Extreme Temperatures and Precipitation in Europe: Analysis of a High-Resolution Climate Change Scenario

Future climate change is generally believed to lead to an increase in climate variability and in the frequency and intensity of extreme events. In this report we analyse the changes in variability and extremes in temperature and precipitation in Europe by the end of this century, based on high-resolution (12 km) simulations of the regional climate model HIRHAM. The results suggest a general trend towards higher temperatures at the end of the 21st century. The magnitude of the changes is, however, not uniform across Europe and varies between seasons. Higher winter temperatures are prevalent in Eastern Europe and in the Alps, while higher summer temperatures mostly affect southern Europe. Also the changes in temperature variability differ between northern and southern Europe and between seasons. In winter the variability in the mean daily temperature decreases considerably in north-eastern Europe, while in summer there is an increase predominantly in southern Europe. Hot summer days and tropical nights become common in areas where such events were previously rare, e.g. in London and Stockholm. While July remains the hottest month in general, the changes in temperature are larger in August. This is also the month with the largest increase in extreme summer temperatures and the occurrence of heat waves. The changes in precipitation are very different between southern and northern Europe. In the south, the annual rainfall is generally decreasing, there is a higher risk of longer dry spells, the differences between the years are getting larger, and arid and semi-arid areas are expanding. In northern Europe, on the other hand, the precipitation amounts are generally increasing, particularly in winter. In between is a broad region where, on an annual basis, the changes are fairly small, but where the differences between the seasons are more pronounced: winter and spring are getting wetter, while summer and, to a lesser extent, autumn are getting drier. On rain days the intensity and variability of the precipitation shows a general increase, even in areas that are getting much drier on average. What is more, the rise in the precipitation extremes tends to be stronger than in the average intensity. Considerably increases in extreme multiday precipitation amounts may be very local, but occur almost everywhere across Europe and in every season, except for summer in southern and western Europe. These findings support the conclusions of earlier studies that a warmer climate will result in a higher incidence of heat waves, less summer precipitation and at the same time higher rainfall intensities throughout much of Europe.JRC.H.7-Land management and natural hazard

Will Europe See More Frequent and Severe River Flow Droughts in the Future?

For the coming decades climate change is projected not only to result in higher temperatures, hence higher evaporative demands, but also to induce changes in the seasonality of precipitation patterns, with wetter winters and dryer summers, as well as to increase the frequency and intensity of extreme climatic events. The combination of these patterns of change will likely result in more frequent, severe and persistent droughts in large parts of Europe, especially in the south. This study presents a pan-European assessment of the possible impacts of climate change on low flows in Europe by comparison of river flow drought characteristics for current and future climate. We employ high resolution (12 km spatial resolution) regional climate data from the HIRHAM model for the control period and the future period based on the SRES A2 greenhouse gas emission scenario to force a hydrological model. Low flow characteristics for current and future climate are derived from the simulated river flow series using extreme value analysis. More specifically, we employ the methods of block maxima and partial duration series to select minimum flows and maximum flow deficits and fit extreme value distributions by the maximum likelihood method. Regions most prone to an increase in river flow drought are southern and south-eastern Europe, but minimum flows will also decrease significantly in many other parts of Europe, especially in summer. In snow dominated regions winter droughts are projected to be less severe because a lower fraction of precipitation will fall as snow in warmer winters.JRC.H.7-Land management and natural hazard

Development and Testing of Methods to Assess the Impact of Climate Change on Flood and Drought Risk at the European Scale

