Climatic changes of extreme precipitation in Denmark from 1874 to 2100

Abstract

This study presents the results of a coordinated effort to estimate past, present and future changes and uncertainties in Danish design rainfall for urban drainage systems. The performed analyses cover long historical precipitation records, observations from a high‐resolution rain‐gauge network, an ensemble of climate model simulations, and two high‐end climate scenarios. During the past 30 years rather dramatic changes in extreme precipitation have been observed in Denmark. These changes are mainly in the frequency of extreme events, but there is also a tendency towards more severe events. Both are considered effects of anthropogenic climate change. The increase in precipitation extremes has led to inundations in most of the larger cities during the last 10 years. To establish cities that are resilient to pluvial floods, robust projections of the frequency and intensity of extreme precipitation events in a changing climate are needed. Additionally, it is equally important to understand the natural variation onto which the anthropogenic changes are imposed. Trend analysis of observations from the high‐resolution rain‐gauge network currently applied for estimation of design intensities shows that the frequency of extreme events has increased by approximately 2% per year during the last three decades. Additional analyses of five long daily precipitation series show that the frequency of extreme events in the past has oscillated with a cycle of 25‐35 years, a behavior that can in part be explained by sea level pressure differences over the Atlantic. On this basis the precipitation extremes in the Eastern part of Denmark are projected to be ascending in the last two decades. However, the increase has continued longer than expected and with larger amplitude in the most recent years. This indicates a likely influence from anthropogenic greenhouse gas emissions. With the complex combination of general increase and natural variation several additional years of observation are needed before this hypothesis can be evaluated by statistical means. Extensive analysis of 17 different regional climate model (RCM) simulations shows that anthropogenic activity very likely will contribute to a significant increase in extreme precipitation amount and occurrence in Denmark. It is argued that climate models are incapable of simulating extreme precipitation at the temporal scales relevant for evaluation of the urban pluvial inundation risk. Hence different statistical downscaling methods have been applied. Furthermore, the effect of the emission scenario, the spatial resolution of the RCM and the interdependency between RCMs are discussed. Taking this information into account a 2‐year event is expected to increase by 20% over a projection period of 100 years. This approximates the variation within one natural oscillation cycle, indicating that it is crucial to understand and account for the future multi‐decadal variations of extreme precipitation. The study estimates the expected magnitude of variation in design rainfall for urban drainage design due to anthropogenic climatic changes and natural variation. The analyses show that the most recent increase in design intensities is not attributed to anthropogenic climate change alone, but also heavily influenced by the natural variation of extreme rainfall. Together with a robust sign of increase in the design intensities, derived from an ensemble of climate models combined with different statistical downscaling methods, this gives confidence to the climate models? ability to project future change of extreme rainfall over Denmark. The potential interaction between the natural variability and changes driven by the anthropogenic forcing is still to be better understood. However, the generated knowledge can assist the design of robust adaptation measures for changes in pluvial flood risk.