The CORDEX.be initiative as a foundation for climate services in Belgium

The CORDEX.be project created the foundations for Belgian climate services by producing high-resolution Belgian climate information that (a) incorporates the expertise of the different Belgian climate modeling groups and that (b) is consistent with the outcomes of the international CORDEX ("COordinated Regional Climate Downscaling Experiment") project. The key practical tasks for the project were the coordination of activities among different Belgian climate groups, fostering the links to specific international initiatives and the creation of a stakeholder dialogue. Scientifically, the CORDEX.be project contributed to the EURO-CORDEX project, created a small ensemble of High-Resolution (H-Res) future projections over Belgium at convection-permitting resolutions and coupled these to seven Local Impact Models. Several impact studies have been carried out. The project also addressed some aspects of climate change uncertainties. The interactions and feedback from the stakeholder dialogue led to different practical applications at the Belgian national level

Heat stress increase under climate change twice as large in cities as in rural areas : a study for a densely populated midlatitude maritime region

Urban areas are usually warmer than their surrounding natural areas, an effect known as the urban heat island effect. As such, they are particularly vulnerable to global warming and associated increases in extreme temperatures. Yet ensemble climate-model projections are generally performed on a scale that is too coarse to represent the evolution of temperatures in cities. Here, for the first time, we combine unprecedented long-term (35years) urban climate model integrations at the convection-permitting scale (2.8km resolution) with information from an ensemble of general circulation models to assess temperature-based heat stress for Belgium, a densely populated midlatitude maritime region. We discover that the heat stress increase toward the mid-21st century is twice as large in cities compared to their surrounding rural areas. The exacerbation is driven by the urban heat island itself, its concurrence with heat waves, and urban expansion. Cities experience a heat stress multiplication by a factor 1.4 and 15 depending on the scenario. Remarkably, the future heat stress surpasses everywhere the urban hot spots of today. Our results demonstrate the need to combine information from climate models, acting on different scales, for climate change risk assessment in heterogeneous regions. Moreover, these results highlight the necessity for adaptation to increasing heat stress, especially in urban areas

Impact of Climate Change on Precipitation Extremes and Pluvial Flooding in Europe

Extreme precipitation and flood are among the major climate-related disasters which cause thousands of fatalities and billions of euros in damages each year. The risk of these events is expected to increase remarkably under changing climatic and socioeconomic conditions, increasing pressure on vulnerable societies and ecosystems. Yet, the impact of the changing conditions on sub-daily extreme precipitation and pluvial flooding and their related risk has not been well studied due primarily to the scarcity of sub-daily precipitation observations and model simulations, a limited availability of detailed and consistent data on future vulnerability components and the computationally expensive continental flood modeling. This PhD dissertation therefore explores the impact of future climatic and socioeconomic changes on sub-daily extreme precipitation, intensity-duration-frequency (IDF) curves and pluvial flooding over Europe. First, the impact of future anthropogenic climate change on 3-hourly extreme precipitation with return periods ranging between 5 and 50 years over Europe is investigated using the simulations from regional climate models (RCMs) with 0.11° resolution from the Coordinate Downscaling Experiment over Europe (EURO-CORDEX) ensemble. The robustness of the signals is examined based on a regionalized signal-to-noise (S2N) technique by taking into account a spatial dependence incorporated spatial pooling (regionalization) to decrease the noise due to internal variability and to achieve improved statistics. The effectiveness of the spatial pooling is evaluated by a sensitivity analysis to precipitation times scale, precipitation intensity, season, climate model resolution and greenhouse gas concentration. The results indicate an intensification of 3-hourly extreme precipitation over Europe at the end of the 21st century for all seasons except summer for which a bipolar pattern (increase in the north and decrease in the south) is discerned. For the non-mitigation scenario of Representative Concentration Pathway (RCP) 8.5, the regionalized winter 3-hourly extreme precipitation changes over 9×9 model grid cells are statistically significant in roughly 72%, 65%, 59% and 48% of the European area for 5-, 15-, 25- and 50-year return periods respectively, whereas 16-21% of the area will experience significant changes in summer. The S2N values for 3-hourly extreme precipitation changes rise after the spatial pooling by about a factor of 1.4-1.7 for all seasons except summer when they decline by about a factor of 0.78. The results of sensitivity analysis reveal that the regionalization influence is sensitive - in order of decreasing importance - to season, precipitation time scale, precipitation intensity, future greenhouse gas concentration scenario and climate model spatial resolution. Whenever and wherever short-duration convective precipitation is dominant, the precipitation time scale is found to be more important, which is the case seasonally for summer and regionally for south Europe. Subsequently, the impact of future anthropogenic climate change on precipitation IDF relationships is examined. As a pilot study, future IDF curves are firstly developed for Belgium by using large ensembles of global climate models (GCMs) and RCMs. Building upon the findings from the case study in Belgium, the methodology is extended for Europe and the future IDF curves are developed considering duration ranging from 30 minutes to 24 hours and return periods ranging between 1 and 100 years. The future continental-scale IDF curves are developed in the framework of the quantile perturbation downscaling by applying climate change signals from the EURO-CORDEX RCMs on current IDF curves obtained from high spatial and temporal resolution remote sensing based precipitation products. The results reveal that under climate change, future IDF curves will be uplifted (16-27%, depending on duration and return period) and steepened (17-25%, depending on return period), with a larger degree under RCP8.5 compared to RCP4.5. The upward shift in IDF curves is caused by the intensification of rainstorms under climate change, while a larger increase for shorter duration rainstorms leads to a steepening of the curves. In addition to the intensity of rainstorms, their frequency is also projected to increase such that the frequency of 50-year and 100-year events will be doubled under RCP4.5 and tripled under RCP8.5. Furthermore, this PhD dissertation analyzes the impact of future climatic and socioeconomic changes on pluvial flooding over Europe. To this end, pluvial flooding is quantified using a satellite-based data driven approach for 20-, 30-, 50- and 100-year return periods. To elucidate the impact of changes in both climatic and socioeconomic conditions on floods at the continental, regional and national levels, the Shared Socioeconomic Pathways (SSPs) are merged with RCPs, integrating hazard and several social, economic and agricultural exposure-vulnerability proxy indicators. The results reveal that future flood hazard increases for different return periods and scenarios by the end of this century. A ubiquitous drastic increase up to 87% is also projected for future flood risks of different return periods over Europe, with eastern and southern regions experiencing the highest risk increase. A fossil-fuel based development (SSP5 combined with RCP8.5) in the future would lead to 14-15% higher flood risk compared to a sustainable development (SSP1 combined with RCP4.5), which goes up to 23% in north Europe. The amplified future flood risk is predominantly driven by climate change, although with a large uncertainty, rather than socioeconomic drivers. In almost all the European countries, the changes in flood hazard are far larger than the exposure-vulnerability changes. As the knowledge on the uncertainty of future projections is indispensable for future planning in the face of climatic and socioeconomic changes, special attention is paid to this matter throughout this dissertation. The uncertainty is quantified in the future projections of different variables including extreme precipitation, IDF curves and flood risk. The uncertainty analysis shows GCM uncertainty as the dominant source in extreme precipitation projections for Belgium for all return periods and durations compared to GCM initial conditions and RCPs. By taking an ensemble of the EURO-CORDEX RCMs, the uncertainty associated with the choice of RCMs is found to be larger than the other uncertainty components in the projection of more extreme precipitation. Similar to the Belgian case study, the uncertainty in the projections of extreme precipitation and IDF curves at the European scale is larger for longer return periods. The uncertainty is, with a lesser extent, dependent on duration, being larger for shorter durations. The uncertainty assessment in the projections of European pluvial flood risk indicates a spatially and temporally varying magnitude of total and fractional uncertainties over the continent, with climate model uncertainty as the dominant source.status: publishe

