Weather and climate extremes and drought ¶

The water balance of catchments will, in many cases, strongly depend on its state in the recent past (e.g. previous days). Processes causing significant hydrological memory may persist at longer timescales (e.g. annual). The presence of such memory could prolong drought and flood risks and affect water resources over long periods, but the global universality, strength, and origin of long memory in the water cycle remain largely unclear. Here, we quantify annual memory in the terrestrial water cycle globally using autocorrelation applied to annual time series of water balance components. These time series of streamflow, global gridded precipitation, and GLEAM potential and actual evaporation, along with a GRACE (Gravity Recovery and Climate Experiment)-informed global terrestrial water storage reconstruction, indicate that, at annual timescales, memory is typically absent in precipitation but strong in terrestrial water stores (root zone moisture and groundwater). Outgoing fluxes (streamflow and evaporation) positively scale with storage, and so they also tend to hold substantial annual memory. As storage mediates flow extremes, such memory often also occurs in annual extreme flows and is especially strong for low flows and in large catchments. Our model experiments show that storage–discharge relationships that are hysteretic and strongly non-linear are consistent with these observed memory behaviours, whereas non-hysteretic and linear drainage fails to reconstruct these signals. Thus, a multi-year slow dance of terrestrial water stores and their outgoing fluxes is common; it is not simply mirroring precipitation memory and appears to be caused by hysteretic storage and drainage mechanisms that are incorporable in hydrological models.

Trockenheit gefährdet auch die Schweiz. Weil es im Umgang mit Trockenheit an Erfahrung fehlt, verlangt das Postulat 22.3007 die Überprüfung der Fähigkeiten des Bevölkerungsschutzes zur Bewältigung klimabedingter Naturgefahren wie Trockenheit. Die in der Folge in Auftrag gegebene Fähigkeitsanalyse empfiehlt die Durchführung gemeinsamer Übungen. Im November 2024 wurde erstmals eine Tabletop-Übung zu Trockenheit mit Partnerorganisationen des Bevölkerungsschutzes1 durchgeführt. Anhand von vier kantonalen Trockenheitsszenarien analysierten die fachübergreifenden Teams die verfügbaren sowie zusätzlich notwendigen Ressourcen für die Bewältigung einer langanhaltenden Trockenheit in der Schweiz. Während der Bevölkerungsschutz personell, materiell und strukturell gut vorbereitet ist, muss die interkantonale Zusammenarbeit und der Wissensaustausch verbessert werden.

Tabletop “Drought”: Assessing the capacity of the Swiss Civil Protection Service to cope with extreme drought
Drought events are set to become more frequent and severe in the future and pose an increasing threat to Switzerland. Because of the current lack of experience in dealing with severe droughts in Switzerland, postulate 22.3007 calls for a review of the civil protection system’s capabilities for coping with climate-related natural hazards such as drought. The resulting capability analysis recommends carrying out joint exercises. In November 2024, a tabletop exercise on drought was conducted for the first time with civil protection partners. Using four cantonal drought scenarios, the interdisciplinary teams analyzed the available and additional resources required to cope with a one-and-a-half-year drought in Switzerland. The exercise showed that while civil protection is well prepared in terms of personnel, equipment and structure, the necessary channels and platforms for inter-cantonal cooperation and knowledge exchange still need to be improved.

