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Droughts are often long-lasting phenomena, without a distinct start or end and with impacts cascading across sectors and systems, creating long-term legacies. Nevertheless, our current perceptions and management of droughts and their impacts are often event-based, which can limit the effective assessment of drought risks and reduction of drought impacts. Here, we advocate for changing this perspective and viewing drought as a hydrological–ecological–social continuum. We take a systems theory perspective and focus on how “memory” causes feedback and interactions between parts of the interconnected systems at different timescales. We first discuss the characteristics of the drought continuum with a focus on the hydrological, ecological, and social systems separately, and then we study the system of systems. Our analysis is based on a review of the literature and a study of five cases: Chile, the Colorado River basin in the USA, northeast Brazil, Kenya, and the Rhine River basin in northwest Europe. We find that the memories of past dry and wet periods, carried by both bio-physical (e.g. groundwater, vegetation) and social systems (e.g. people, governance), influence how future drought risk manifests. We identify four archetypes of drought dynamics: impact and recovery, slow resilience building, gradual collapse, and high resilience–big shock. The interactions between the hydrological, ecological, and social systems result in systems shifting between these types, which plays out differently in the five case studies. We call for more research on drought preconditions and recovery in different systems, on dynamics cascading between systems and triggering system changes, and on dynamic vulnerability and maladaptation. Additionally, we advocate for more continuous monitoring of drought hazards and impacts, modelling tools that better incorporate memories and adaptation responses, and management strategies that increase societal and institutional memory. This will help us to better deal with the complex hydrological–ecological–social drought continuum and identify effective pathways to adaptation and mitigation.

• Precipitation has increased over recent decades, especially incold seasons, and is associated with an increase in rainfallin all seasons and a decrease in snowfall in summer, withspatially varying trends in winter. This century, precipitationevents presently regarded as extremes are expected to becomeroutine. Snow mass has decreased across northern NorthAmerica, but in Eurasia the trend has been negligible andsnow depth has increased in parts of Eurasia.

• Permafrost thaw is likely to drive changes in the waterbalance in Arctic areas, but the relevant subsurface processesare difficult to observe directly at the catchment scale.However, observed changes in streamflow dynamics andwater chemistry indicate that permafrost thaw is influencinghydrological connectivity by creating deeper and longerwaterflow pathways through catchments across the Arctic.

• Increasing trends in annual river discharge to the Arctic Oceanfrom both continents have continued, providing compellingevidence of intensification of the Arctic water cycle. Asignificant increase in base streamflow during the cold seasonis observed across most regions of the pan-Arctic drainagebasin. The magnitude of maximum river discharge has notchanged significantly; however, the timing of snowmelt freshethas become earlier almost everywhere across the pan-Arctic.

• Lake area is declining across the discontinuous permafrostzone. In the continuous permafrost zone, however, the numberof sites with decreasing lake area is similar to the numberwith increasing lake area. Stronger lake area declines in thediscontinuous permafrost zone is consistent with permafrostthaw being further advanced there than in the continuouspermafrost zone.

• Ice-cover duration on rivers has declined significantly in coldregions over the past several decades due to later freeze-upand earlier breakup. The observed decline in river ice is likelyto continue in the future due to the projected increase in airtemperature. Maximum river-ice thickness has decreasedsignificantly on most pan-Arctic rivers over the last 50 to60 years, with the greatest decrease observed before 2000.

• Lakes are rapidly losing ice across the Northern Hemisphere,with later ice-on dates, earlier ice-off dates, and in some years,some lakes not freezing at all.

• Freshwater delivery from Arctic land ice is roughly equivalentto that from North American rivers. Eurasian river dischargeis roughly three times higher. However, the increase in Arcticriver discharge was 1.6 times smaller than the increase infreshwater flux from Arctic land ice. Most of the increasedland-ice freshwater discharge originated from Greenland andArctic Canada. A further increase in freshwater flux fromland ice reduction is likely to continue with the projectedfuture increase in Arctic warming.

