Lins and Slack (1999) analyzed secular streamflow trends in 395 different parts of the United States that were derived from more than 1500 individual streamgauges, some of which had continuous data stretching all the way back to 1914.  In the mean, they found that "the conterminous U.S. is getting wetter, but less extreme."  That is to say, as the near-surface air temperature of the planet gradually rose throughout the course of the 20th century, the United States became wetter in the mean but less variable at the extremes, which is where floods and droughts occur, leading to what could well be called the best of both worlds, i.e., more water with less floods.

In a similar but more regionally-focused study, Molnar and Ramirez (2001) conducted a detailed analysis of precipitation and streamflow trends for the period 1948-1997 in the semiarid Rio Puerco Basin of New Mexico.  At the annual timescale, they reported finding "a statistically significant increasing trend in precipitation," which was driven primarily by an increase in the number of rainy days in the moderate rainfall intensity range, with essentially no change at the high-intensity end of the spectrum.  In the case of streamflow, however, there was no trend at the annual timescale; but monthly totals increased in low-flow months and decreased in high-flow months, once again reducing the likelihood of both floods and droughts, courtesy of 20th-century warming.

Think of the implications of these findings.  Increased precipitation in a semiarid region is a real plus.  Having most of the increase in the moderate rainfall intensity range also sounds like a plus.  Increasing streamflow in normally low-flow months sounds good too, as does decreasing streamflow in high-flow months.  In fact, all of the observed changes in precipitation and streamflow in this study would appear to be highly desirable, leading to more water availability but a lowered probability of both floods and droughts, which we could now well call the best of all worlds.

Knox (2001) identified an analogous phenomenon in the more mesic Upper Mississippi River Valley, but with a slight twist.  Since the 1940s and early 50s, the magnitudes of the largest daily flows in this much wetter region have been decreasing at the same time that the magnitude of the average daily baseflow has been increasing, once again manifesting simultaneous trends towards both lessened flood and drought conditions.

Much the same story is told by the research of Garbrecht and Rossel (2002), who studied the nature of precipitation throughout the U.S. Great Plains over the period 1895-1999.  For the central and southern Great Plains, the last two decades of this period were found to be the longest and wettest of the entire 105 years of record, due primarily to a reduction in the number of dry years and an increase in the number of wet years.  Once again, however, the number of very wet years - which would be expected to produce flooding - "did not increase as much and even showed a decrease for many regions."

The northern and northwestern Great Plains also experienced a precipitation increase near the end of Garbrecht and Rossel's 105-year record; but it was primarily confined to the final decade of the 20th century.  And again, as they report, "fewer dry years over the last 10 years, as opposed to an increase in very wet years, were the leading cause of the observed wet conditions."

In spite of the general tendencies described in these several papers, there still were some significant floods during the last decade of the past century, such as the 1997 flooding of the Red River of the North, which devastated Grand Forks, North Dakota, as well as parts of Canada.  However, as Haque (2000) reports, although this particular flood was indeed the largest experienced by the Red River over the past century, it was not the largest to occur in historic times.  In 1852, for example, there was a slightly larger Red River flood; and in 1826 there was a much larger flood that was nearly 40% greater than the flood of 1997.  And the temperature of the globe, we hasten to add, was much colder at the times of these earlier catastrophic floods than it was in 1997, indicating that one cannot attribute the strength of the 1997 flood to the degree of warmth experienced that year or throughout the preceding decade.

Analogously, Olsen et al. (1999) report that some upward trends in flood-flows have been found in certain places along the Mississippi and Missouri Rivers, which is not at all surprising, as there will always be exceptions to the general rule.  At the same time, however, they note that many of the observed upward trends were highly dependent upon the length of the data record and when the trends began and ended.  Hence, they say of these trends that they "were not necessarily there in the past and they may not be there tomorrow."

Expanding the scope of our vision somewhat, we move next to a consideration of studies that have concerned themselves with much longer intervals of time, beginning with that of Fye et al. (2003), who developed multi-century reconstructions of summer (June-August) Palmer Drought Severity Index over the continental United States from annual proxies of moisture status provided by 426 climatically-sensitive tree-ring chronologies.  This exercise indicated that the greatest 20th-century wetness anomaly across the United States was the 13-year pluvial that occurred in the early part of the century, when it was considerably colder than it is now.  In addition, Fye et al.'s analysis revealed the existence of a 16-year pluvial from 1825 to 1840 and a prolonged 21-year wet period from 1602 to 1622, both of which anomalies occurred during the Little Ice Age, when, of course, it was colder still.

St. George and Nielsen (2002) likewise used "a ringwidth chronology developed from living, historical and subfossil bur oak (Quercus macrocarpa (Michx.)) in the Red River basin to reconstruct annual precipitation in southern Manitoba since A.D. 1409."  Their analysis indicated, in their words, that "prior to the 20th century, southern Manitoba's climate was more extreme and variable, with prolonged intervals that were wetter and drier than any time following permanent Euro-Canadian settlement."

