Significant increase in heat-related human death toll due to climate change during 2009 Victoria heatwave

In recent decades, anthropogenic greenhouse gas emissions have led to heatwaves becoming significantly more intense, many of which resulting in substantial impacts on human health. In 2009, the state of Victoria, Australia, experienced several days of maximum temperatures soaring 12-15°C above the climatological mean and a considerable rise in excess heat-related human mortality. We attempt to directly quantify the heat-related human fatalities of the 2009 heatwave attributable to anthropogenic climate change.

For our analysis, we focus on changes in return values of the heat-related death toll. Furthermore, we use a combination of two types of modeling tools. The first is a collection of large initial-condition ensembles of atmosphere-only model simulations from the weather@home/ANZ and C20C+ D&A projects as well as large initial-condition ensembles of simulations from models taking part in the Coupled Model Intercomparison Project Phase 6. For the attribution assessment we compare factual model outcomes from year-2009 era periods from historical simulations to counterfactual outcomes. The second modeling tool is an empirical function linking heat-related human deaths to exceedance of temperature percentile thresholds. This function categorizes heatwave days based on three consecutive percentile windows starting at the 95th, 97.5th, and the 99th percentile, respectively.

The climate-mortality model combinations show considerable agreement with most models attributing approximately one third of excess heat-related deaths during conditions comparable to the 2009 Victoria heatwave to anthropogenic climate change. Our analysis indicates that without substantial climate change mitigation and adaptation efforts, to reduce exposure and vulnerability, further increases in heat-related mortality risk are to be expected.