During the last 100 years global climate has warmed by an average of 0.6ºC, owing in part to human induced greenhouse gas emissions. Based on different scenarios of future greenhouse gas emissions projections of climate models indicate another 1.4 to 5.8 ºC of warming over the next century (IPCC, 2001a). The projected change in climate will significantly impact the hydrological cycle. A warmer climate will increase evaporation, the intensity of water cycling, and result in greater amounts of moisture in the air. It is expected that the magnitude and frequency of extreme weather events will increase, and that hydrological extremes such as floods and droughts will likely be more frequent and severe. The Joint Research Centre aims to develop knowledge and tools in support of the EU Climate Change Strategy that was recently put forward in the Commission’s Communication “Winning the Battle Against Global Climate Change” (COM(2005) 35). In view of this, an important research topic of the Land Management Unit of the IES is to assess the impact of climate change on the occurrence of hydrological extremes such as floods and droughts. This will be accomplished by developing an integrated modelling framework that combines regional climate predictions for Europe with the LISFLOOD model. LISFLOOD is a distributed, partially physically-based rainfall-runoff model that has been devised to simulate the hydrological behaviour in large European catchments (De Roo et al., 2000), with emphasis on predicting floods and droughts. Owing to its general nature, LISFLOOD is optimally suited for simulating the different hydrological regimes across Europe. Predicted climate for current conditions and for different scenarios of greenhouse gas emissions by the end of the 21st century will be used as input to LISFLOOD, after taking due account of any systematic bias in the climate forcing data obtained from climate models. Runoff statistics for the two periods will provide a means to estimate changes in the frequency and severity of hydrological extremes under different scenarios of future greenhouse gas emissions. Projections of future climate change are typically obtained from coupled Atmosphere-Ocean General Circulation Models (AOGCM). Because they require time steps of minutes but are used to predict climate change on time scales of months to centuries, their horizontal resolution is typically at least 100 km and hence their treatment of physical processes is approximate. Due to their coarse spatial resolution AOGCMs fail to explicitly capture fine-scale climatic structures needed for climate change impact studies and policy planning at the regional or sub-regional scale (e.g., catchment or basin scale). To resolve this problem, regionalization or downscaling methods have been developed that enhance regional detail and provide climatic information at smaller scales. The aim of this document is to provide an overview on existing methods for downscaling global climate information. Also, this document gives an overview of existing regional climate data sets for Europe, and details on how to use regional climate data for impact studies at the European and regional scale. The document is organised as follows. Section 2 presents a general overview of existing downscaling methods, with details of the underlying principles to generate regional climate information. In Section 3 an overview is given of regional climate data that are currently available to be used for impact studies at the European scale. In Section 4 some details are provided about the integrated modelling framework that couples the regional climate model data with the hydrological model LISFLOOD. Conclusions and an overview of current and further work are presented in Section 5.JRC.H.7-Land management and natural hazard

Sub-arctic hydrology and climate change : a case study of the Tana River Basin in Northern Fennoscandia

The most significant changes in climate, due to the well-known enhanced greenhouse effect, are generally expected to occur at northern high latitudes. Sub-arctic environments, that are dominated by the presence of a seasonal snow cover, may therefore be particularly sensitive to global warming. The impact of human-induced climate changes on the hydrological system of sub-arctic environments was analysed in a series of studies, undertaken in the Tana River Basin in northernmost Finland and Norway. Central to the approach was a large-scale hydrological model of the study area, that has been coupled to a regional climate model. The hydrological model was based on a conceptual water balance model originally developed for the River Rhine. In this study, special attention was paid to the spatial distribution of snow coverage, snowmelt and evapotranspiration, in addition to river discharge. In order to be able to make proper estimates of the consequences of climate change, the empirical methods of estimating snowmelt and evapotranspiration in the water balance model were replaced by two physically-based models. In developing the hydrological model, field measurements were used that have been collected in a small catchment in Northern Finland. The model performance was evaluated by comparing the results with satellite observations of snow cover depletion in the Tana Basin, as well as observations on snow depth and river discharge. The improved, physically-based model version appeared to simulate the hydrological behaviour of the Tana Basin realistically. The impact of changes in climate was analysed by driving the hydrological model with climate change scenarios. For the Tana Basin, these scenarios imply that the mean annual temperature in the coming century may rise by more than 5 degrees Celsius, and precipitation may increase by 25 %. As a consequence, the snow-free season is extended by more than 30 days at higher elevations, to 70 days or more days close to the Barents Sea. Consequently, the amount of solar radiation that is received during the snow-free season, increases by about 16 %. This increase in the annual radiation sum may be of importance for the tundra vegetation, that consists mainly of low shrubs and heath. Due to the shorter snow season, the annual amount of sublimation decreases by 30 %, while the amount of evapotranspiration in summer increases by about 15 %. River discharge may increase as well, by almost 40 % on an annual basis. Changes in climate, as predicted for the coming century, will therefore have a significant impact on snow coverage, evapotranspiration and freshwater runoff in the Tana Basin. These changes will influence the ecological processes in the sub-arctic landscape. Due to changes in albedo and the amount of freshwater runoff towards the Arctic Ocean, changes in the hydrological cycle of sub-arctic areas may also provide a feedback to the global climate system. Understanding the hydrological processes operating in sub-arctic environments, and evaluating their sensitivity to climate change, is therefore an essential part of global change research

Climate Change Impact on Flood Hazard in Europe: An Assessment Based on Regional Climate Scenarios