Satellite-based data driven quantification of pluvial floods over Europe under future climatic and socioeconomic changes

Flooding is one of the major threats jeopardizing lives and properties of the people, and its risk is expected to increase remarkably under changing climatic and socioeconomic conditions. Yet, future flood risk has not been well studied due primarily to a limited availability of detailed and consistent data on future vulnerability components and the computationally expensive continental flood modeling. Here we perform a top-down data driven flood risk assessment for 20-, 30-, 50- and 100-year return periods over Europe at the continental, regional and national levels for the late 21st century. To account for the impact of changes in both climatic and socioeconomic conditions on floods, the Shared Socioeconomic Pathways (SSPs) are merged with Representative Concentration Pathways (RCPs), integrating hazard and several social, economic and agricultural exposure-vulnerability proxy indicators. Our results show a ubiquitous drastic increase up to 87% in future flood risks of different return periods over Europe, with eastern and southern regions experiencing the highest risk increase. A fossil-fuel based development in the future would lead to 14-15% higher flood risk compared to a sustainable development, which goes up to 23% in north Europe. The amplified future flood risk is predominantly driven by climate change, although with a large uncertainty, rather than socioeconomic drivers.status: publishe

Precipitation intensity–duration–frequency curves for central Belgium with an ensemble of EURO-CORDEX simulations, and associated uncertainties

An ensemble of 88 regional climate model (RCM) simulations at 0.11° and 0.44° spatial resolutions from the EURO-CORDEX project is analyzed for central Belgium to investigate the projected impact of climate change on precipitation intensity–duration–frequency (IDF) relationships and extreme precipitation quantiles typically used in water engineering designs. The rate of uncertainty arising from the choice of RCM, driving GCM, and radiative concentration pathway (RCP4.5 & RCP8.5) is quantified using a variance decomposition technique after reconstruction of missing data in GCM ×RCM combinations. A comparative analysis between the historical simulations of the EURO-CORDEX 0.11° and 0.44° RCMs shows higher precipitation intensities by the finer resolution runs, leading to a larger overestimation of the observations-based IDFs by the 0.11° runs. The results reveal that making a temporal stationarity assumption for the climate system may lead to underestimation of precipitation quantiles up to 70% by the end of this century. This projected increase is generally larger for the 0.11° RCMs compared with the 0.44° RCMs. The relative changes in extreme precipitation do depend on return period and duration, indicating an amplification for larger return periods and for smaller durations. The variance decomposition approach generally identifies RCM as the most dominant component of uncertainty in changes of more extreme precipitation (return period of 10 years) for both 0.11° and 0.44° resolutions, followed by GCM and RCP scenario. The uncertainties associated with cross-contributions of RCMs, GCMs, and RCPs play a non-negligible role in the associated uncertainties of the changes.status: publishe

