GRACE/GRACE-FO, remote sensing, land use modelling, and coupled simulations for understanding the causes of regional climate change

Several continental regions on Earth are getting wetter, while others are drying out not only in terms of precipitation but also measured by the increase or decrease in surface water, water stored in the soils, the plant root zone, and in groundwater. Drying and wetting as seen in terrestrial e.g. rainfall or soil moisture, GRACE/GRACE-FO and remote sensing data are generally ascribed to combined effects of global warming due to greenhouse gas forcing, natural variability, and anthropogenic modification of the water cycle. Existing climate models that account for these effects fail to explain observed patterns of hydrological change sufficiently. Contrary to common beliefs, observations also do not support a simple dry-gets-dryer and wet-gets-wetter logic. Instead, the observed trends, in total water storage TWSA but also precipitation, soil moisture, or river discharge, differ considerably from a simplified logic, and characteristic regions not necessarily match each other. The new collaborative research centre CRC1502, funded by the German Research Foundation and run at the Universities Bonn, Cologne, Göttingen and the FZ Jülich and DWD, targets at closing this gap of understanding. To better comprehend the origin of these patterns, we have started to build a modelling framework that explains past observations as realistically as possible (reanalysis), accounts for potential drivers of change that may have been understudied in the past, and that can predict future changes.

 

Climate change and anthropogenic interactions are already affecting the frequency of extreme events such as heat waves, droughts and floods. More intense, more frequent, and longer-lasting heat waves are projected for the 21st century; surface and ground water buffer the effects of such heat waves, but large-scale drying may amplify them to an as yet unknown extent. Societal, environmental and economic consequences include increased risk in agricultural production, threats to agricultural productivity and food security and increasing health risks. This CRC proposes the hypothesis that humans – through several decades of land use change, and intensified water use and management – have caused persistent modifications in the coupled land and atmospheric water and energy cycles. These human-induced modifications contribute considerably, compared to greenhouse gas forcing and natural variability, to the observed trends in water storage at the regional scale. We hypothesize that land management and land and water use changes have modified the regional atmospheric circulation and related water transports. These changes in the spatial patterns of the water balance are hypothesized to have created and magnified imbalances that lead to excessive drying or wetting in more remote regions. We test this hypothesis for Europe in the 1st phase. In later phases, we evaluate the transferability of our approach for regions with different environmental conditions. We propose to develop evidence-based sustainability criteria for land and water use activities.

 

The presentation will introduce the objectives of the new CRC and the approaches that we will pursue, including how we will utilize GRACE/GRACE-FO data in combination with other remote sensing data and with coupled modelling.