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416 ROGER A. PIELKE, JR. creates obstacles to proper understanding of the flood problem and, consequently, the development of effective responses. The I DO-year standard refers to a flood that has a one percent chance of being exceeded in any given year. It does not refer to a flood that occurs 'once every 100 years'. In fact, for a home in a I OO~year flood zone there is a greater than 26% chance that it will see at least one lOG-year flood over a period of 30 years (and, similarly, more than a 74% chance over 100 years). The general formula for the cumulative probability of at least one flood of annual probability Pis (1- p)N ~ C where N equals the number of years from now, and C is the cumulative probability over period N (P is assumed to be constant and events are independent from year to year). By choosing values for P and C one can compute the number of years that the cumulative probability (C) covers. The concept and tenninology of the' IOO-year floodplain' was formally adopted by the federal government as a standard for all public agencies in 1977 under Exec- utive Order I 1988. In 1982 FEMA reviewed the policy and found that it was being used in the agencies and, lacking a better alternative, concluded that the policy should be retained (FIFMTF, 1992, p. 8-3). However, despite the FEMA review, use of the concept of the I DO-year flood is encumbered by a number of logical and practical difficulties (d. Lord, 1994). First, there is general t.:oni"usion among users or the term about whal it rneuns. Some use the term to refer to a flood that occurs every 100 years, as did the Mid- western mayor who stated that 'after the 1965 flood, they told us this wouldn't happen again for another 100 years' (lFMRC, 1994. p, 59), Public confusion is widespread: A fanner suffering through Midwest flooding for the second time in three years complained that 'Two years ago was supposed to be a lOO~year flood, and they're saying this is a 75-year flood, What kind of sense does that make? You'd think they'd get it right' (Peterson, 1995), Second, the' lOO-year flood' is only one of many possible probabilistic mea- sures of an area's flood risk, For instance, in the part of the floodplain that is demarcated as the' I OO-year floodplain' it is only the outer edge of that area that is estimated to have an annual probability of flooding of 0.01, yet confusion ex- ists (Myers, 1994). Areas closer to the river have higher probabilities of flooding, e.g., there are areas of a floodplain with a 2% annual chance of flooding (50-year fioodplain). 10% annual chance (la-year fioodplain). 50% annual chance (2-year floodplain) etc., and similarly, areas farther from the river have lower probabilities of flooding. The '1 DO-year floodplain' is arbitrarily chosen for regulatory reasons and does not reflect anything fundamentally intrinsic to the floodplain. Third, the' lOO-year floodplain' is determined based on past flood records and is thus subject to considerable errors with respect to the probabilities of future floods. According to Burkham (1978) errors in determination of the' lOG-year flood' may be off by as much as 50% of flood depth. Depending on the slope of the flood plain, this could translate into a significant error in terms of distance from the river channel. A FEMA press release notes that 'in some cases there is a differenc~ of NINE FALLACIES OF FLOODS 417 only inches between the 10- and the lOa-year fiood levels' (FEMA. 1996), Further, researchers are beginning to realize an 'upper limit' on what can be known about flood frequencies due to the lack of available trend data (Bobee and Rasmussen, 1995), Fourth, the IOO-year floodplain is not a natural feature. but rather is defined by scientists and engineers based on the historical record. Consequently, while the 'I OO-year floodplain' is dynamic and subject to redefinition based on new flood events that add to the historical record, the regulatory definition is much more difficult to change. For instance, following two years of major flooding on the Salt River in Phoenix, Arizona, the previously estimated lOO-year flood was reduced to a 50-year fiood (FIFMTF. 1992, p, 9-7), What happens to the structures in redefined areas? Any changes in climate patterns, especially precipitation, will also modify the expected probabilities of inundation, For example, some areas of the upper Midwest have documented a trend of increasing precipitation this cen- tury (Changnon and Kunkel, 1995; Bhowmik et aI., 1994). Furthermore, human changes to the river environment, e.g., levees and land use changes, can also alter the hydraulics of floods. Finally, the extensive use of the term 'IOO-year flood' focuses attention on that aspect of flooding. sometimes to the neglect of the area beyond the lOG-year Oeod plain (Myers, 1994). What can be done? Given the pervasive use of the concepf or the 'IOO-year flood' in flood insurance and regulatory decision-making it seems that adoption of an alternative concept is unlikely. Nevertheless. there are a number of steps that can be taken by those who use the concept when dealing with poliCy makers and the public, First. we need to be more precise with language, The FlFMTF (1992) recommends the phrase 'one percent annual chance flood' as a preferred altema~ tive to 'IOO-year flood', 'base flood', or 'one percent flood'. Another alternative is 'national base flood standard' which removes reference to probability (Thomas, 1996, personal communication). Second, when communicating with the public and the media, flood experts could take care to convert annual exceedances into annual probabilities. And third, policy documents could rely less on the' IOO.year flood' to illustrate examples and propose policies, and at the very least explicitly discuss floods of different magnitudes. 2.2. DAMAGING FLOODING IN RECENT YEARS IS UNPRECEDENTED BECAUSE OF 'GLOBAL WARMING' The phrase 'global warming' refers to the possibility that the earth's climate may change because human activities are altering the composition of the atmosphere. Scientists first raised this possibility more than a century ago, and in recent decades policy makers have begun to express concern about the possibility of climate change. Possible changes that have been discussed in the context of global warm- ing include increasing or decreasing tropical cyclone activity, increased spread of infectious diseases, change in mean global temperature and regional and local tem-