In the water sector, it is important to distinguish between the needs of those who wish to estimate the potential magnitude of climate change impacts on hydrology and water resources—to meet IPCC concerns, for example—and the more pragmatic needs of water managers who need to consider how best to adapt to climate change. The two sets of requirements are linked, but there are some important differences in emphasis.

Some climate change analysts are essentially concerned with estimating what would actually happen under different climate futures: What are the impacts, for example, of continued growth of emissions of GHGs at 1% yr ^-1 , and what would be the impact of stabilizing CO₂ concentrations at, say, 550 ppmv by 2150? How do changes in variability affect the water environment? These impacts—and their costs—then could be compared with the impacts, costs, and benefits of mitigation. Such studies, in principle, could allow identification of “dangerous” levels of climate change. There also are important science questions concerning the processes by which climate change might impact the water environment. For example, how might flow pathways through soils change?

Such research questions need developments in the following areas:

• Creation of credible climate change scenarios. This involves improvements to GCMs so that they simulate present climate and its multi-decadal variability even better, development of conceptually sound downscaling techniques (in the absence of high-resolution global climate models), and characterization of potential changes in variability at time scales from daily to decadal. These requirements are common to all impact sectors and (with the exception of downscaling) are central to improving the understanding of climate change in the most general sense.
• Characterization of natural climatic and hydrological variability. Potential future climate changes resulting from increasing concentrations of GHGs need to be placed in context by appreciation of “natural” climatic and hydrological variability. Much needs to be learned about linkages between different components of the climate system in different parts of the world, which requires joint use of observational data (including remotely sensed data), palaeoclimatic data, and model simulations. Palaeoclimatic and palaeohydrological reconstructions can provide very useful information on the variability in “natural” hydrological systems, as well as insights into nonlinear relationships between climate forcing and hydrological response.
• Improved hydrological models. Particularly important is development and application of process-based models of hydrological processes that include realistic representations of processes that generate streamflow and recharge and determine water quality. Key issues include development of models that do not need catchment calibration (but may require remotely sensed inputs) to assess the effects of climate change in parts of the world with limited hydrological data and development of coupled climate-hydrology models (which also are important for the improvement of climate model performance and for seasonal forecasting). The international collaborative research efforts summarized in Section 4.2.2 are extremely important.
• Characterization of uncertainty. How important are the different sources of uncertainty—in emissions, global climate response, and regional climate change— for estimated effects of climate change? Is downscaling cost-effective, given the wide range of changes in climate that might result from different emissions scenarios, for example? What can ensemble climate model experiments contribute? There has been little systematic analysis to date of the relative importance of different sources of uncertainty.
• Impacts on real-world water systems. Section 4.5 notes that there have been relatively few published studies on the impacts of climate change on real-world water resources systems, and inferences about impacts generally have been made from estimates of changes in streamflow alone. This may give a very misleading impression of the actual impacts of change because the characteristics of the water management system are a very important buffer between hydrological effect and impact on users and the environment. Therefore, more studies into real-world systems are needed.
• Effects of adaptation. Most impact studies have ignored adaptation by water managers, and in opposition it often is asserted that water managers will be able to adapt. However, how will managers make adaptation decisions in practice on the basis of incomplete information, and what would be the effects of inefficient adaptation on the impacts of climate change?

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