Simulations with global and regional climate models predict that future climate change will lead to an increase in frequency and intensity of extreme precipitation events in Europe, especially in the north and in the winter also in central Europe. Some models project an increase in heavy rainfall amounts even in areas that in general are expected to become much drier. This trend is likely to lead to more frequent and more intense river flooding in many parts of Europe. To analyse changes in flood hazard at the European scale we employed the hydrological model LISFLOOD that has been developed for operational flood forecasting using a grid scale of 5 km. This model was driven by data from several regional climate models, including an experiment of the RCM HIRHAM that was performed with a very high horizontal resolution of 12 km. It was found that, under the SRES A2 emissions scenario of the IPCC, in many European rivers the extreme discharge levels may have increased in magnitude and frequency by the end of this century. In several rivers, most notably in the west and parts of eastern Europe, the probability of what is currently a 100-year flood may double or increase even more, meaning that the return period decreases to 50 years or less. A notable exception to this was found in the northeast, where warmer winters and a shorter snow season reduce the magnitude of the spring snowmelt peak. Also in other rivers in central and southern Europe a considerable decrease in extreme river flows was found. The results from the 12-km HIRHAM simulation were compared with those obtained with two experiments of the same model at a lower resolution of about 50 km for the SRES A2 and B2 scenarios. Disagreements between these model experiments indicate that the effect of the horizontal resolution of the regional climate model is comparable in magnitude to the choice for a particular greenhouse gas scenario.JRC.H.7-Land management and natural hazard

Improving the health forecasting alert system for cold weather and heat-waves in England: a proof-of-concept using temperature-mortality relationships

Objectives In this study a prototype of a new health forecasting alert system is developed, which is aligned to the approach used in the Met Office’s (MO) National Severe Weather Warning Service (NSWWS). This is in order to improve information available to responders in the health and social care system by linking temperatures more directly to risks of mortality, and developing a system more coherent with other weather alerts. The prototype is compared to the current system in the Cold Weather and Heatwave plans via a case-study approach to verify its potential advantages and shortcomings. Method The prototype health forecasting alert system introduces an “impact vs likelihood matrix” for the health impacts of hot and cold temperatures which is similar to those used operationally for other weather hazards as part of the NSWWS. The impact axis of this matrix is based on existing epidemiological evidence, which shows an increasing relative risk of death at extremes of outdoor temperature beyond a threshold which can be identified epidemiologically. The likelihood axis is based on a probability measure associated with the temperature forecast. The new method is tested for two case studies (one during summer 2013, one during winter 2013), and compared to the performance of the current alert system. Conclusions The prototype shows some clear improvements over the current alert system. It allows for a much greater degree of flexibility, provides more detailed regional information about the health risks associated with periods of extreme temperatures, and is more coherent with other weather alerts which may make it easier for front line responders to use. It will require validation and engagement with stakeholders before it can be considered for use

Exploring the value of machine learning for weighted multi-model combination of an ensemble of global hydrological models

This study presents a novel application of machine learning to deliver optimised, multi-model combinations (MMCs) of Global Hydrological Model (GHM) simulations. We exemplify the approach using runoff simulations from five GHMs across 40 large global catchments. The benchmarked, median performance gain of the MMC solutions is 45% compared to the best performing GHM and exceeds 100% when compared to the EM. The performance gain offered by MMC suggests that future multimodel applications consider reporting MMCs, alongside the EM and intermodal range, to provide endusers of GHM ensembles with a better contextualised estimate of runoff. Importantly, the study highlights the difficulty of interpreting complex, non-linear MMC solutions in physical terms. This indicates that a pragmatic approach to future MMC studies based on machine learning methods is required, in which the allowable solution complexity is carefully constrained

