A study of the effects of high-end climate change scenarios on the global hydrological regime

Summarization: In recent years, there has been a strong consensus on the changes in climate caused by increased concentrations of anthropogenic greenhouse gas emissions. Global CO2 emission rates have been following high-end climate change pathways leading to a future global temperature that is likely to surpass the target limits of +1.5oC and +2oC, and reach levels of +4oC and higher at the end of the 21st century. Freshwater availability under such conditions is a key issue of concern and thus, scientific research has focused on estimating the range of changes in the future climate and the effectiveness of different adaptation strategies. The main tool for the investigation of future climate is the utilization of global climate models (GCMs). GCMs are based on physical principles that describe the components of the climate system. The next step for hydrological impacts’ assessments is to force global hydrological models (GHMs) or land surface models (LSMs) with GCM outputs. Due to the systematic biases they feature, GCM outputs need some kind of bias correction prior to their application as forcing to impact models, especially for hydrological studies. Most bias correction techniques focus mainly on the variables of precipitation and temperature. However most state-of-art hydrological models require more forcing variables, additionally to precipitation and temperature, such as radiation, humidity, air pressure and wind speed. The biases in these additional variables can hinder hydrological simulations, but the effect of the bias of each variable is unexplored. In the present thesis, a methodological framework of a multi-faceted assessment of the effects of high-end climate change on the global hydrological regime is presented. The tool for the hydrological simulations in our study is the LSM JULES, a physically based model operating at the global scale. The first component of our methodological framework is the evaluation of the model for a historical period and the assessment of the model’s sensitivity to input forcing. A runoff routing algorithm is designed and implemented, to allow the comparison of the model output with discharge measurements. The second part of the methodological framework aims to assess the effect of the GCM biases on the performed runoff simulations, with the scope of deciding on the meteorological variables that should be included in bias correction. To this end, a methodology for the classification of the effect of biases in four effect categories (ECs), based on the magnitude and sensitivity of runoff changes, is developed and applied. The final part of the methodological framework of this thesis is the assessment of hydrological climate change impacts under high-end warming scenarios. Assessment of impacts focuses on water availability and droughts at the global, European, regional and basin scale, employing a number of different and complementary methods. Climate change impacts are examined for different levels of warming (+1.5, +2 and +4oC) and the uncertainty in the projected changes is assessed throughout this analysis. The results of this study could assist scientists make informed decisions on variables and methods that should be considered in future climate change impacts’ studies, focusing on the uncertainty component of the impact analysis, by examining a wide range of “hydrologically opposing” future climates. The results of the present study could also be useful for policy makers, who need information relevant to this thesis, in order to decide on planning and legislations regarding climate change adaptation and mitigation