Quantifying the contributions of climate change and adaptation to mortality from unprecedented extreme heat events

Understanding the mortality effects of the most extreme heat events is central to climate change risk analysis and adaptation decision-making. Accurate representation of these impacts requires accounting for the effects of prolonged sequences of hot days on mortality, the change in that mortality due to anthropogenic forcing, and the potential compensating effects of adaptation to heat. Here, we revisit the August 2003 heat wave in France, a canonical event in a region with rich climate and mortality data, to understand these influences. We find that standard heat mortality exposure-response functions underpredict excess deaths in August 2003 by 60%, but that accounting for the temporally compounding effects of hot days better matches observed mortality. After accounting for compounding effects and applying a machine learning approach to single-event climate attribution, we attribute 6,038 deaths in August 2003 to climate change, ten times higher than previous estimates. Finally, we show that recent adaptation to heat has reduced the projected death tolls of future 2003-like events by more than 80%.