flood events can be represented by probability distributions based on relative frequency of events in historical records. By contrast, there are very few historical records of environmental restoration projects in particular ecological settings. Decision analysis is one approach that offers at least a conceptual framework for such an analysis. Decision trees can be used to map the various pathways to which sequences of decisions and their outcomes could lead. A combination of objective and subjective probabilities of following each path can be assessed. Physical, chemical, biological, and economic consequences are then predictable in quantitative and probabilistic terms, and strategies may be devised to cope with those uncertainties. Applications of this type of decision analysis would require research, development, testing, and evaluation. A critical issue is the process by which subjective probabilities would be assigned.
The climate change issue strongly relates to risk-based analysis. Extreme climate events and changes in variability can skew the hydrologic parameters upon which Corps projects are based. These changes, in turn, can change the reliability of Corps projects. For example, the degree of protection afforded by a Corps flood damage reduction project can change if flood events occur more or less frequently in the future.
While the specter of climate change hangs over many of the Corps' planning and management activities, it is not known how climate might change in the future. The available evidence suggests that 20th century global mean temperature, which has increased between 0.3°C and 0.6°C since the late 19th century (Houghton et al., 1996) is at least as warm as any century since 1400 A.D. The climate record also shows that three years in the 1990s—1990, 1995, and 1997—were warmer than any other year since (at least) 1400 A.D. (Mann et al., 1998).
It is difficult to prepare for possible future changes in climate, the direction and magnitude of which are not known. However, the possible consequences of dramatic shifts in climate, especially extreme weather events, suggest that the issue be taken seriously. The Corps has been studying the climate change issue extensively for years and continues to keep current with changes and advances in global warming research (e.g., Stakhiv, 1998). The Corps should remain abreast of research on climate change and variability issues, such as El Niño-Southern Oscillation, and their implications for hydrology and water management. Federal agencies such as the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey are among the organizations the Corps can call upon to help stay well informed.