Methodology

HeatWatch shows the historic and projected increase in the annual number of extreme heat days with maximum temperatures over certain thresholds that are considered extreme in relation to human health. These include 35, 37 and 40 degrees Celsius. Temperatures that are considered extreme vary by region with people’s capacity to acclimatise.

HeatWatch uses historic maximum temperature data from the Bureau of Meteorology, and CSIRO–BoM temperature projections for future extreme heat days.

For the historic change in the number of days over the various thresholds, HeatWatch uses published maximum temperature records for Bureau of Meteorology weather stations where these are available for the relevant locations. Where there are no records for that location we use the CSIRO–BoM time series data from the Australian Water Availability Project (AWAP) where the average temperature was compiled in five kilometre by five kilometre spatial grids between 1981 and 2010. These are indicated as flat line showing the average annual days with maximum temperatures over the threshold over that period.

The projections use CSIRO–BoM temperature projections for future extreme heat days provided by the CSIRO, and are also compiled in the five kilometre by five kilometre spatial grids of the AWAP data. The projections use between five and eight climate models selected for their historical accuracy, to predict the average number of days over the 35, 37 and 40 degree thresholds in 2030, 2050, 2070 and 2090.

The CSIRO–BoM data is a time series from the Australian Water Availability Project (AWAP) where the average temperature was compiled in five kilometre by five kilometre spatial grids between 1981 and 2010.11 This time series uses between five and eight models to predict days over 35 degrees, over 37 degrees and over 40 degrees in 2030, 2050, 2070 and 2090.

The models use IPCC scenarios for global climate action: RCP 2.6 (“low emissions”) and RCP 8.5 (“high emissions/current government policies”). RCP 2.6 equates roughly to what is required to keep the world below 1.5 degrees warming, and RCP 8.5 is the “business as usual” scenario where the world fails to act decisively on climate change. RCP 8.5 is the current trajectory due to the world’s failure to implement necessary climate policies.

The results of the different models are reduced to three sets for each of the three scenarios:

• a minimum result (the result from the model with the fewest hot days predicted)
• a maximum result (the result from the model with the most hot days predicted)
• a mean result (the average result across all models)