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Figure 1 Century-scale changes in a broad array of water cycle variables contribute to the scientific evidence for a detectable greenhouse warming signal. Taken together, these variables indicate acceleration of the hydrologic cycle and the non-uniform spatial distribution of these changes. SOURCE: From the U.S. Global Change Research Program’s national assessment of climate impacts. Reprinted, with the permission of Cambridge University Press, from Karl et al. (2009). © 2009 by University Corporation of Atmospheric Research.

1998). But for the associated hydrologic variables, results are mixed using standard hydrologic measures. Analysis of flood occurrence (i.e., the annual maxima series) shows essentially no trends at a set of U.S. Geological Survey (USGS) stream gages that were carefully selected to minimize any influences of water management (USGS, 2005). This phenomenon was also noted at a 2008 workshop hosted by the COHS, both by speakers citing the same USGS report as well as by participants citing other research (NRC, 2008). Yet, evidence of changes in U.S. drought characteristics is mixed. Across much of the eastern and central United States, trends in increasing precipitation appear to have resulted in reductions in drought severity and length. In contrast, in parts of the West a general warming appears to have increased evaporative demand more rapidly than precipitation, with the result that these areas tend toward more, longer, and more severe droughts (Groisman et al., 2004). These results point again to difficulties in interpreting climate- and weather-oriented extremes in a hydrologic context.

The breakout discussions noted that major uncertainties have presented themselves but have yet to be reconciled. Why have continental U.S. streamflow changes over the past 50-60



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