Ensemble flood and drought simulations for Europe

The projected changes in precipitation extremes under enhanced greenhouse conditions, with more frequent and prolonged periods of intense or, conversely, lack of precipitation, suggest more frequent and severe river floods and droughts for the future. We present a pan-European assessment of changes in extreme high and low flows by examining extreme discharge levels as simulated by a hydrological model when driven by a multi-model ensemble of climate simulations. The ensemble consists of simulations from two regional climate models (RCMs), both run with boundary conditions from two global climate models (GCMs), and for two scenarios of greenhouse gas emissions. Streamflow droughts will become more severe and persistent in most parts of Europe by the end of this century, except in the most northern and north-eastern regions. Most prone to an increase in drought hazard are the southern parts of Europe, which already suffer most from water stress. For floods we find a tendency toward a higher flood hazard in the majority of the model experiments in several major European rivers. A notable exception is northeastern Europe, where a consistent decrease in extreme river discharge is projected, suggesting a reduction in the hazard of extreme snowmelt floods due to warmer winters. At the scale of individual river basins the magnitude of change in flood hazard can vary strongly between climate models and scenarios, especially with respect to the driving GCM. Changes in streamflow drought seem less sensitive to the decadal-scale internal variability that is usually present in climate simulations and that may partially or completely obscure the climate change signal in extreme events. Our results suggest that combining climate models through a multi-model ensemble increases the reliability and consistency of the predictions, and is indispensable to better identify the climate signal amidst large variability.