There are several layers of uncertainty inherent in assessing climate change impacts. For example, uncertainties in projected greenhouse gas emissions, limitations of climate models, information loss when climate projections are downscaled to watershed resolution, and imperfections in hydrological models all contribute to the uncertainty.  Perhaps even more frustrating is the fact that there is no universally accepted standard for quantifying these uncertainties.  This means that it is difficult to define a meaningful confidence level for these projections.  Given the uncertain nature of climate impact analysis, it may be tempting to disregard climate change in decision analysis.  However, much is known regarding climate change and the remaining uncertainties do not provide a valid excuse to dismiss all aspects of climate change in water resource planning.  Rather, the uncertainty introduced by climate emphasizes the importance of incorporating flexibility or no-regrets options in water resource planning.

Water managers are accustomed to adapting to changing circumstances, many of which are analogs of future climate change, and they have developed a wide range of adaptive options. Supply-side options are more familiar to most water managers, but demand-side options are becoming increasingly prevalent. Water management is evolving continually, and this evolution will affect the impact of climate change in practice. For reasons noted above, climate change will inevitably challenge existing water management practices, especially in countries with less experience in incorporating uncertainty into water planning. The current challenge is to incorporate climate change uncertainty along with the other types of uncertainty traditionally treated in water planning.

A review of the scientific and water planning literature suggests that most water resource and water utility studies have incorporated climate change information into their planning process using a top-down approach (Figure 1). This approach typically begins by establishing the scientific credibility of human-caused climate change, develops future climate scenarios to be used at the regional level, and then imposes those potential changes on water resource systems to assess, for example, system reliability. The problems with a top-down approach are: 1) it does not always address the unique vulnerabilities and information needs of a utility, and 2) the approach may become mired in the uncertainty of the future climate projections. There is a danger that utility managers will disregard the results and view them as lacking relevancy and credibility. Alternatively, the bottom-up approach begins by identifying a water utilities’ most critical vulnerabilities; articulates the causes for those vulnerabilities; suggests how climate change, climate variability, and climate extremes might or might not exacerbate those vulnerabilities; and finally designs an analytic process to better address and solve the vulnerability in the face of the climatic uncertainty (e.g. a precautionary approach) (Figure 1).  In either top-down or bottom-up approaches, Integrated Water Resource Management (IWRM) can be the most effective method for assessing adaptation options and their implications in the context of an evolving regulatory environment with its competing demands (Figure 1).

Figure 1. Bottom-up and top-down approaches to climate change assessment