Event-dependent changes of recent European droughts in a warming climate using event-based storyline simulations.

Climate change is increasing the intensity, frequency, and persistence of droughts, as observed by the recent record-breaking events in Europe. Two factors contribute to this intensification: dynamic changes —changes in the likelihood of weather patterns— and thermodynamic changes. While the former are uncertain in future climate projections, the latter are characterized by a high signal-to-noise ratio, as there is a robust and ubiquitous rise in land-surface temperatures.

To better analyze the contributions of these factors, we utilize the so-called "event-based storyline approach". This involves nudging our global CMIP6 coupled climate model (AWI-CM1) towards the observed (according to ERA5) large-scale free-troposphere winds using various climate background conditions and initial states. In contrast, the model reproduces the circulation in the atmospheric boundary layer and the rest of the variables. This enables us to simulate the same weather conditions (including jet streams, ridges and blockings) in different climates: preindustrial, present, and 2 °C, 3 °C, and 4 ºC warmer worlds. This method creates an efficient way of showing experts and non-experts the consequences of climate change by describing the storyline of specific extreme events in different plausible climates using moderate computational resources.

Focusing on Europe, our simulations successfully replicate recent hot and dry extreme events, like the 2019 or 2022 European heatwaves and the record-breaking 2018 or 2022 drought. Our experiments reveal that these extremes have intensified from preindustrial to present climates (which answers the conditional attribution question), mainly in southern Europe, with no significant changes in Western and Northern Europe. However, we project that this exacerbation will expand northward in future warmer climates, leading to even more severe drought in Central Europe and the Mediterranean by the end of the century. These changes are not only associated with an increase in atmospheric evaporative demand in a warmer atmosphere but, in some cases, with significant decreases in the accumulated precipitation.

However, the response to climate change depends on the event, with events with similar present-day values showing a different climate change response. With our methodology, we can explore the physical mechanisms, including weather regimes, changes in vertical stability, or humidity profiles, that help explain the event-dependent response to climate change.