The research question focuses on determining how soon end-of-century levels of extreme heat and drought stress could occur over Europe and the role of North Atlantic variability. The context is the increasing frequency and intensity of extreme heat events globally, with Europe being potentially vulnerable due to the influence of North Atlantic variability. The purpose is to systematically assess the likelihood of successive and compound extreme heat and drought events in Europe under different warming scenarios and considering internal climate variability. The importance lies in the significant socio-economic and ecological impacts of extreme heat and drought, including increased mortality, reduced labor productivity, economic losses, wildfires, crop failures, and potential regional uninhabitability. Understanding the timing and likelihood of these events is crucial for effective adaptation and mitigation strategies.

The introduction references several studies on the increased frequency and intensity of extreme heat under global warming, including the record-breaking summer of 2010 in Europe. It also cites research on the impacts of extreme heat, such as increased mortality and morbidity, economic losses, and ecological damage. Studies on the role of North Atlantic variability in influencing European climate and its potential for multi-year successive extremes are also reviewed. Existing research indicates a link between North Atlantic Multidecadal Variability (AMV) and European temperature and precipitation, but the exact mechanisms and implications for successive extremes remain unclear. The authors mention several studies utilizing other large ensembles, the limitations of the multi-model studies, and the overall consistency of their results based on the MPI-GE simulations.

The study utilizes the 100-member Max Planck Institute Grand Ensemble (MPI-GE) under historical and RCP4.5 forcing scenarios. MPI-GE is chosen for its large ensemble size and adequate representation of historical internal variability. The researchers employ cumulative excess metrics to quantify heat and drought stress, encompassing Excess Heat, Humid Heat, Night Heat, and Rain Deficit. Compound metrics like Compound Heat Stress, Compound Heat and Drought, and Drought-Rain Volatility are also analyzed. Extreme years are defined as those exceeding the 50th percentile of the ensemble spread in 2090–2099. The study also incorporates observational data from E-OBS for comparison and validation. The Atlantic Multidecadal Variability (AMV) index, calculated as a 10-year mean SST in a specified North Atlantic region, is used to assess its influence on extreme events. Specific calculation methods for each heat and drought metric are detailed, highlighting the use of region- and time-specific thresholds to define extreme conditions and the accumulation of excess above these thresholds. The study also includes a multi-model comparison using ACCESS-ESM1.5, CanESM5, and MIROC6, focusing on changes in extreme temperature and rain-deficit drought across models under a common warming level of 2°C above pre-industrial levels.

The MPI-GE simulations project a significant intensification of heat and drought stress in Europe. End-of-century levels of extreme heat, previously considered virtually impossible, could reach 1-in-10 likelihoods as early as the 2030s for all heat metrics. By 2050–2074, the likelihood of two successive years of end-of-century extreme heat surpasses 15%, and 5-year megadroughts become plausible. The likelihood of end-of-century compound extreme events also increases substantially, reaching 1-in-10 for single years and exceeding 1-in-10 for two consecutive years by 2050–2074. The decadal variability in heat and drought stress significantly increases under warming, with the possibility of experiencing decades exceeding end-of-century levels starting as early as 2040 for heat and 2020 for drought. A warm North Atlantic state significantly increases the likelihood of exceeding end-of-century levels of both single and compound extreme heat and drought, doubling the likelihood of exceeding typical end-of-century levels in decades starting as early as 2030. The multi-model comparison shows relative agreement in the timing and likelihood of exceeding end-of-century heat levels, while drought projections showed more model dependence.

The findings highlight the potential for unprecedented heat and drought extremes in Europe in the coming decades, even under moderate warming. The results underscore the role of internal climate variability and the significant influence of the North Atlantic's decadal variability in accelerating the arrival of end-of-century conditions. The high likelihood of successive and compound extremes indicates increased risks of cascading impacts. The relatively predictable nature of the North Atlantic's decadal variability offers a potential avenue for improved prediction and preparedness. The study's use of cumulative excess metrics and region-specific thresholds enhances the relevance of the results for assessing potential impacts. The authors acknowledge the limitations of relying on a single model and note the greater uncertainty associated with drought projections compared to temperature extremes.

This study demonstrates that unprecedented heat and drought, characteristic of a late 21st-century climate, could become frequent in Europe within the next few decades. Even under moderate warming, the likelihood of end-of-century extreme heat and drought events increases dramatically, with the chance of successive extremes becoming non-negligible. The North Atlantic's influence further exacerbates this risk. The study's findings emphasize the urgency of implementing effective adaptation and mitigation strategies. Future research could focus on refining the multi-model comparison for drought, especially given the large intermodel variability.

The study relies primarily on the MPI-GE simulations, which while comprehensive, represents a single model. The multi-model comparison highlights the larger uncertainties associated with drought projections compared to temperature projections. The study's focus on Europe limits the generalizability of the findings to other regions. The use of specific thresholds to define extreme conditions and the specific choice of metrics for quantifying heat and drought may influence the results.