WorkBlock 3 - The Global Water Cycle in the 21st century

Co-ordinator: Stefan Hagemann, Max Planck Institute for Meteorology (MPI-M)
Deputy co-ordinator: Jens H. Christensen, Danish Meteorological Institute (DMI)

In a changing future climate, European decision-makers in government, non-governmental organisations and industry, as well as the general public, need information as detailed and reliable as possible on the hydrological cycle. This will provide them with the means to evaluate the hydrological risks of climate change due to anthropogenic emissions of greenhouse gases. While projections of future hydrological changes already exist, they are deficient concerning the complexity of the considered components of the hydrological cycle and their related uncertainties. To date, the assessment of potential impacts of climate change on the hydrological cycle has generally relied on projections from atmosphere-ocean general circulation models (GCMs) at the coarse global scale (~200-300 km), and from regional climate models (RCMs) forced by the GCMs for selected regions at spatially finer scales of about 25-50 km.

In climate models (CMs), hydrological processes are often only crudely represented. Thus, future changes in some components, such as precipitation, evapotranspiration, runoff, and precipitable water content may be captured in a general fashion, i.e. for large areas and large catchments. But detailed changes, especially in the terrestrial components of the hydrological cycle, are largely uncertain or cannot even be tackled at all, such as groundwater, snowmelt, permafrost hydrology, and wetlands. In addition, certain anthropogenic influences on the hydrological cycle are generally not considered within current CMs, such as irrigation, dams, river regulation, and agricultural land use changes and management. This limited physical representation of the hydrological cycle precludes the realistic simulation of all of its components in full detail in space and time.

The need of information for long-term investments and for planning of the users of water resources drives the following major questions:

  • Will the hydrological cycle change in the 21st century?
  • Where will these changes be significant for the use of water resources?
  • How can we attribute the causes of these changes?

Answering the 3rd question requires the understanding of the role of climate within the context of global changes, e.g. the shift of the ITCZ (Intertropical Convergence Zone), changes in the polar front, changes of the storm tracks, or increases in sea surface temperatures.

Fig. 1 Relative changes in precipitation (in percent) for the period 2090–2099, relative to 1980–1999.
Values are multi-model averages based on the SRES A1B scenario for December to February (left) and June to August (right). White areas are where less than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change (IPCC 2007).

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