Attribution of excess heat-related death toll during 2009 heat wave in Victoria, Australia

In recent decades, anthropogenic climate change has led to significant increases in heat wave length and intensity. Many of these heat waves have resulted in substantial impacts on human health. In 2009, the state of Victoria, Australia, experienced several days with maximum temperatures rising 12-15°C above the climatological mean and a marked rise in the human death toll. This study attempts to directly quantify the heat-related human fatalities of the 2009 heatwave attributable to anthropogenic climate change.

We focus on changes in return values of heat wave-related mortality. Furthermore, we combine two types of modeling tools. The first is a set of large initial-condition ensembles of simulations from atmosphere-only models from the weather@home/ANZ and C202C+ D&A projects, and large initial-condition ensembles of simulations from atmosphere-ocean models from CMIP6.  We compare factual outcomes from year-2009 era periods from historical simulations against counterfactual outcomes from either naturalised (non-anthropogenic) simulations or pre-industrial times. The second tool is an empirical model linking heat-related mortality to exceedance of temperature percentile thresholds from daily climate simulation output. This mortality model categorizes heat waves based on three consecutive percentile windows starting at the 95th, 97.5th, and the 99th percentile.

Our analysis shows considerable agreement among the climate-mortality model combinations indicating significant increases in human fatalities during conditions comparable to the 2009 Victoria heat wave under anthropogenic climate change. Most models attribute approximately one third to one half of excess heat-related deaths to anthropogenic greenhouse gas emissions. These findings demonstrate that unless significant climate change mitigation and adaptation efforts are undertaken, further increases in heat-related mortality risk can be expected.