Assessing the occurrence of compound hot and dry events from pre-industrial conditions to present-day extremes

Heatwaves and dry spells are major climate hazards that severely impact human health, economy, agriculture, and natural ecosystems. Compound hot and dry summers have become more frequent and intense in recent years in Europe. What remains unclear is, however, to which extent the observed trend can be explained by climate change or as a feature of internal climate variability. In this study, we assess the frequency and intensity of compound hot and dry events in Europe by analyzing recent historical events from reanalysis data 1960-2022 and comparing it to i) a counterfactual reference (corresponding to pre-industrial climate conditions), and ii) model data derived from a Single Model Initial-condition Large Ensemble (SMILE).

We use data from the fifth generation of the European Reanalysis (ERA5) to assess the current frequency of the compound hot and dry summers like 2003, 2015, 2018, and 2022 and analyze the intensity of the events. We use the data from the 50-member SMILE Canadian Regional Climate Model Large Ensemble (CRCM5-LE) and calculate the probability of event occurrence for those events in Europe’s current climate. Employing the ensemble allows us to assess the influence of internal climate variability vs. climate change for those events. Additionally, we use pre-industrial conditions (pi-control runs) simulated with CRCM5 to compare the probability of recent hot and dry compound events to a counterfactual world without climate change. 

Our analysis shows that climate change increases the frequency and intensity of compound hot and dry events. We see a substantial increase in occurrence probabilities compared to a pre-industrial world and draw to emerging hotspots of new compound extremes in several European regions. We illustrate the added value of using pi-control runs in a regional SMILE as a novel approach for impact quantification. It provides the means to understand better the already prominent role of climate change on the occurrence, frequency, and intensity of extreme events in a world of still limited warming.