In evaluating the climate-alarmist claim that anthropogenic-induced global warming will lead to intensified flooding around the globe, it is instructive to see how flood activity has responded to the global warming of the past century or so, much of which is claimed by climate alarmists to be due to anthropogenic greenhouse gas emissions. In this summary, we thus review several studies of the subject that have been conducted in North America.
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, it should be noted, 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."
Another study testing for long-term changes in flood magnitudes and frequencies in the Mississippi River system was conducted by Pinter et al. (2008), who "constructed a hydrologic database consisting of data from 26 rated stations (with both stage and discharge measurements) and 40 stage-only stations." Then, to help "quantify changes in flood levels at each station in response to construction of wing dikes, bendway weirs, meander cutoffs, navigational dams, bridges, and other modifications," the researchers put together a geospatial database consisting of "the locations, emplacement dates, and physical characteristics of over 15,000 structural features constructed along the study rivers over the past 100-150 years." And as a result of these operations, Pinter et al. say that "significant climate- and/or land use-driven increases in flow were detected," but they indicate that "the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction."
In discussing the implications of their findings, Pinter et al. write that "the navigable rivers of the Mississippi system have been intensively engineered, and some of these modifications are associated with large decreases in the rivers' capacity to convey flood flows." Given such findings, it would appear that man may indeed have been responsible for the majority of the enhanced flooding of the rivers of the Mississippi system over the past century or so, but not in the way suggested by the world's climate alarmists. The question that needs addressing by the region's inhabitants, therefore, has nothing to do with CO2, but everything to do with how to "balance the local benefits of river engineering against the potential for large-scale flood magnification."
Similar findings have been reported for the Upper Midwest (consisting of North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin and Illinois) by Villarini et al. (2011), who "analyzed the annual maximum instantaneous flood peak distributions for 196 U.S. Geological Survey streamflow stations with a record of at least 75 years over the Midwest U.S." According to the four U.S. researchers who conducted this study, in the vast majority of cases where streamflow changes were observed, they were "associated with change-points (both in mean and variance) rather than monotonic trends," and they indicated that "these non-stationarities are often associated with anthropogenic effects." But rather than associate the increases with anthropogenic CO2 emissions, they cite such things as "changes in land use/land cover, changes in agricultural practice, and construction of dams and reservoirs" as the primary cause(s). As a result, and, as they note, "in agreement with previous studies (Olsen et al., 1999; Villarini et al., 2009)," they conclude that "there is little indication that anthropogenic climate change has significantly affected the flood frequency distribution for the Midwest U.S." And as they make doubly clear in the abstract of their paper, they say that "trend analyses do not suggest an increase in the flood peak distribution due to anthropogenic climate change."
Moving across the continent, Villarini and Smith (2010) "examined the distribution of flood peaks for the eastern United States using annual maximum flood peak records from 572 U.S. Geological Survey stream gaging stations with at least 75 years of observations." This work revealed that, "in general, the largest flood magnitudes are concentrated in the mountainous central Appalachians and the smallest flood peaks are concentrated along the low-gradient Coastal Plain and in the northeastern United States." They also found that "landfalling tropical cyclones play an important role in the mixture of flood generating mechanisms, with the frequency of tropical cyclone floods exhibiting large spatial heterogeneity over the region." And they additionally write that "warm season thunderstorm systems during the peak of the warm season and winter-spring extratropical systems contribute in complex fashion to the spatial mixture of flood frequency over the eastern United States."
Of even greater interest to the climate change debate, however, were their more basic findings that (1) "only a small fraction of stations exhibited significant linear trends," that (2) "for those stations with trends, there was a split between increasing and decreasing trends," and that (3) "no spatial structure was found for stations exhibiting trends." Thus, they concluded, most importantly of all, that (4) "there is little indication that human-induced climate change has resulted in increasing flood magnitudes for the eastern United States," providing no support for the claim that global warming will lead to more frequent, more widespread and more serious floods.
