Physical storylines of future European drought events like 2018 based on ensemble climate modelling


Karin van der Wiel, Geert Lenderink & Hylke de Vries

Weather and Climate Extremes
Work Package 2



The impacts of future climate change are important to understand and communicate to those who might be affected or can limit them. This study uses a storyline approach, describing plausible future weather events and putting them in the context of recent events. This link to recent events is useful to make future climate change more tangible. Here, analogues of the severe drought seen in western Europe in 2018 are used to describe how such events may change in the future. Similar events are found to be more severe in a warmer world, due to decreased rainfall, increased evaporation and less water available for plants in the soil. These impacts are more severe still if temperatures rise by 3°C over pre-industrial conditions than 2°C, highlighting the importance of limiting global warming. Results like these are useful for informing policymakers and developing effective strategies to mitigate the impacts of climate change and help communities adapt to them.

It is important to understand the impacts of future climate change in order to mitigate and adapt to them, as well as to communicate these impacts to communities who may be affected by them. Climate projections are inherently uncertain, for example due to uncertain future greenhouse gas emissions and differences between climate models. Besides this uncertainty, projections can be rather abstract, making it difficult to effectively communicate them. Storylines are a useful tool here and involve taking plausible scenarios from an ensemble of model predictions and describing these in detail. This study looks at a new method for developing these storylines; choosing analogues of past events, further helping link projections to the real world. Storylines analogous to the 2018 drought in western Europe are used to assess how climate change may affect similar future events. This is key to effectively prepare for the impacts such severe events bring.

The team found that future events with similar characteristics to the 2018 drought are likely to be more severe. This is linked to decreased rainfall and increased evaporation, the latter due to higher temperatures and stronger sunlight. The storylines studied here also show that future droughts will lead to significantly lower amounts of water available for plants in the soil; this can affect crops. The droughts will affect a larger area in a warmer future, though the timing of when this occurs during the year differs between storylines. Droughts in these storylines are more severe in a world with 3°C of warming than with 2°C, highlighting the importance of mitigating climate change. Adaptation will also be required to limit the effects of these more severe drought events.

The climate simulations used in this paper were produced using the EC-Earth global coupled climate model v2.3. Three, 2000-year long ensembles of model runs were produced, one with a global temperature similar to the present day and two with levels of global warming at 2°C and 3°C above pre-industrial temperatures. Within each of these ensembles the 20 instances of droughts most similar to that seen in 2018 were selected and averaged, producing the storylines. Similarity to the 2018 event was defined by rainfall deficit (the difference between rain and evaporation), assessed using three different metrics based on when and how the deficit occurred during the year. These three metrics allow for different requirements or vulnerabilities to be accounted for. Differences between these storylines were then assessed, showing the effect of different levels of global warming. This paper serves as a useful proof-of-concept for this methodology.

Policy relevance
These results highlight the potential for more severe droughts in a warmer future, events that can have serious impacts on communities and agriculture. By putting these future projections in the context of a recent drought event, communicating them to the public and policymakers is more valuable. Such projections are crucial for policymakers in planning effective adaptation and mitigation strategies to limit the impacts of future climate change.


In the aftermath of observed extreme weather events, questions arise on the role of climate change in such events and what future events might look like. We present a method for the development of physical storylines of future events comparable to a chosen observed event, to answer some of these questions. A storyline approach, focusing on physical processes and plausibility rather than probability, improves risk awareness through its relation with our memory of the observed event and contributes to decision making processes through their user focus. The method is showcased by means of a proof-of-concept for the 2018 drought in western Europe. We create analogues of the observed event based on large ensemble climate model simulations representing 2 °C and 3 °C global warming scenarios, and discuss how event severity, event drivers and physical processes are influenced by climate change. We show that future Rhine basin meteorological summer droughts like 2018 will be more severe. Decreased precipitation and increased potential evapotranspiration, caused by higher temperatures and increased incoming solar radiation, lead to higher precipitation deficits and lower plant available soil moisture. Possibly, changes in atmospheric circulation contribute to increased spring drought, amplifying the most severe summer drought events. The spatial extent of the most severe drought impacts increases substantially. The noted changes can partly be explained by changes in mean climate, but for many variables, changes in the relative event severity on top of these mean changes contribute as well.