Anne F. Van Loon’s research while affiliated with Vrije Universiteit Amsterdam and other places

As extreme event attribution (EEA) matures, explaining the impacts of extreme events has risen to be a key focus for attribution scientists. Studies of this type usually assess the contribution of anthropogenic climate change to observed impacts. Other scientific communities have developed tools to assess how human activities influence impacts of extreme weather events on ecosystems and societies. For example, the disaster risk reduction (DRR) community analyses how the structure of human societies affects exposure, vulnerability, and ultimately the impacts of extreme weather events, with less attention to the role of anthropogenic climate change. In this perspective, we argue that adapting current practice in EEA to also consider other causal factors in attribution of extreme weather impacts would provide richer and more comprehensive insight into the causes of disasters. To this end, we propose a framework for EEA that would generate a more complete picture of human influences on impacts and bridge the gap between the EEA and DRR communities. We provide illustrations for five case studies: the 2021–2022 Kenyan drought; the 2013–2015 marine heatwave in the northeast Pacific; the 2017 forest fires in Portugal; Acqua Alta (flooding) events in Venice and evaluation of the efficiency of the Experimental Electromechanical Module, an ensemble of mobile barriers that can be activated to mitigate the influx of seawater in the city; and California droughts and the Forecast Informed Reservoir Operations system as an adaptation strategy.

Persistent drought conditions may alter catchment response to precipitation, both during and after the drought period, hindering accurate streamflow forecasting of high flows and floods. Yet, the influence of drought characteristics on the catchment response to precipitation remains unclear. In this study, we use a comprehensive dataset of global observations of streamflow and remotely sensed precipitation, soil moisture, total water storage and normalized difference vegetation index (NDVI). Using multivariate statistics on 4487 catchments with a stationary streamflow-to-precipitation ratio, we investigate the influence of drought on fluctuations of streamflow sensitivity to precipitation. Our analysis shows that generally droughts with streamflow or soil moisture anomalies below the 15th percentile lead to around 20 % decrease in streamflow sensitivity to precipitation during drought compared to the historical norm, with up to a 2 % decrease one year after the drought. Negative NDVI anomalies are the only exception, resulting in a 3 % increase in sensitivity. These effects are more pronounced when droughts are longer and more severe. Most changes were found in arid and warm-temperate regions, whereas snow-influenced regions exhibit less sensitivity changes due to drought. In addition, we used step-change analyses on 1107 catchments with non-stationary streamflow-to-precipitation ratio to identify significant abrupt shifts on the timeseries, examining the role of drought in driving these shifts. This analysis revealed both positive and negative shifts in streamflow sensitivity after severe and persistent drought conditions regardless of climate and catchment characteristics. Positive shifts occur only when the drought propagated through the hydrological system after extended dry periods, while negative shifts are usually linked to shorter, intense dry periods. This study sheds light on the importance of considering climate characteristics in predicting dynamic catchment response to precipitation during and after persistent drought conditions.

Flooding during or after droughts poses significant challenges to disaster risk management. However, interactions between droughts and floods are often overlooked as studies typically analyse these events in isolation. Here we explore historical occurrences of compound and consecutive drought-flood events and drought effects on flood severity and timing by analysing global datasets of hydrometeorological and biophysical variables for 8255 catchments worldwide. These data show that 24% of floods globally (anomalies above the 85th percentile) are preceded by, or happen during, drought conditions. Flood events occurring during drought conditions are typically of lower magnitude, especially in arid regions, while floods following drought events have a severity distribution comparable to single flood events. For most drought-flood events, flood timing appears relatively unaffected by drought conditions, but almost a quarter of the drought-flood events had flood timings occurring two to three months later than expected. These shifts in flood timing suggest droughts potentially affect flood-generating processes. As both drought and flood occurrences are projected to increase in a warming climate, interactions between them may become more common and need to be accounted for in flood risk assessment and management.