Much the same was reported for Canada by Cunderlik and Ouarda (2009), who evaluated trends in the timing and magnitude of seasonal maximum flood events across that country, based on pertinent data obtained from 162 stations of the Reference Hydrometric Basin Network established by Environment Canada over the 30-year period 1974 to 2003. In spite of the supposedly unprecedented warming experienced over the period of time they studied, the Canadian researchers report finding that "only 10% of the analyzed stations show significant trends in the timing of snowmelt floods during the last three decades (1974-2003)," and they say these results imply that "the occurrence of snowmelt floods is shifting towards the earlier times of the year," as would be expected in a warming world. However, they note that most of the identified trends "are only weakly or medium significant results," and they add that "no significant trends were found in the timing of rainfall-dominated flood events."
With respect to flood magnitudes, the two scientists state that the trends they observed "are much more pronounced than the trends in the timing of the floods," but they say that most of these trends "had negative signs, suggesting that the magnitude of the annual maximum floods has been decreasing over the last three decades." In addition, they found that "the level of significance was also higher in these trends compared to the level of significance of the trends in the timing of annual maximum floods."
Expanding the temporal scope of the subject somewhat, there are a number of studies that have examined floods over much longer intervals of time. Wolfe et al. (2005), for example, conducted a multi-proxy hydro-ecological analysis of Spruce Island Lake (58°51'N, 111°29'W), a shallow, isolated, upland lake in a bedrock basin located in the northern Peace sector of the Peace-Athabasca Delta in northern Alberta, Canada, in an attempt to assess the impacts of both natural variability and anthropogenic change on the hydro-ecology of the region over the past 300 years. Specifically, their research was designed to answer the following three questions: (1) Have hydro-ecological conditions in Spruce Island Lake since 1968 (the year in which river flow became regulated from hydroelectric power generation at the headwaters of the Peace River) varied beyond the range of natural variation of the past 300 years? (2) Is there evidence that flow regulation of the Peace River has caused significant changes in hydro-ecological conditions in Spruce Island Lake? (3) How is hydro-ecological variability at Spruce Island Lake related to natural climatic variability and Peace River flood history?
Wolfe et al.'s research efforts revealed that hydro-ecological conditions varied substantially over the past 300 years, especially in terms of multi-decadal dry and wet periods. With respect to the three research questions posed above, for example, the authors found for question #1 that hydro-ecological conditions after 1968 have remained well within the broad range of natural variability observed over the past 300 years, with both "markedly wetter and drier conditions compared to recent decades" having occurred prior to the time of Peace River flow regulation. With respect to question #2, they note that the current drying trend is not the product of Peace River flow regulation, but rather the product of an extended drying period that was initiated in the early to mid-1900s. Lastly, with respect to question #3, Wolfe et al. showed that the multi-proxy hydro-ecological variables they analyzed were well correlated with other reconstructed records of natural climate variability, indicating a likely climatic influence on Spruce Island Lake hydro-ecological conditions over the period of record.
It is important to note that there is nothing unusual about recent trends in the hydro-ecology of the Spruce Island Lake region. Wet and dry conditions of today fall well within the range of natural variability and show no fingerprint of anthropogenic global warming. What is more, they even bear no fingerprint of anthropogenic flow control on the Peace River since 1968, demonstrating, in the words of the authors, that "profound changes in hydro-ecological conditions are clearly a natural feature of this ecosystem, independent of human influence or intervention."
Moving southward, Shapley et al. (2005) developed a 1000-year hydroclimate reconstruction from local bur oak (Quercus macrocarpa) tree-ring records and lake sediment cores from the Waubay Lake complex located in eastern South Dakota. During the 1990s, broad areas of the US Northern Great Plains experienced notable lake highstands, including Waubay Lake, which rose by 5.7 meters and more than doubled in area from 1993 to 1999, severely flooding roads, farms and towns, and prompting the Federal Emergency Management Agency to declare the region a disaster area on 1 June 1998. In their paper, therefore, Shapley et al. set out to determine the historical context of that 1990s lake-level rise.
In doing so, the researchers found that "prior to AD 1800, both lake highstands and droughts tended towards greater persistence than during the past two centuries," such that "neither generally low lake levels occurring since European settlement (but before the recent flooding) nor the post-1930s pattern of steadily increasing water availability and favorableness for tree growth are typical of the long-term record." In this particular part of the world, therefore, it is clear that longer-lasting floods and droughts of equal or greater magnitude than those of modern times occurred repeatedly prior to 1800.
