Heat-mortality impacts under 1.5°C overshoot pathways

Within the 2015 Paris Agreement, the international community committed to limiting the long-term rise of global temperature to 1.5°C. As our planet continues to heat as a result of continued greenhouse gas emissions, it has become highly likely that the world is entering a period where global mean temperatures exceed this 1.5°C limit - a period referred to as Overshoot. Despite their importance for society and policymakers, health impacts of this overshoot remain understudied, particularly the different consequences of following different overshoot pathways.

In this study we therefore combine climate model output with a well-established epidemiological model to quantify the increase of heat-related mortality under pathways that overshoot the 1.5°C target. This analysis is conducted for over 850 locations across 52 countries, for which daily city-level mortality data is available through the MCC (Multi-Country Multi-City) Collaborative Research Network. The epidemiological analysis relies on quasi-Poisson regression time series analyses and requires daily city-level mortality data to establish location specific temperature-mortality relationships. We then project heat-related mortality levels across all 540 Paris Agreement–aligned scenarios available in the IPCC AR6 Scenario Database. To this end, we estimate local heat-mortality impacts for each location as a function of global mean surface temperature, by sampling data from five fully coupled earth system model initial condition large ensembles (SMILEs). In addition, we validate our approach using bespoke earth system model simulations that represent physically consistent overshoot and stabilization pathways which follow the recently developed Adaptive Emission Reduction Approach (AERA) methodology

We find a robust linear increase of heat-mortality with the cumulative temperature exceedance above 1.5°C (“overshoot-degree-years”) of each future global mean surface temperature (GMST) scenario. Hence, both the length (time) and intensity (temperature) of the overshoot is relevant for levels of heat-mortality as the impacts scale with the integral of GMST above 1.5°C over time. The linear increase of heat-mortality is in the range of 1-2 % / °C year, with larger increases found in tropical countries. While the linear scaling is apparent in nearly all countries and within all five SMILEs used, the slope of the linear relationship depends on the SMILEs. Comparing the sampled results to the physically consistent AERA runs reveals a good agreement, although the sampling approach slightly overestimates heat-mortality after the peak of GMST. Our results thus lay an important foundation for law and policy makers, as we clearly show that delaying climate action leads to increased heat-mortality.