Extreme weather and climate in Europe

This report describes the current scientific knowledge of extreme weather and climate events in Europe for the following variables: temperature, precipitation, hail, and drought (with the following types of drought: meteorological, hydrological and soil moisture). The content summarises key literature drawn from peer reviewed journals and other sources (business and government reports), and builds upon the synthesised results presented in international assessments such as IPCC reports. It describes the recorded observations and modelled projections for extreme events including definitions, frequency, trends, spatial and temporal distribution. The report also presents an overview of the indices used to characterise extreme events as well as their main uses, before going on to describe the datasets where they are recorded, and provides information on the strengths and weaknesses of the indices and the datasets. Extra consideration is given to indices that are relevant to socio-economic impacts resulting from climate change and relevant statistical techniques for analysing extreme events. Observed changes in global climate and extreme events provide the context to the changes in extreme events observed in Europe, which are described for much of the 20th century. Modelled projections of extreme events are also given, under different emissions scenarios and time horizons, including results from regional models covering the European domain, such as EURO-CORDEX. The report is written for climate scientists, climate researchers and experts who use climate information in a professional role. There are four case studies (Appendix 2) which provide an anatomy of different recent European extreme weather/climate events including meteorological impacts and synoptic context. Observed global temperature trends show the number of warm extremes has increased and number of cool extremes has decreased over the last 100 years, and the length and frequency of summer heat waves has increased during the last century. In Europe these trends are most pronounced in the last 40 years although regional variations exist. For Europe, 2014 was the warmest year on record, although it had fewer hot days than recent years. Under future climate change with continued warming, the number of heat waves is projected to increase, along with their duration and intensity. Under all emissions scenarios, summers like the hot summer experienced in 2003 will become commonplace by the 2040s. The global trend in precipitation is generally for wetter conditions over the 20th century although changes are less temporally and spatially coherent than those observed for temperature. The general trend in precipitation for Europe in the 20th century is of increases over northern Europe and decreases over southern Europe. Extreme precipitation is becoming more intense and more frequent in Europe, especially in central and eastern Europe in winter, often resulting in greater and more frequent flooding. Since 1950 winter wet spells increased in duration in northern Europe and reduced in southern Europe, while summer wet spells became shorter in northern and eastern Europe. An increasing proportion of total rainfall is observed to fall on heavy rainfall days. Extreme precipitation (including short intense convective or longer duration frontal types) demonstrates complex variability and lacks a robust spatial pattern. Climate models project that events currently considered extreme are expected to occur more frequently in the future. For example a 1-in-20 year annual maximum daily precipitation amount is likely to become a 1-in-5 to 1-in-15 year event by the end of the 21st century in many parts of Europe. There are few ground based hail observation networks, so satellite measurements and weather models are used to identify hail forming conditions. In Europe most extreme hail events occur in the summer over Central Europe and the Alps where convective energy is greatest. Intense hail events are linked to increases in convective energy in the atmosphere observed over the last 30 years. Hailstorm projection studies, although limited to France, northern Italy and Germany, show increases in the convective conditions that lead to hail and some areas show a rise in damage days although this is not statistically significant. Recent severe droughts include Italy (1997-2002), the Baltic countries 2005-2009, the European heatwave of summer 2003, and the widespread European drought of 2011. The 1950s were prone to long, intense, Europe-wide meteorological and hydrological droughts. In northern and eastern Europe the highest drought frequency and severity was from the early 1950s to the mid-1970s. Southern and Western Europe (especially the Mediterranean) show the highest drought frequency and severity since 1990. There has been a small but continuous increase of the European areas prone to drought from the 1980s to the early 2010s. Regional climate models project a decrease in summer precipitation until 2100 of 17%. Dry periods are expected to occur 3 times more often at the end of this century and to last longer by 1 to 3 days compared to the period of 1971-2000. There is significant uncertainty associated with future projections of drought, with climate variability being the dominant source of uncertainty in observed and projected soil moisture drough

Drivers and subseasonal predictability of heavy rainfall in equatorial East Africa and relationship with flood risk

Equatorial East Africa (EEA) suffers from significant flood risks. These can be mitigated with pre-emptive action, however currently available early warnings are limited to a few days lead time. Extending warnings using subseasonal climate forecasts could open a window for more extensive preparedness activity. However before these forecasts can be used, the basis of their skill and relevance for flood risk must be established. Here we demonstrate that subseasonal forecasts are particularly skillful over EEA. Forecasts can skillfully anticipate weekly upper quintile rainfall within a season, at lead times of two weeks and beyond. We demonstrate the link between the Madden-Julian Oscillation (MJO) and extreme rainfall events in the region, and confirm that leading forecast models accurately represent the EEA teleconnection to the MJO. The relevance of weekly rainfall totals for fluvial flood risk in the region is investigated using a long record of streamflow from the Nzoia river in Western Kenya. Both heavy rainfall and high antecedent rainfall conditions are identified as key drivers of flood risk, with upper quintile weekly rainfall shown to skillfully discriminate flood events. We additionally evaluate GloFAS global flood forecasts for the Nzoia basin. Though these are able to anticipate some flooding events with several weeks lead time, analysis suggests action based on these would result in a false alarm more than 50% of the time. Overall, these results build on the scientific evidence base that supports the use of subseasonal forecasts in EEA, and activities to advance their use are discussed