There is now undisputed evidence that the Earth’s climate has changed at an unusually rapid pace during the last decades, that these changes bear a clear human signature, and that they will intensify if anthropogenic emissions of greenhouse gases continue unabated. Climate changes are most frequently communicated in terms of means (increasing global mean temperature, rising mean sea level), while in fact they often have a larger impact on the occurrence of extreme events, such as persistent heat waves and coastal floods, that tangibly affect our societies.

Characterizing such events and quantifying their occurrence probabilities are significant challenges to be addressed in order to equip society with resilient adaptation solutions. Attribution science aims at evaluating the relative contributions of multiple causal factors or drivers to an extreme weather event (e.g., an individual heat wave) or a climate feature (e.g., a long-term trend in temperature) with an assignment of statistical confidence.

Attribution studies being concerned with low-probability weather and climate events, they often make use of Extreme Value Theory (EVT), the branch of statistics that provides models for events with low frequency yet potentially severe impact. Unfortunately, in their standard form, the statistical distributions of EVT are appropriate for stationary data only, while the core of the problem is that the climate is changing over time.

Another challenging aspect is that extreme climate events often result from a combination of interacting processes, resulting in compound events. For instance, high summer temperatures coupled with a lack of precipitation may cause soil moisture deficit, potentially leading to an unusually persistent drought. Thus, a significant challenge is to model two or more random variables of interest simultaneously, taking the dependence between their extremes (the tail dependence) into account.

**The overall objective of EXALT is to introduce a novel comprehensive framework for extreme event attribution and to apply it to the most influential events in the mid-high latitudes**, namely **heatwaves and compound drought-heat events over Europe, floods in southern and northern Europe, and sea ice extent over the Antarctic**.

Climate change effects on compound effects. From Zscheischler et al., 2020.