• Changes in the terrestrial hydrological system have importantimpacts on ecosystems and Arctic livelihoods. Declining snowcover, permafrost, lake area, and lake ice have implications forecosystems, as well as for hunting, fishing, reindeer herding,transportation, and drinking water availability. Impactsalso include feedbacks to the climate and ocean circulationthrough increased freshwater fluxes to the Arctic Ocean andchanges in lake area and ice cover.

Human mobility from droughts is multifaceted and depends on environmental, political, social, demographic and economic factors. Although droughts cannot be considered as the single trigger, they significantly influence people's decision to move. Yet, the ways in which droughts influence patterns of human settlements have remained poorly understood. Here we explore the relationships between drought occurrences and changes in the spatial distribution of human settlements across 50 African countries for the period 1992-2013. For each country, we extract annual drought occurrences from two indicators, the international disaster database EM-DAT and the standardized precipitation evapotranspiration index (SPEI-12) records, and we evaluate human settlement patterns by considering urban population data and human distance to rivers, as derived from nighttime lights. We then compute human displacements as variations in human distribution between adjacent years, which are then associated with drought (or non-drought) years. Our results show that drought occurrences across Africa are often associated with (other things being equal) human mobility toward rivers or cities. In particular, we found that human settlements tend to get closer to water bodies or urban areas during drought conditions, as compared to non-drought periods, in 70%-81% of African countries. We interpret this tendency as a physical manifestation of drought adaptation, and discuss how this may result into increasing flood risk or overcrowding urban areas. As such, our results shed light on the interplay between human mobility and climate change, bolstering the analysis on the spatiotemporal dynamics of drought risks in a warming world. Prolonged water shortages induced by droughts can have severe consequences on both the environment and society. For instance, the mobility of people can be influenced by drought events. In order to test this assumption, we relate the movement of people to drought occurrences, without considering any additional factor. We focus on Africa, since it is one of the most drought-prone continents and the movement of people is more prominent compared to other areas. We find that people tend to move closer to rivers and to urban centers during droughts, as compared to non-drought periods. This pattern is found for the majority of African countries, which suggests a large-scale signal. The increased movement of people toward rivers during droughts might generate larger human losses if flood events take place in the future. A new methodology integrating satellite data is developed for evaluating drought-induced human displacements in AfricaWe found that 70%-81% of African countries exhibit larger displacements during droughts, as compared to non-drought periodsHuman displacement toward rivers and urban centers is triggered, other things being equal, by drought occurrences

As the adverse impacts of hydrological extremes increase in many regions of the world, a better understanding of the drivers of changes in risk and impacts is essential for effective flood and drought risk management and climate adaptation. However, there is currently a lack of comprehensive, empirical data about the processes, interactions, and feedbacks in complex human-water systems leading to flood and drought impacts. Here we present a benchmark dataset containing socio-hydrological data of paired events, i.e. two floods or two droughts that occurred in the same area. The 45 paired events occurred in 42 different study areas and cover a wide range of socio-economic and hydro-climatic conditions. The dataset is unique in covering both floods and droughts, in the number of cases assessed and in the quantity of socio-hydrological data. The benchmark dataset comprises (1) detailed review-style reports about the events and key processes between the two events of a pair; (2) the key data table containing variables that assess the indicators which characterize management shortcomings, hazard, exposure, vulnerability, and impacts of all events; and (3) a table of the indicators of change that indicate the differences between the first and second event of a pair. The advantages of the dataset are that it enables comparative analyses across all the paired events based on the indicators of change and allows for detailed context- and location-specific assessments based on the extensive data and reports of the individual study areas. The dataset can be used by the scientific community for exploratory data analyses, e.g. focused on causal links between risk management; changes in hazard, exposure and vulnerability; and flood or drought impacts. The data can also be used for the development, calibration, and validation of socio-hydrological models. The dataset is available to the public through the GFZ Data Services (Kreibich et al., 2023,https://doi.org/10.5880/GFZ.4.4.2023.001).