Klimabedingte Trockenperioden werden in Zukunft häufiger, intensiver und langandauernder (IPCC 2023). Sie werden auch die Schweiz betreffen. Es liegt im Interesse der Behörden und der Wirtschaft, durch geeignete Massnahmen vor, während und nach einem Trockenheitsereignis die Auswirkungen zu begrenzen. In der Schweiz spielen dabei die Verwaltung, die Versicherungsindustrie sowie die Forschung eine wichtige Rolle. Vertreterinnen und Vertreter dieser Sektoren trafen sich im Frühsommer 2024, um Schnittstellen und mögliche Synergien in der Trockenheitsbewältigung zu identifizieren. In weiteren Schritten schärften sie das gemeinsame Verständnis, klärten Gemeinsamkeiten im Umgang mit Extremereignissen und identifizierten Aktivitäten, von denen alle Parteien profitieren könnten. Der Trialog zeigte, dass eine dauerhafte nationale Plattform mit Akteuren aus der Verwaltung, Forschung sowie weiteren Branchen wie der Versicherungsindustrie notwendig ist, um die Herausforderungen zukünftiger Trockenheit zu bewältigen. Eine solche Plattform sollte den Austausch und die Bereitstellung von trockenheitsrelevantem Wissen stärken sowie die Koordination, Zusammenarbeit und einheitliche Kommunikation vor, während und nach Trockenheitsperioden fördern.

Trialogue on coping with extreme drought in Switzerland
Climate-related droughts will become more frequent, more intense and longer-lasting in the future (IPCC 2023). They will also affect Switzerland. To limit the impacts, it is in the interest of the authorities and the economy to take appropriate measures before, during and after a drought event. In Switzerland, the Federal administration, the insurance industry, and academia play an important role in this. Representatives of these sectors met in early summer 2024 to identify interfaces and possible synergies in drought management. In further steps, they sharpened their common understanding, clarified commonalities in dealing with extreme events and identified activities from which all parties could benefit. The trialogue showed that a permanent, national platform with stakeholders from administration, research, and other sectors such as the insurance industry is necessary to overcome the challenges of future droughts. Such a platform should strengthen the exchange and provision of drought-relevant expertise and promote coordination, cooperation and mutually agreed communication before, during and after drought periods.

Die Folgen des Klimawandels, insbesondere Hitzewellen und Trockenheit, sind auch in der Schweiz zunehmend sichtbar. Die Forschung schafft Grundlagen, um klimabedingte Extreme reignisse und deren Auswirkungen besser vorhersagen und sich darauf vorzubereiten zu können. Die darauf basierenden Lösungsansätze für den  Umgang mit Extremereignissen erfordern die enge Zusammenarbeit zwischen akademischen und nicht-akademischen Fachleuten, wie sie beispielsweise im WSL-Forschungsprogramm EXtremes (2020–2025) gepflegt wird. Wie die Beiträge in diesem Tagungsband deutlich machen, ermöglicht die frühzeitige und stetige Einbindung der Praxis in die Forschungsaktivitäten nicht nur die Entwicklung anwendungsorientierter Produkte und Entscheidungshilfen, sondern auch den Dialog zwischen den betroffenen Akteuren über notwendige Anpassungsmassnahmen und deren Optimierung.

Future extremes
The consequences of climate change, particularly heatwaves and drought, are also becoming increasingly visible in Switzerland. Research is creating a basis for better predicting climate-related extreme events and their effects, and for preparing for them. The resulting solutions for dealing with extreme events require close cooperation between academic and non-academic experts, as is the case in the WSL EXtremes research program (2020–2025). As the contributions in this conference volume make clear, the early and continuous involvement of practitioners in research activities not only enables the development of application-oriented products and decision-making aids, but also facilitates dialogue between the stakeholders concerned about necessary adaptation measures and how these can be optimized.

Persistent multiyear drought (MYD) events pose a growing threat to nature and humans in a changing climate. We identified and inventoried global MYDs by detecting spatiotemporally contiguous climatic anomalies, showing that MYDs have become drier, hotter, and led to increasingly diminished vegetation greenness. The global terrestrial land affected by MYDs has increased at a rate of 49,279 ± 14,771 square kilometers per year from 1980 to 2018. Temperate grasslands have exhibited the greatest declines in vegetation greenness during MYDs, whereas boreal and tropical forests have had comparably minor responses. With MYDs becoming more common, this global quantitative inventory of the occurrence, severity, trend, and impact of MYDs provides an important benchmark for facilitating more effective and collaborative preparedness toward mitigation of and adaptation to such extreme events.