Compound drought and heatwave events: A case study on historical and future perspective in the Slovenian pre-Alpine region

In the last two decades, several severe to extreme droughts have been recorded in parts of the Slovenian pre-Alpine region, with the most pronounced events occurring in the summers of 2003, 2013 and 2022, which also affected Slovenia on the scale of a natural disaster. These droughts typically resulted from prolonged periods of below-average precipitation, while on shorter timescales, their development and intensification were strongly influenced by concurrent heatwaves. When droughts and heatwaves occur simultaneously or in close succession, their combined impact to ecosystems and related economic sectors, such as agriculture and forestry, often exceeds the sum of their individual impacts. These compound extreme events can lead to significant crop losses, heightened wildfire risk and cascading disruptions across interconnected systems.

Compound drought and heatwave (CDHW) events can be analysed using various combinations of drought and heatwave indicators. In the framework of the Interreg Alpine Space project X-RISK-CC, we defined CDHW events by combining the Standardized Precipitation-Evapotranspiration Index (SPEI) on a monthly, 2-monthly and 3-monthly timescale with the heatwave index based on daily maximum temperature exceeding a high threshold over three consecutive days. For the case study area in the Slovenian Prealps, we examined past and future characteristics of CDHW events through historical trends from observations and through projections using the global warming level (GWL) approach for warming scenarios of 2 °C, 3 °C and 4 °C relative to the pre-industrial period 1850–1900.

Several CDHW events have been observed in the case study area in recent decades, with trends indicating an increase in both frequency and magnitude since 1970 at certain locations. Future projections suggest that the annual frequency of CDHW events could increase by 1 to 5 events per year relative to the current climate. These events are also expected to intensify under all global warming levels, with the increase in annual maximum magnitude ranging from 40 % to 150 %. The probability of occurrence of individual droughts and heatwaves, as well as the probability of CDHW events is projected to rise. A CDHW event as extreme as the 1-in-50-year event in the current climate is expected to become up to 8 times as likely under a global warming of 3 °C and up to 13.5 times as likely under a global warming of 4 °C.