Grand Challenges Related to the Assessment of Climate Change Impacts on Freshwater Resources

The present contribution reviews a suite of grand challenges related to assessment of climate change impacts on freshwater resources. Among them are challenges related to the detection and attribution of changes in observed records, projections for the future, changes in hydrologic extremes (floods and droughts), assessing and reducing uncertainty, and adaptation to change under uncertainty. The global water system is very complex, so that it is difficult to disentangle individual contributions of various factors to changes in freshwater variables at any scale. As for detection and attribution of changes in global river discharge in twentieth century, variations in precipitation were the main force. Other major factors were temperature effects on evapotranspiration, direct effects of rising atmospheric CO2 concentration on the physiology and abundance of vegetation, and anthropogenic changes in land cover and land use. A general finding regarding possible future trends in the water cycle is that wet regions will likely become wetter and dry regions to become drier in a warming world. Climate-driven hydrologic changes combine with other pressures on water resources, such as population growth, land use change, changes in life styles increasing water demand, and environmental pollution. A grand global challenge is to provide an adequate basis for adaptation decisions that must be made under strong uncertainty, without reliance on precise projections of changes in hydrologic variables.