Quantification of uncertainty in reference evapotranspiration climate change signals in Belgium

Projections of evapotranspiration form the basis of future runoff and water availability assessment in a climate change context. The scarcity of data or insufficiency of time/funds compels the application of simple reference evapotranspiration (ETo) methods requiring less meteorological inputs for ETo projections which adds uncertainty to the projected changes. This study investigates the bias in ETo climate change signals derived from seven simple temperature- and radiation-based methods (Blaney–Criddle, Hargreaves–Samani, Schendel, Makkink, Turc, Jensen–Haise, Tabari) compared with that from the standard Penman–Monteith FAO 56 method on the basis of 12 general circulation model (GCM) outputs from the Coupled Model Intercomparison Project Phase 5 for central Belgium for four future greenhouse gas scenarios (RCP2.6, RCP4.5, RCP6.0, RCP8.5). The results show the lack of conformity on the amount of ETo changes between the simple and standard methods, with biases of over 100% for some simple methods. The uncertainty affiliated with ETo methods for monthly ETo changes is smaller but of comparable magnitude to GCM uncertainty, which is usually the major source of uncertainty, and larger for daily extreme ETo changes. This emphasizes the imperative of addressing the uncertainty associated with ETo methods for quantifying the hydrological response to climate change.status: publishe

Uncertainty assessment for climate change impact on intense precipitation: How many model runs do we need?

Precipitation projections are typically obtained from general circulation model (GCM) outputs under different future scenarios, then downscaled for hydrological applications to a watershed or site-specific scale. However, uncertainties in projections are known to be present and need to be quantified. Although GCMs are commonly considered the major contributor of uncertainty for hydrological impact assessment of climate change, other uncertainty sources must be taken into account for a thorough understanding of the hydrological impact. This study investigates uncertainties related to GCMs, GCM initial conditions and representative concentration pathways (RCPs) and their sensitivity to the selection of GCM runs in order to quantify the impact of climate change on extreme precipitation and intensity/duration/frequency statistics. The results from a large ensemble of 140 CMIP5 GCM runs including 15 GCMs, 3–10 GCM initial conditions and 4 RCPs are analysed. Albeit the choice of GCM is the major contributor (up to 65% for some cases) to intense precipitation change uncertainty for all return periods (1 year, 10 years) and aggregation levels (1-, 5-, 10-, 15- and 30-day), uncertainties related to the GCM initial conditions and RCPs of up to 38 and 23%, respectively, are found in some cases. The sensitivity analysis reveals that the GCM, RCP and GCM initial condition uncertainties are greatly influenced by the set of climate model runs considered, especially for more extreme precipitation at finer time scales.Programme’s Working Group on Coupled Modelling – phase 5 (CMIP5), and the climate modeling centres involved in that programme. The work has been funded by projects for the Belgian Science Policy Office (CORDEX.be), the Flemish Environment Agency, and the European Union’s Horizon 2020 research and innovation programmes under grant agreement No 700699, project BRIGAID (BRIdges the GAp for Innovations in Disaster project PUCS (Pan-European Urban Climate Service).status: publishe

Regionalization of anthropogenically forced changes in 3hourly extreme precipitation over Europe

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Identical hierarchy of physical drought types for climate change signals and uncertainty

Climate change may have different impacts on different types of drought through its influence on the mechanisms of the propagation of a precipitation lack into a hydrological or agricultural drought. The involvement of additional processes in runoff and soil moisture modeling potentially leads to discrepancies in the projection uncertainties and signal-to-noise ratios between different drought types. This global study compares climate change signals, uncertainty, and signal-to-noise ratios between meteorological, hydrological, and agricultural droughts characterized by standardized precipitation index (SPI), standardized runoff index (SRI), and standardized soil moisture index (SSI), respectively. The comparison is made for five drought characteristics including median and peak intensity, median and longest duration, and frequency using 18 Coupled Model Intercomparison Project Phase 6 (CMIP6) models for four Shared Socioeconomic Pathways (SSPs) SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. We find that the spatial extent and magnitude in all five drought characteristics increase from meteorological to hydrological to agricultural drought. This increase manifests itself, however, at the expense of augmented uncertainty, to the extent that uncertainty for agricultural drought is up to sevenfold larger compared to meteorological drought. Despite the augmentation of uncertainty from meteorological to agricultural drought, the hierarchy of drought types for climate change signals still holds for the spatial extent of significant signal-to-noise ratios