Human activities have increasingly affected hydrological processes in many river basins worldwide leading to changes in the severity of droughts and floods. A number of modelling studies have used system dynamic models to represent the complex dynamics generated by the interplays between the social and physical systems. Yet, attitudes towards drought and flood risk leading to the implementation of individual and collective adaptation measures are not included in current system dynamics modelling approaches. To address this gap, we developed a system dynamic model to represent the dynamics between human societies, decision-making processes, adaptation measures, and hydrological extremes. The model accounts for a society characterized by the coexistence of four types of risk attitudes and management responses: risk-neglecting, risk-controlling, riskdownplaying, and risk-monitoring. Our findings show that the presence of a prevalent social group with a certain risk attitude has a strong influence on the awareness, preparedness, and consequent losses of the other social groups. On the other hand, the homogenous presence of social groups with diverse risk attitudes leads to higher drought and flood losses due to the unsustainable water use of risk-neglecting and risk-downplaying groups. Finally, our results highlight that societies characterized by high heterogeneity in risk-attitudes tend to implement less collective measures, opting instead for individual measures by specific social groups. This emphasizes the importance of accounting for different social groups when modelling human-water dynamics to promote an integrated risk management and design more sustainable and resilient future adaptation pathways.

In the past decades, and notably the last few years, droughts have severely impacted various interconnected socioeconomic sectors and ecosystems across the EU. These impacts encompass, among others, extensive losses in both rain-fed and irrigated agriculture, challenges and constraints in public water supply, disruptions in inland shipping, diminished production of hydropower and thermoelectric energy, impaired functioning of terrestrial and freshwater ecosystems, and implications for the tourism industry. In order to better prepare for future drought events in Europe, knowledge on the drivers, spatial patterns and dynamics of drought risks is urgently needed. The European Drought Risk Atlas responds to that need by mapping hotspots and risk drivers across diverse systems and regions within the EU. Combining conceptual risk models (impact chains) and a data-driven quantitative drought risk assessment based on machine learning, this Atlas represents a significant stride toward impact-driven drought risk analysis in present and projected global warming levels (+1.5°C, +2.0°C, +3.0°C). It provides a detailed and disaggregated perspective on the risks posed by droughts to societies and ecosystems, with a particular focus on agriculture, public water supply, energy, river transportation, freshwater, and terrestrial ecosystems. The data-driven analysis reveals that current levels of drought risk in the EU are already notable, with average annual losses presenting economic and environmental threats in nearly all regions. As expected, the Mediterranean region, particularly the Iberian Peninsula, faces high drought risk under both current and projected climate conditions, driven by the escalating dry conditions associated with global warming. However, while drought risk of certain sectors in Europe follows a north-south gradient of overall mean drying (south) and wetting (north) under climate change, the analysis underscores that each sector reacts distinctly to current and projected hazard conditions, exhibiting sector-specific sensitivity. Eastern and Western Europe may experience complex dynamics due to the interplay between drying and wetting patterns and precipitation variability.

In the last few years, the world has experienced numerous extreme droughts with adverse impacts on coupled human and natural systems. While agriculture is the most affected sector, the lack of water due to droughts in our highly interconnected world also affects ecosystems, public water supply, power generation, tourism, water-borne transport and buildings, often with nonlinear cascading and systemic impacts. Moreover, droughts also interact with other hazards in complex ways, for example leading to compound heat-drought events, wildfires or aggravated impacts when concurring with other non-climatic hazards and shocks, such as the COVID-19 pandemic. At the same time, responses to droughts can also lead to response risks, for example when the establishment of reservoirs in response to droughts leads to overreliance on these reservoirs and in turn increases the vulnerability of communities, sectors and systems to droughts. Combined, these characteristics pose a serious challenge to our ability to grasp the complexities of drought risks and to manage them in a comprehensive way. To avoid ineffective risk management and maladaptation, a paradigm shift in how we look at, assess and manage drought risks is urgently needed – from a siloed, single-risk (e.g. drought risks for agriculture, energy, transport) to a systemic perspective. However, despite more frequent and severe events, systemic drought risk assessment is still incipient compared to that of other meteorological and climate hazards. This is mainly due to the outlined complexity of drought, the high level of uncertainties in its analysis, and the lack of community agreement on a common framework to tackle the problem. Addressing this gap, we propose a novel drought risk framework that highlights the systemic nature of drought risks, and show its operationalization using the example of the 2022 drought in Europe. Our research emphasizes that solutions to tackle growing drought risks should not only consider the underlying drivers of drought risks for different sectors, systems or regions, but also be based on an understanding of sector/system interdependencies, feedbacks, dynamics, compounding and concurring hazards, as well as possible tipping points and globally and/or regionally networked risks.