The influence of anthropogenic climate change on wet and dry summers in Europe

Understanding the role of anthropogenic forcings in regional hydrological changes can help communities plan their adaptation in an informed manner. Here we apply attribution research methods to investigate the effect of human influence on historical trends in wet and dry summers and changes in the likelihood of extreme events in Europe. We employ an ensemble of new climate models and compare experiments with and without the effect of human influence to assess the anthropogenic contribution. Future changes are also analysed with projections to year 2100. We employ two drought indices defined relative to the pre-industrial climate: one driven by changes in rainfall only and one that also includes the effect of temperature via changes in potential evapotranspiration. Both indices suggest significant changes in European summers have already emerged above variability and are expected to intensify in the future, leading to widespread dryer conditions which are more extreme in the south. When only the effect of rainfall is considered, there is a distinct contrast between a shift towards wetter conditions in the north and dryer in the south of the continent, as well as an overall increase in variability. However, when the effect of warming is also included, it largely masks the wet trends in the north, resulting in increasingly drier summers across most of the continent. Historical index trends are already detected in the observations, while models suggest that what were extremely dry conditions in the pre-industrial climate will become normal in the south by the end of the century.