4.5 gives a rise of 0.28 to 0.65 m and RCP8.5 gives a rise of 0.39 to 0.84m. These ranges are considered likely (at least 66% probability). However, if a collapse in the marine based sectors of the Antarctic ice sheet were initiated, these projections could be several tenths of a metre higher by late in the century.
  • Taking into account the nature of extreme sea levels along the southern and south-west flatlands coastlines and the uncertainty in the sea level rise projections, an indicative extreme sea level ‘allowance’ is provided. The allowance being the minimum distance required to raise an asset to maintain current frequency of breaches under projected sea level rise. In 2030, the vertical allowances along the cluster coastline are in the range of 11 to 13 cm for all RCPs, and by 2090, 48 to 56 cm for RCP4.5 and 66 to 76 cm for RCP8.5.


Warmer and more acidic oceans in the future.

  •  very high confidence – sea surface temperature (SST) has increased significantly across the globe over recent decades and warming is projected to continue. 
  • Across the coastal waters of the southern and south western flatlands region in 2090, warming is projected in the range of 1.5 to 3.9˚C for RCP8.5.
  • very high confidence –  around Australia the ocean will become more acidic
  • high confidence – the rate of ocean acidification will be proportional to carbon dioxide emissions.
  • By 2030, pH is projected to fall by up to additional 0.08 units in the coastal waters. By 2090, pH is projected to fall by up to 0.15 units under CP4.5 and up to 0.33 units under RCP8.5. These values would represent an additional 40% and 110% in acidity respectively.
  • Continued acidification will compromise the ability of calcifying marine organisms such as corals, oysters and some plankton to form their shells or skeletons.