Changing climate both increases and decreases European river floods.

Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere1. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe2. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe3, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results-arising from the most complete database of European flooding so far-suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century4,5, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management.

Long-term patterns and changes of unglaciated High Arctic stream thermal regime.

Although water temperature is one of the most important factors influencing hydrochemistry and river ecology, long-term monitoring and modelling of stream thermal temporal variability are uncommon. There is sparse research regarding the thermal regimes of Arctic rivers, especially in Svalbard, a geographical hotspot affected by extreme climate change and Arctic amplification. There is a need for improvement and better understanding of the factors influencing the stream water temperature regime. To address this research gap, we present a study of the non-glaciated arctic catchment, Fuglebekken (Spitsbergen, Svalbard). We propose methods for reconstructing the thermal regime of the Arctic stream based on available in-situ data. This study evaluates different sets of input variables with hourly time steps required to explain the variability in water temperature. The study comprises two modelling approaches, a stochastic transfer function Multiple Input Single Output and a supervised machine learning technique, Gaussian Process Regression, to simulate the water temperature in the years 2005-2022. The ground temperature at a depth of 20 cm and total solar radiation were found to be the main forcings that explain most of the water temperature variability. The outputs of both models showed similar tendencies and patterns. A diurnal warming trend of 0.5-3.5 °C per decade has been detected in stream water temperature throughout the summer season. The highest increase of 6.0 °C in the water temperature in 2005-2022 was found to be present in the second part of June. The outcomes prove that the thermal regime of the Fuglebekken stream is sensitive to ongoing climatic changes. This variability is an important factor with many environmental implications.

Linking drought indices to atmospheric circulation in Svalbard, in the Atlantic sector of the High Arctic.

Based on long-term climatological data from Ny-Ålesund, Svalbard Airport-Longyearbyen and the Polish Polar Station at Hornsund, we undertook an analysis of drought indices on Spitsbergen Island, Svalbard, for the period 1979-2019. The features and causes of spatiotemporal variability of atmospheric drought in Svalbard were identified, as expressed by the standardized precipitation evapotranspiration index (SPEI). There were several-year periods with SPEI indicating the dominance of drought or wet conditions. The long-term variability in the annual and half-year (May-October) SPEI values showed a prevalence of droughts in the 1980s and the first decade of the twenty-first century, while wet seasons were frequent in the 1990s and in the second decade of the twenty-first century. The seasonal SPEIs were characteristic of interannual variability. In MAM and JJA, droughts were more frequent after 2000; during SON and DJF of the same period, the frequency of wet seasons increased. The most remarkable changes in the scale of the entire research period occurred in autumn when negative values of SPEI occurred more often in the first part of the period, and positive values dominated in the last 20 years. The long-term pattern of the variables in consecutive seasons between 1979 and 2019 indicates relationships between the SPEI and anomalies of precipitable water and somewhat weaker relationships with anomalies of sea level pressure. The three stations are located at distances of more than 200 km from each other in the northern (Ny-Ålesund), central (Longyearbyen) and southern parts of Svalbard (Hornsund), and the most extreme values of drought conditions depended on the atmospheric circulation which could have been modified by local conditions thus droughts developed under various circulation types depending on the station. However, some similarities were identified in the atmospheric circulation patterns favouring drought conditions at Ny-Ålesund and Hornsund, both having more maritime climates than Longyearbyen. Extremely dry seasons were favoured by anticyclonic conditions, particularly a high-pressure ridge (type Ka) centred over Svalbard, air advection from the eastern sector under an influence of cyclone and negative precipitable water anomalies. During wet seasons anomalies of precipitable water were positive and cyclonic conditions dominated. These results were corroborated by the frequency of regional circulation types during JJA and DJF with the lowest and highest values of SPEI.

Changes in the flow regime of High Arctic catchments with different stages of glaciation, SW Spitsbergen.

This study investigates the response of four High Arctic catchments with differing proportions of glacierization to changing climatic conditions. The study area located in SW Spitsbergen, has experienced a warming of 4.5 °C in the last 40 years along with a large increase in autumn rainfall. The changes in flow regime were determined based on available hydro-meteorological data and simulations of a semi-distributed rainfall-runoff model, which allowed reconstruction of streamflow in the period 1979-2020 and trend analyses for each calendar day. Similar trends of change were detected in all studied catchments. These include earlier onset of snowmelt driven floods (by two weeks over the period of analysis), large increases in autumn flows, prolongation of the hydrologically active season (starts earlier and lasts longer), decrease in flows in the latter half of June and the early part of August (except for the Breelva catchment). As a result of increases in autumn precipitation, the flood regime in these catchments has changed from snowmelt-dominated to the bi-modal with peaks in both July/August and September. The results also indicate differences in the magnitude of hydrological response depending on the percentage of glacial coverage in the catchments. The larger the glacierized area is, the larger the changes in the flow regime. The hydrological regime of SW Spitsbergen catchments has undergone dramatic changes over the last four decades. Such a shift in hydro-climatic conditions is larger than that observed in lower latitudes and has significant environmental implications.

Changing climate shifts timing of European floods.

A warming climate is expected to have an impact on the magnitude and timing of river floods; however, no consistent large-scale climate change signal in observed flood magnitudes has been identified so far. We analyzed the timing of river floods in Europe over the past five decades, using a pan-European database from 4262 observational hydrometric stations, and found clear patterns of change in flood timing. Warmer temperatures have led to earlier spring snowmelt floods throughout northeastern Europe; delayed winter storms associated with polar warming have led to later winter floods around the North Sea and some sectors of the Mediterranean coast; and earlier soil moisture maxima have led to earlier winter floods in western Europe. Our results highlight the existence of a clear climate signal in flood observations at the continental scale.