A new study led by ANU has warned that Melbourne and Sydney should prepare for 50 degree Celsius days under the Paris Agreement global warming limit of 2 degrees.
Lead researcher Dr Sophie Lewis said the study assessed the potential magnitude of future extreme temperatures in Australia under Paris targets of an increase in global temperatures of 1.5 and 2 degrees Celsius above pre-industrial levels.
"Major Australian cities, such as Sydney and Melbourne, may experience unprecedented temperatures of 50 degrees Celsius under 2 degrees of global warming," said Dr Lewis from the Fenner School of Environment and Society and the Centre of Excellence for Climate System Science at ANU.
"The increase in Australian summer temperatures indicates that other major cities should also be prepared for unprecedented future extreme heat.
"Our climate modelling has projected daily temperatures of up to 3.8 degrees Celsius above existing records in Victoria and New South Wales, despite the ambitious Paris efforts to curb warming."
Dr Lewis said immediate climate action internationally could prevent record extreme seasons year after year.
"Urgent action on climate change is critical - the severity of possible future temperature extremes simulated by climate models in this study poses serious challenges for our preparedness for future climate change in Australia," she said.
Dr Lewis said the record hot Australian summer in 2012 and 2013 was made more likely due to human-caused greenhouse warming, and such an event was expected to occur more frequently under future warming.
"One of the hottest years on record globally in 2015 could be an average year by 2025," she said.
Co-researcher Dr Andrew King from the University of Melbourne said the research team used a combination of observations and modelling to assess how the magnitude of record-breaking events may change in the future.
"Previous scientific studies have focused on how current temperature extremes have been impacted by climate change, or on how the frequency of these current extremes will change in the future," said Dr King from the School of Earth Sciences and the Centre of Excellence for Climate System Science at the University of Melbourne.
"This study takes a different approach and examines how the severity of future temperature extremes might change in the future."
The research, supported by the Centre of Excellence for Climate System Science, is published in Geophysical Research Letters.