Tailored approaches to analyse cumulative drought-related climate risks and associated impact cascades in Switzerland

Cumulative extreme events pose substantial risk to society and nature, as they can propagate through various socio-economic systems via process cascades. Adaptation to future climates requires estimations of the likelihood and possible combined impacts of cumulating meteorological/climatic extremes events. Due to the very rare occurrence of low probability events, such estimations remain challenging.

In response to this knowledge gap, a collaborative effort of academic and government institutions at different administrative levels is undertaken. It aims at analysing the potential of such cumulative, complex risks and to suggest actions needed to manage them in Switzerland. The project is based on two case studies, which were developed in collaboration with stakeholders from science, policy making and practice at the national and sub-national level. The case studies assess rare but plausible combined risks of extreme drought events and other meteorological extremes, e.g. heat, as projected by the recently published Swiss Climate Scenarios CH2018. One case study is conducted in the alpine region of southern Grisons, the second one in the urban area of Basel.

Currently, there are only limited approaches available to quantitatively model the manifold cascading effects that may propagate through natural and human systems after the occurrence of combined drought-related extremes. We therefore adapt methods from the field of civil protection and use expert knowledge to develop impact storylines and estimate probabilities and magnitudes of adverse effects on societies and ecosystems.

To estimate the feasibility of a combined drought event leading to the loss of the protective function of forests in the southern Swiss Alps (case study 1), we developed an extensive expert survey. 29 experts from science, administration and practice provided quantitative estimates of drought thresholds and damage probabilities. The survey was split into a top-down and a bottom-up approach, allowing to characterize the possible impacts from two different angles and thereby also assess the robustness of the results.

In contrast, urban areas consist of diverse interlinked systems with very different characteristics, which does not allow to assess impact cascades with a single expert survey. Instead, we used a three-step approach based on semi-quantitative storylines informed by literature and expert interviews. In a first step, experts for individual systems, such as water, transport or health were interviewed about possible weaknesses and blind-spots with regard to the trigger-event. Second, we characterized possible storylines of impact cascades using process diagrams along with quantitative estimates of drought related variables such as river discharge, air and water temperature. In a third step, the plausibility of these storylines was discussed once more with the experts. We report on the advantages and challenges of our approach compared to traditional modelling-based methods in light of transformative adaptation measures to future climates.