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North Pacific/North American sector (rather than the tropics for ENSO events). Both "warm" and <br />"cool" PDO cycles can be documented, producing results broadly similar to weak ENSO events. <br />Understanding the combined impact of ENSO, PDO, and possible global climate change is an <br />important and active area of study. <br />SCIENCE, UNCERTAINTY AND BEST GUESSES <br />A better understanding of past and present climate phenomenon is expected, ultimately, to lead to <br />more sophisticated projections of future conditions. The extent to which current research satisfies <br />this need is largely tied to the expectations of the user. For example, if an accurate prediction of year <br />2100 water availability in a given basin is the goal, then the user is likely to be disappointed by even <br />the most acclaimed studies, as the output is the product of several stages of compounded errors and <br />uncertainties. Models with similar assumptions (and in many cases, identical computer source code) <br />may in fact yield similar results, but this consistency cannot safely be interpreted as accuracy. <br />In order to make use of this research exploring the long -term relationship between climate change <br />and water management, therefore, it is probably most useful to view the model output as <br />"scenarios " —i.e., plausible alternative futures based on a given set of assumptions— rather than <br />predictions or forecasts. By using a reasonable range of assumptions, a group of scenarios can be <br />generated that, collectively, describe a range of potential futures that are likely to encompass our <br />actual future. In many cases, that range may first appear too broad to help guide actual planning and <br />management decisions, but ultimately, the scenarios can help managers assess the vulnerability of <br />their systems, and can provide useful sidebars within which to explore the utility of adaptation and <br />mitigation mechanisms.$ <br />Already, a few consistent findings are emerging from the suite of available scenarios that should <br />merit immediate consideration in the water community. These are discussed in detail in the literature <br />provided (described later) and are the subject of several conference presentations; thus, they are only <br />mentioned briefly here. They include: <br />• A continued rise in global temperatures, perhaps in the range of 1.4 to 5.8 °C (2.5 to 10.4 °F) <br />from 1990 to 2100. This range is largely explained by different emissions scenarios; it <br />would likely constitute the most dramatic temperature increase in at least 10,000 years. <br />• In basins where most of the water supply is associated with snowmelt, the result of warmer <br />temperatures is likely to be a reduced ratio of snow to rain, earlier snowmelt, increased <br />winter stream flows, and lower summer flows. These impacts are likely to be most <br />noticeable in lower mountainous areas, where just a modest temperature increase can <br />noticeably shift the snowline. <br />• Precipitation trends are highly region - specific and among the most difficult climate variables <br />to forecast, but for most basins in the American West, many researchers believe that the 21St <br />century will continue the late -20th century trend toward wetter conditions. This trend is <br />7 More information about PDO can be found at: "The Pacific Decadal Oscillation: A Brief Overview for Non - <br />Specialists." Nathan Mantua. hqp:// tao. atnios .washinjzton.edu/PNWimpacts /Publications /Publ29.htm <br />8 The distinction between adaptation and mitigation is important. Members of the water management <br />community are likely confined to seeking adaptation measures that attempt to limit vulnerability and impacts <br />associated with a given range of possible climate futures. The term mitigation is usually reserved for national <br />or global strategies aimed at reducing human contributions to climate change. <br />7 <br />