Also working with tree-ring chronologies, Ni et al. (2002) developed a 1000-year history of cool-season (November-April) precipitation for each climate division in Arizona and New Mexico, USA.  In doing so, they found that several wet periods comparable to the wet conditions seen in the early 1900s and post-1976 occurred in 1108-20, 1195-1204, 1330-45 (which they denominate "the most persistent and extreme wet interval"), the 1610s, and the early 1800s, all of which wet periods are embedded in the long cold expanse of the Little Ice Age, which is clearly revealed in the work of Esper et al. (2002).

Doubling the temporal extent of Ni et al.'s investigation, Schimmelmann et al. (2003) analyzed gray clay-rich flood deposits in the predominantly olive varved sediments of the Santa Barbara Basin off the coast of California, USA, which they accurately dated by varve-counting.  Their analysis indicated that six prominent flood events occurred at approximately AD 212, 440, 603, 1029, 1418 and 1605, "suggesting," in their words, "a quasi-periodicity of ~200 years," with "skipped" flooding just after AD 800, 1200 and 1800.  They further note that "the floods of ~AD 1029 and 1605 seem to have been associated with brief cold spells," that "the flood of ~AD 440 dates to the onset of the most unstable marine climatic interval of the Holocene (Kennett and Kennett, 2000)," and that "the flood of ~AD 1418 occurred at a time when the global atmospheric circulation pattern underwent fundamental reorganization at the beginning of the 'Little Ice Age' (Kreutz et al., 1997; Meeker and Mayewski, 2002)."  As a result, they hypothesize that "solar-modulated climatic background conditions are opening a ~40-year window of opportunity for flooding every ~200 years," and that "during each window, the danger of flooding is exacerbated by additional climatic and environmental cofactors."  They also note that "extrapolation of the ~200-year spacing of floods into the future raises the uncomfortable possibility for historically unprecedented flooding in southern California during the first half of this century."  Consequently, if such flooding does occur in the near future, there will be no need to suppose it came as a consequence of what climate alarmists call the unprecedented warming of the past century, although they will surely claim it did.

Once again doubling the length of time investigated, Campbell (2002) analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada, to provide a non-vegetation-based high-resolution record of streamflow variability for this part of North America over the past 4000 years.  This work revealed that the highest rates of stream discharge during this period occurred during the Little Ice Age, approximately 300-350 years ago, at which time grain sizes were about 2.5 standard deviations above the 4000-year mean.  In contrast, the lowest rates of streamflow were observed around AD 1100, during the Medieval Warm Period, when median grain sizes were nearly 2.0 standard deviations below the 4000-year mean.

Further extending the temporal scope of our review, Brown et al. (1999) analyzed various properties of cored sequences of hemipelagic muds deposited in the northern Gulf of Mexico for evidence of variations in Mississippi River outflow over the past 5300 years.  This group of researchers found evidence of seven large megafloods, which they describe as "almost certainly larger than historical floods in the Mississippi watershed."  In fact, they say these fluvial events were likely "episodes of multidecadal duration," five of which occurred during cold periods similar to the Little Ice Age.

Last of all, in a study that covered essentially the entire Holocene, Noren et al. (2002) employed several techniques to identify and date terrigenous in-wash layers found in sediment cores extracted from thirteen small lakes distributed across a 20,000-km² region in Vermont and eastern New York that depict the frequency of storm-related floods.  Their results indicated, in their words, that "the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr."  Specifically, they found there were four major peaks in the data during this period, with the most recent upswing in storm-related floods beginning "at about 600 yr BP [Before Present], coincident with the beginning of the Little Ice Age."  In addition, they note that several "independent records of storminess and flooding from around the North Atlantic show maxima that correspond to those that characterize our lake records [Brown et al., 1999; Knox, 1999; Lamb, 1979; Liu and Fearn, 2000; Zong and Tooley, 1999]."

Taken together, the research described in this Summary suggests that, if anything, North American flooding tends to become both less frequent and less severe when the planet warms, although there have been some exceptions to this general rule.  Hence, although there could also be exceptions to this rule in the case of future warming, on average, we would expect that any further warming of the globe would tend to further reduce both the frequency and severity of flooding in North America, which, of course, is just the opposite of what the world's climate alarmists continue to claim would occur.

References
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Campbell, C.  2002.  Late Holocene lake sedimentology and climate change in southern Alberta, Canada.  Quaternary Research 49: 96-101.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Fye, F.K., Stahle, D.W. and Cook, E.R.  2003.  Paleoclimatic analogs to twentieth-century moisture regimes across the United States.  Bulletin of the American Meteorological Society 84: 901-909.

Garbrecht, J.D. and Rossel, F.E.  2002.  Decade-scale precipitation increase in Great Plains at end of 20th century.  Journal of Hydrologic Engineering 7: 64-75.

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