WorkBlock 3 The Global Water Cycle in the 21st century


Climate change is expected to affect the mean hydrological cycle and its variability as well as the frequency and magnitude of hydrological extremes, such as droughts and floods. A major limitation in previous studies of climate change impacts on the land surface water balance has been the scale mismatch of the large scale climate models and fine scale process-based hydrological models. This limitation is thoroughly addressed in this WorkBlock, by bringing together state-of-the-art GCMs and RCMs with state-of-the-art, process-based hydrological models, which eventually will contribute substantially to the development of a complex global hydrological model that couples atmosphere and hydrosphere (as a final goal of WATCH). In this context, methodologies will be developed and applied to establish the link between climate information and the related hydrological realisations to serve the needs of European society and economy.

To characterize and understand future changes in the hydrological cycle, the following specific objectives will be executed:

  • Objective 1: Produce multiple-model based projections for the terrestrial components of the hydrological cycle for the 21st century (links to WB 5 and WB 6)
  • Objective 2: Construct global maps identifying regions where the global water cycle is significantly impacted upon by climate change, and which regions are vulnerable to these changes in terms of e.g. water availability, and to understand this in detail in selected regions
  • Objective 3: Identify critical drivers for changes in the regional as well as in the global water cycle
  • Objective 4: Identify thresholds (if possible) that potentially may lead to dramatic or irreversible changes in the hydrological cycle and the local climate as well

These objectives complement each other to produce an assessment of the impact of future climate on the global hydrological cycle, its intra-annual variability and the related uncertainties at a hitherto unprecedented resolution. This will provide useful information for decision makers and the users of water resources on the characteristics and spatial distribution of likely future changes in the hydrological cycle. Furthermore, a better appreciation of the uncertainty range in calculations of future impacts on water resources from climate change may offer new insights into the range of adaptation and mitigation responses to climate change.

The main deliverables of WB3 are:

  • Archive of data produced within the project
  • Assessment report tackling the above-mentioned scientific questions addressing the future changes in the hydrological cycle
  • Definition of a strategy to apply gridded hydrological models to a large ensemble of climate change simulations
  • Set of global simulations where gridded hydrological models are forced by GCM simulations
  • Set of regional simulations where gridded hydrological models are forced by RCM simulations for specific regions
  • Set of sensitivity simulations that represent certain sensitivity tests
  • Set of high-resolution time-slice RCM simulations for specific regions under current and future climate conditions

WB3 comprises three main work packages:

  • WP3.1 Changes in the global water cycle. Work package leader: Stefan Hagemann, MPI-M
  • WP3.2 Changes in the regional water cycle. Work package leader: Jens H. Christensen, DMI
  • WP3.3 Sensitivity of changes to human activity. Work package leader: Dieter Gerten, PIK

Work Performed and Results achieved during Year 2 (February 2008 – 31 January 2009)
Methodologies have been developed to adequately handle biases in climate model output and to quantify the resulting uncertainties in estimates of future components of the global water cycle. Comprehensive analyses of projected changes in the hydrological cycle over Europe have been conducted as they are represented in existing global and regional climate model simulations. Further work comprises discussions on defining scenarios for the 21st century that reflect regional and global changes in socio-economic conditions, which shall be used to prescribe temporal, and spatially distributed, boundary conditions for global and regional hydrological sensitivity simulations. Main results of the first two years were compiled in technical reports. In addition, the first RCM test simulations were conducted for the second WATCH focus region over the Indian subcontinent.

Achievements have included: