For example, January 2017 saw the highest monthly mean temperatures on record for the cities of Sydney and Brisbane, and the highest daytime temperatures on record for Canberra. Overall, Australia experienced its 12th hottest summer on record.
There were three distinct heatwaves in southeast Australia during January and February, with the highest temperatures recorded from February 9th to the 12th. For much of the country, the heat peaked on the weekend of February 11th and 12th, when many places hit upwards of 113°F (45°C). The 2016-2017 heatwaves broke long-standing records in central New South Wales that were originally set back in January of 1939 (Figure 2).
The WWA team and colleagues from the University of New South Wales conducted a rapid attribution analysis to see how climate change factored into the exceptionally warm summer (December to February) of 2016-2017. The team also looked at the hottest three-day average February temperatures in Canberra and Sydney.
Regional level: New South Wales
The New South Wales record hot summer can be linked directly to climate change. Two different methods were used to reach this conclusion. First, drawing from a previously published analysis using coupled model simulations, we see that average summer temperatures like those seen during 2016-2017 are now at least 50 times more likely in the current climate than in the past, before global warming began. The team also performed an analysis based on the observational series from ACORN-SAT. This approach is similar to previous analyses used for record heat in the Arctic in 2016 and Central England in 2014. Comparing the likelihood of this record in the climate of today compared with the climate of around 1910 (before global warming had a big impact on our climate system and when reliable observations are available), the team again found at least a 50-fold increase in the likelihood of this hot summer.
The team then looked at the maximum summer temperature for New South Wales (see graphic below). Based on climate model simulations (weather@home and CMIP5) and observational data analysis (ACORN-SAT), maximum summer temperatures like those seen during 2016-2017 are now at least 10 times more likely in the current climate than in the past, before global warming began. In the past, a summer as hot as 2016-2017 was a roughly 1 in 500-year event. Today, climate change has increased the odds to roughly 1 in 50 years — a 10-fold increase in frequency. In the future, a summer as hot as this past summer in New South Wales is likely to happen roughly once every five years. In addition, climate change has increased the intensity of an exceptionally hot summer like this by roughly 1ºC (1.8°F). In the future, the intensity increases by roughly 2°C (3.6°F).
Local level: Canberra and Sydney heatwaves
The team also looked at the local scale to see if a climate change role could be measured in the heat waves that hit Canberra (population ~380,000) and Sydney (population ~4.9 million). Climate has much larger variability at the city level compared to a big area like New South Wales. This can make it more difficult to see the influence of climate change within the overall noise of the weather system.
In Canberra, temperatures hit 96.8oF (36°C) on February 9th and 104oF (40oC) on both February 10th and 11th. Using the weather@home model and ACORN-SAT observations, we analyzed three-day average maximum temperature. Both the observational data and the climate model simulations show that climate change increased the likelihood of the kind of extreme three-day heat observed in Canberra. The weather@home results point to at least a 50 percent increase in the chance of a heatwave like that.
For Sydney, a coastal city, the effect of climate change on this heat wave is less clear. Observations show that climate change increased the chance of such a heat wave occurring, but the high year-to-year variability makes identifying a clear human influence more difficult.
The heat seen this past summer across parts of Australia is still rare in our current climate. However, if greenhouse gas emissions are not dramatically reduced, intense summer heat will become the norm in the future.