The last 16 record-breaking hot years globally clearly show the influence of human caused climate change with the first signs appearing as far back as the 1930s, according to new Australian research released today.
"Globally all the record-breaking hot years we've had since the 1990s are so much outside natural variability that they would be almost impossible without climate change caused by humans," said Dr Andrew King, from the ARC Centre of Excellence for Climate System Science.
But even at country and regional scales, where it is often much harder to detect global warming signals, the influence of human caused climate change still became unmistakable in many regions in the 1990s and for Australia as far back as 1980.
"In Australia our research showed the last six record-breaking hot years and last three record-breaking hot summers were made much more likely due to global warming," said lead author, Dr King.
"We were able to see climate change more clearly in Australia because of its position in the Southern Hemisphere in the middle of the ocean, far away from the cooling influence of high concentrations of industrial aerosols."
Previous research has shown that aerosols in high concentrations over specific regions had a cooling effect, reflecting more heat back into space. However, when those aerosols were removed from the atmosphere, the warming returned rapidly.
This cooling impact was seen very clearly by the researchers when they looked at five different regions, Central England, Central Europe, Central US, East Asia and Australia.
Cooling periods, likely caused by aerosols, occurred in Central England, Central US, Central Europe and East Asia during the 1960s and 1970s before accelerated warming returned. These heightened aerosol concentrations also delayed the emergence of a clear human caused climate change signal in all regions studied except Australia.
"In regards to a regional human caused climate change signal, Australia was the canary in the coal mine for the rest of the world. The signal appeared there first and then over the coming years it became apparent elsewhere," Dr King said.
"Recent increases in aerosols over East Asia has started to slow the rise in the number of the region's record hot years and summers, again masking the clear climate change signal we are finding in other areas.
"High aerosol concentrations also significantly delayed the climate signal in the Central US."
To get their results the researchers took a new approach. In the past, most researchers searching for a human caused climate change signal selected specific events and then tried to determine the role of climate change in those events.
By contrast, this study looked at when events started exceeding the range of natural variability. Using climate models, they looked at a world without human produced greenhouse gases and compared it to a world where the composition of the atmosphere corresponded with those found over time in the real world.
Where real world observations rose above the range of natural variability produced in climate models showing temperatures in a world without industrial activity, this indicated the unequivocal influence of human caused global warming.
"Everywhere we look the climate change signal for extreme heat events is becoming stronger. The key now is to determine how much warmer the climate will continue to get, so we can respond to the impacts this will inevitably bring," said Dr King.
"This is particularly true for Australia, which appears to have one of the strongest climate change signals for a populated country. As a nation, it will need to respond more quickly and understand clearly what future climate change brings."