Swedish Meteorological and Hydrological Institute

The monitoring and modeling of riverine floods have been covered extensively in the scientific literature with a substantial number of scientific contributions related to calibration/validation of hydraulic and hydrological models and assimilation of Earth Observation (EO) data into them. These models, when used for flood forecasting purposes, rely heavily on ground-based hydrological networks along with numerical weather models which, particularly in data-scarce regions, are often challenged by data sparsity. In these situations, EO data offer a viable solution to enhance the skill of these flood forecasting systems by providing global-scale observations of key hydrological variables such as precipitation, soil moisture, river discharge, water levels, and flood extent. This manuscript reviews and discusses the capability of these EO data in enhancing flood forecasting systems, by analyzing their accuracy, lead time, and reliability, while at the same time highlighting key challenges such as data latency, spatial–temporal resolution trade-offs, and model assimilation constraints. By leveraging recent advancements in remote sensing, data assimilation techniques, and artificial intelligence, EO-based flood forecasting has the potential to bridge existing observational gaps, particularly in vulnerable regions. The paper also outlines future research directions and technological developments needed to maximize the impact of satellite data in operational flood forecasting systems.

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the Earth’s climate system, has long been considered vulnerable to irreversible weakening or collapse under global warming and related Greenland Ice Sheet (GrIS) melt, yet its resilience remains uncertain. Here, we use a CO2-emission-driven Earth system model with an interactive GrIS to assess AMOC reversibility under idealised CO2 emission pathways that produce near-linear global warming up to 10 K, stabilisation across 1.5-9 K, and subsequent cooling. We find that although the AMOC attains “collapsed” states by commonly used threshold definitions, these weakened states do not represent dynamical tipping: the overturning weakens quasi-linearly with global temperature increase, yet consistently and promptly recovers under cooling. In contrast, GrIS mass loss accelerates with warming, continues through stabilisations, and is only slowed by cooling, committing the planet to long-term sea-level rise. These results reveal a striking asymmetry in Earth-system resilience: under transient CO2 forcing, the AMOC strength remains dynamically reversible even under continued Greenland meltwater input, whereas the GrIS is locked into persistent decline. Our findings underscore the urgency of rapid emission cuts to limit climate overshoot, AMOC weakening, and irreversible ice-sheet loss.

Understanding the impact of climatic drivers on future catchment runoff is crucial for assessing how climate change will influence hydrological regimes. This knowledge is essential for evaluating water resources at the local and regional scales. Here, we assessed the contributions of climatic drivers to runoff changes under future conditions across Europe by analyzing the sensitivity of runoff to changes in precipitation and potential evapotranspiration using the Budyko framework. This analysis was conducted for 35,408 basins and based on ensembles of bias-adjusted climate projections and hydrological data. We further explored the link between runoff changes and catchment hydrological similarities, investigating the impact of climatic drivers on runoff across different hydrological regimes in three future periods (early, mid and late century) and emission scenarios. Results indicate precipitation as the primary driver of runoff changes across Europe under low and medium emission scenarios. However, by the mid and late century in high emission scenarios, potential evapotranspiration’s contribution becomes comparable to precipitation, significantly influencing runoff variations in central and southern Europe. Finally, the sensitivity of the runoff changes to climatic drivers is linked to the regime of the river systems. Runoff changes in basins with slow response to precipitation tended to show a more balanced contribution from both precipitation and potential evapotranspiration under high emission scenarios, while basins that are very responsive to precipitation, changes in precipitation primarily led to runoff changes. This Budyko-based diagnosis provides new insights into attribution analysis by linking hydrological regimes to runoff changes under global warming, supporting more effective local adaptation strategies.

Droughts in Europe are becoming increasingly frequent and severe, with the 2022 drought surpassing previous records and causing widespread socio-economic impacts. Using a Europe-wide survey (n = 481 across 30 countries) combined with hydroclimatic data (i.e., Standardized Precipitation Evapotranspiration Index; SPEI), we quantify how forecasting systems and Drought Management Plans (DMPs) affected response timing and perceived effectiveness. It specifically assesses the role of forecasting systems and Drought Management Plans (DMPs) in improving preparedness and in facilitating more effective and timely responses. Our findings show that organisations with forecasting systems or DMPs in place implemented drought response measures on average two and one months earlier respectively than those without, and rated their effectiveness higher. Additionally, the study investigates how drought management practices and awareness have evolved as a consequence of the 2018 European drought and how recent experiences shape water managers' perceptions, with 35 % of the respondents indicating introducing or updating their DMPs after the 2018 drought. The findings emphasize the necessity of a standardized, continent-wide drought risk management coordination to address the multifaceted nature of drought risk by integrating climatic and societal factors, and advocates for a Drought Directive as a means to achieve this. This research aims to inform policy development towards sustainable and holistic drought risk management, highlighting the crucial roles of preparedness, awareness, and adaptive strategies in mitigating future drought impacts. This study and its companion paper The 2022 drought needs to be a turning point for European drought risk management are the result of a study carried out by the Drought in the Anthropocene (DitA) network, an IAHS initiative.

An improved understanding of how forest trees may respond individually and differentially to climate across broad environmental gradients, due to adaptation or physiological acclimation, may facilitate more robust forecasts of forest resilience under climate change. We present a framework for modeling stem diameter growth in adult canopy trees that accounts for responses to climate that may be unique for individuals in different ecological settings. We used data from > 10,000 tree cores from 888 forest inventory plots distributed across wide climatic gradients in two mountain ranges in Europe. We formulated a suite of nonlinear models for each of the four species to understand factors regulating annual radial growth. The models accounted for the effects of tree ontogeny, competition, nitrogen deposition (Nd), temperature, and precipitation. We compared two approaches to evaluate evidence for adaptation or acclimation in the growth–climate relations of trees. One method tested whether growth responses diverged for individual trees associated with distinct climate regimes. An alternate method fitted climate response functions with the deviation of climate in a given year from the prevailing average conditions at a tree location. We also tested whether the peak height of this function, representing the maximum growth capacity of a tree, depended on local average climate. For all taxa, models that incorporated within-species variation received stronger support relative to simpler models that assumed a consistent species-average growth response to climate. Growth in all but one species was best predicted by models fitted with climate deviations. Trees differed markedly in terms of their peak growth potential and climate optima, and in some cases, occupied suboptimal environments. Growth responses to nitrogen (N) inputs were also modulated by climate. Our framework offers a flexible approach for integrating individual-level climate sensitivity into tree demography models, which may allow for more rigorous investigations of forest dynamics, the outcomes of which may inform adaptive management strategies for mitigating climate change impacts.