Economic inequality is rising within many countries globally, and this can significantly influence the social vulnerability to natural hazards. We analysed income inequality and flood disasters in 67 middle- and high-income countries between 1990 and 2018 and found that unequal countries tend to suffer more flood fatalities. This study integrates geocoded mortality records from 573 major flood disasters with population and economic data to perform generalized linear mixed regression modelling. Our results show that the significant association between income inequality and flood mortality persists after accounting for the per-capita real gross domestic product, population size in flood-affected regions and other potentially confounding variables. The protective effect of increasing gross domestic product disappeared when accounting for income inequality and population size in flood-affected regions. On the basis of our results, we argue that the increasingly uneven distribution of wealth deserves more attention within international disaster-risk research and policy arenas.

Action toward strengthened disaster risk reduction (DRR) ideally builds from evidence-based policymaking to inform decisions and priorities. This is a guiding principle for the Sendai Framework for Disaster Risk Reduction (SFDRR), which outlines priorities for action to reduce disaster risk. However, some of these practical guidelines conceal oversimplified or unsubstantiated claims and assumptions, what we refer to as “truisms”, which, if not properly addressed, may jeopardize the long-term goal to reduce disaster risks. Thus far, much DRR research has focused on ways to bridge the gap between science and practice while devoting less attention to the premises that shape the understanding of DRR issues. In this article, written in the spirit of a perspective piece on the state of the DRR field, we utilize the SFDRR as an illustrative case to identify and interrogate ten selected truisms, from across the social and natural sciences, that have been prevalent in shaping DRR research and practice. The ten truisms concern forecasting, loss, conflict, migration, the local level, collaboration, social capital, prevention, policy change, and risk awareness. We discuss central claims associated with each truism, relate those claims to insights in recent DRR scholarship, and end with suggestions for developing the field through advances in conceptualization, measurement, and causal inference.

Risk management has reduced vulnerability to floods and droughts globally, yet their impacts are still increasing. An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data4,5. On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change.

This study presents a global explanatory analysis of the interplay between the severity of flood losses and human presence in floodplain areas. In particular, we relate economic losses and fatalities caused by floods during 1990-2000, with changes in human population and built-up areas in floodplains during 2000-2015 by exploiting global archives. We found that population and built-up areas in floodplains increased in the period 2000-2015 for the majority of the analyzed countries, albeit frequent flood losses in the previous period 1990-2000. In some countries, however, population in floodplains decreased in the period 2000-2015, following more severe floods losses that occurred in the period 1975-2000. Our analysis shows that (i) in low-income countries, population in floodplains increased after a period of high flood fatalities; while (ii) in upper-middle and high-income countries, built-up areas increased after a period of frequent economic losses. In this study, we also provide a general framework to advance knowledge of human-flood interactions and support the development of sustainable policies and measures for flood risk management and disaster risk reduction.

Riverine flood risk studies often require the identification of areas prone to potential flooding. This modelling process can be based on either (hydrologically derived) flood hazard maps or (topography-based) hydrogeomorphic floodplain maps. In this paper, we derive and compare riverine flood exposure from three global products: a hydrogeomorphic floodplain map (GFPLAIN250m, hereinafter GFPLAIN) and two flood hazard maps (Flood Hazard Map of the World by the European Commission's Joint Research Centre, hereinafter JRC, and the flood hazard maps produced for the Global Assessment Report on Disaster Risk Reduction 2015, hereinafter GAR). We find an average spatial agreement between these maps of around 30 % at the river basin level on a global scale. This agreement is highly variable across model combinations and geographic conditions, influenced by climatic humidity, river volume, topography, and coastal proximity. Contrary to expectations, the agreement between the two flood hazard maps is lower compared to their agreement with the hydrogeomorphic floodplain map. We also map riverine flood exposure for 26 countries across the global south by intersecting these maps with three human population maps (Global Human Settlement population grid, hereinafter GHS; High Resolution Settlement Layer, hereinafter HRSL; and WorldPop). The findings of this study indicate that hydrogeomorphic floodplain maps can be a valuable way of producing high-resolution maps of flood-prone zones to support riverine flood risk studies, but caution should be taken in regions that are dry, steep, very flat, or near the coast.