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such as demographic changes. Ultimately, estimating impacts requires going beyond a purely <br />supply -side investigation of inflows and necessitates a consideration of changing demand patterns. <br />3 <br />Again, this effort usually involves the development of computer models, but also requires the <br />involvement of a much wider variety of experts and disciplines — including the social scientists — <br />than found in the preceding stages. Interdisciplinary assessments are the primary vehicle for <br />determining possible impacts of climate change and variability. <br />(6) Adaptations. An ideal but frequently missing concluding step is the evaluation of <br />possible adaptation strategies. Both supply- oriented and demand- oriented options normally deserve <br />investigation, implicating tools from the realms of law, economics, engineering, and many other <br />areas. <br />CLIMATE VARIABILITY & RECURRING PHENOMENA <br />Some of the most fruitful research in recent years has focused on recurring phenomena that can <br />create or modify extreme events and can alter seasonal precipitation totals. Of particular interest has <br />been the phenomenon of El Nino, or more generally, the El Nino /Southern Oscillation or ENSO. El <br />Nino entails modest (roughly 1°C) increases in sea surface temperatures (SSTs) for several thousand <br />miles along the equator in the eastern Pacifica A cooling of sea surface temperatures is known as La <br />Nina. While La Nina often follows El Nino, this is not always the case. El Nino and La Nina are <br />associated with different phases of the Southern Oscillation — patterns of surface air pressure changes <br />between north - central Australia and Tahiti. The term ENSO is usually used to refer to this entire <br />suite of related climatic phenomena.5 <br />The existence of an El Nino can have dramatic impacts in the "seasonal climate" in a variety of <br />locales throughout the world, although some regions are largely unaffected. In the American West, <br />El Nino generally brings increased precipitation to the Southwest and reduced precipitation to the <br />Northwest 6 A generally opposite effect tends to occur during La Nina. ENSO also influences the <br />probability and magnitude of extreme storms (e.g.., hurricanes) and can modify other seasonal <br />weather phenomena such as monsoons. <br />ENSO events are not a new type of phenomenon and are thus not likely a result of more recent <br />increases in atmospheric carbon dioxide concentrations and global temperatures. However, global <br />climate changes may be influencing the frequency, strength, and length of ENSO events. Exploring <br />these connections is an active area of research, but identifying statistically significant linkages is <br />difficult due to the relatively small numbers of ENSO events for which good data exists. <br />A somewhat more embryonic area of research involves a phenomenon known as the Pacific Decadal <br />Oscillation (PDO). The PDO is an ENSO -like phenomenon that operates on a much longer time <br />frame (20 -30 years for PDO as opposed to 6 to 18 months for ENSO events) and is centered on the <br />3 Irrigation demand, for example, is particularly climate sensitive <br />4 El Nino is Spanish for "the Christ Child." This name derives from the observation that El Nino sea surface <br />temperatures peak around Christmas. <br />5 For more information on ENSO, see: "Frequently (well, at least once)- asked - questions about El Nino." Billy <br />Kessler, Oceanographer, Pacific Marine Environmental Laboratory, NOAA. <br />http: / /www.pmel.noaa.jzov/ kessler /occasionally- asked- guestions.html #ql7; and "ENSO Information" from <br />the Climate Diagnostics Center, http: / /www.edc.noaa.gov/ENSO /. <br />6 The degree to which a region's weather is influenced is known as its ENSO "signal." The stronger the signal, <br />the greater the probability that the seasonal climate will be influenced. <br />0 <br />