In another study, Fye et al. (2003) 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.
In a study designed to determine the environmental origins of extreme flooding events throughout the southwestern United States, Ely (1997) wrote that "paleoflood records from nineteen rivers in Arizona and southern Utah, including over 150 radiocarbon dates and evidence of over 250 flood deposits, were combined to identify regional variations in the frequency of extreme floods," which information "was then compared with paleoclimatic data to determine how the temporal and spatial patterns in the occurrence of floods reflect the prevailing climate." The results of this comparison indicated that "long-term variations in the frequency of extreme floods over the Holocene are related to changes in the climate and prevailing large-scale atmospheric circulation patterns that affect the conditions conducive to extreme flood-generating storms in each region," which changes, in Ely's view, "are very plausibly related to global-scale changes in the climate system."
With respect to the Colorado River watershed, for example, which integrates a large portion of the interior western United States, she writes that "the largest floods tend to be from spring snowmelt after winters of heavy snow accumulation in the mountains of Utah, western Colorado, and northern New Mexico," such as occurred with the "cluster of floods from 5 to 3.6 ka," which occurred in conjunction with "glacial advances in mountain ranges throughout the western United States" during the "cool, wet period immediately following the warm mid-Holocene."
The frequency of extreme floods also increased during the early and middle portions of the first millennium AD, many of which coincided "with glacial advances and cool, moist conditions both in the western U.S. and globally." Then came a "sharp drop in the frequency of large floods in the southwest from AD 1100-1300," which corresponded, in her words, "to the widespread Medieval Warm Period, which was first noted in European historical records." With the advent of the Little Ice Age, however, there was another "substantial jump in the number of floods in the southwestern U.S.," which was "associated with a switch to glacial advances, high lake levels, and cooler, wetter conditions." And in distilling her findings down to a single succinct statement, and speaking specifically of the southwestern United States, Ely states that "global warm periods, such as the Medieval Warm Period, are times of dramatic decreases in the number of high-magnitude floods in this region."
In another 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.
In doubling the length of time investigated in the prior study, 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.
Focusing on the northern Uinta Mountains of northeastern Utah, Carson et al. (2007) developed a Holocene history of flood magnitudes from reconstructed cross-sectional areas of abandoned channels and relationships relating channel cross-sections to flood magnitudes derived from modern stream gage and channel records. Of most interest to the present subject, Carson et al. report that over the past 5,000 years, the record of bankfull discharge "corresponds well with independent paleoclimate data for the Uinta Mountains," and that "during this period, the magnitude of the modal flood is smaller than modern during warm dry intervals and greater than modern during cool wet intervals," noting most particularly that "the decrease in flood magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly."
Based upon the three researchers' graphical results, the three largest negative departures from modern bankfull flood magnitudes (indicating greater than modern warmth) range from approximately 15-22%, as best as can be determined from visual inspection of their plotted data; and they occur between about 750 and 600 cal yr B.P., as determined from radiocarbon dating of basal channel-fill sediments.
In addition to demonstrating that the degree of natural variability in northeastern Utah flood magnitudes throughout the Holocene has been much larger (in both positive and negative directions) than what has been observed in modern times (which demonstrates that possible future occurrences of greater- or smaller-than-modern floods in the region ought not be regarded as "unprecedented," as climate alarmists are typically prone to claim), Carson et al.'s findings demonstrate that the portion of the Medieval Warm Period between about AD 1250 and 1400 was likely significantly warmer than it is at present (which demonstrates that since something other than high concentrations of atmospheric CO2 was responsible for the region's earlier greater-than-present warmth, one need not invoke today's much higher CO2 concentrations as the reason for our actually lower current temperatures).
Further extending the temporal scope of 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 above 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. And although there could also be exceptions to this rule in the future, it is more likely than not that any further warming of the globe would tend to further reduce both the frequency and severity of flooding in North America, which is just the opposite of what the world's climate alarmists claim would occur under such conditions.
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