The 2018 west-central European drought projected in a warmer climate: how much drier can it get?

Projections of changes in extreme droughts un- der future climate conditions are associated with large uncer- tainties, owing to the complex genesis of droughts and large model uncertainty in the atmospheric dynamics. In this study we investigate the impact of global warming on soil mois- ture drought severity in west-central Europe by employing pseudo global warming (PGW) experiments, which project the 1980–2020 period in a globally warmer world. The fu- ture analogues of present-day drought episodes allow for in- vestigation of changes in drought severity conditional on the historic day-to-day evolution of the atmospheric circulation.
The 2018 west-central European drought is the most se- vere drought in the 1980–2020 reference period in this re- gion. Under 1.5, 2 and 3◦C global warming, this drought episode experiences strongly enhanced summer tempera- tures but a fairly modest soil moisture drying response com- pared to the change in climatology. This is primarily because evaporation is already strongly moisture-constrained during present-day conditions, limiting the increase in evaporation and thus the modulation of the temperature response under PGW. Increasing precipitation in winter, spring and autumn limits or prevents an earlier drought onset and duration. Nev- ertheless, the drought severity, defined as the cumulative soil moisture deficit volume, increases considerably, with 20 % to 39 % under 2 ◦C warming.
The extreme drought frequency in the 1980–2020 period strongly increases under 2 ◦ C warming. Several years with- out noticeable droughts under present-day conditions show very strong drying and warming. This results in an increase in 2003-like drought occurrences, compounding with local summer temperature increases considerably above 2 ◦C.
Even without taking into account a (potentially large) dy- namical response to climate change, drought risk in west- central Europe is strongly enhanced under global warm- ing. Owing to increases in drought frequency, severity and compounding heat, a reduction in recovery times between drought episodes is expected to occur. Our physical cli- mate storyline provides evidence complementing conven- tional large-ensemble approaches and is intended to con- tribute to the formulation of effective adaptation strategies.