Floods in North America
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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
In the midst of 2002’s massive flooding in Europe, Gallus Cadonau, the managing director of the Swiss Greina Foundation, called for a punitive tariff on U.S. imports to force cooperation on reducing greenhouse gas emissions, claiming that the flooding “definitely has to do with global warming” and stating that “we must change something now” (Hooper, 2002). Cadonau was joined in this sentiment by Germany’s environment minister, Jurgen Trittin, who implied much the same thing when he said “if we don’t want this development to get worse, then we must continue with the consistent reduction of environmentally harmful greenhouse gasses” (Ibid.).
The IPCC seems to agree with Cadonau and Trittin. Its authors report “a catastrophic flood occurred along several central European rivers in August 2002. The floods resulting from extraordinarily high precipitation were enhanced by the fact that the soils were completely saturated and the river water levels were already high because of previous rain. Hence, it was part of a pattern of weather over an extended period” (IPCC, 2007-I, p. 311). While admitting “there is no significant trend in flood occurrences of the Elbe within the last 500 years,” the IPCC nevertheless says the “observed increase in precipitation variability at a majority of German precipitation stations during the last century is indicative of an enhancement of the probability of both floods and droughts” (Ibid.)
Contents |
Research
Lins and Slack (1999) analyzed secular streamflow trends in 395 different parts of the United States that were derived from more than 1,500 individual streamgauges, some of which had continuous data stretching 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 twentieth 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 fewer floods and droughts.
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, 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.
Knox (2001) identified an analogous phenomenon in the more mesic Upper Mississippi River Valley, but with a slight twist. Since the 1940s and early 1950s, 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 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 twentieth 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 there was a slightly larger Red River flood, and in 1826 there was a flood that was nearly 40 percent greater than the flood of 1997. The temperature of the globe was 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 higher temperatures that year or the warming of the preceding decades. We also note that Red River flooding is also linked to snow melt and ice jams because itflows northward into frozen areas.
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. 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. 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 survey to much longer intervals of time is 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 twentieth century wetness anomaly across the United States was a 13-year period in the early part of the century when it was colder than it is now. Fye et al.’s analysis also 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) 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 1,000-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 the IPCC calls the unprecedented warming of the past century.
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 4,000 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 4,000-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 4,000-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 5,300 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, 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 13 small lakes distributed across a 20,000-km2 region in Vermont and eastern New York that depict the frequency of storm-related floods. They found 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].”
A report released by the US Geological Survey in October 2011 (Hirsch and Ryberg 2011), found that "in none of the four regions defined in this study is there strong statistical evidence for flood magnitudes increasing with increasing [global mean carbon dioxide concentration]." The study actually showed a statistically significant negative relationship with CO2 in the Southwest, where flooding has become less severe with increased greenhouse gas concentrations.
Taken together, the research described in this section suggests that 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. 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.
References
Brown, P., Kennett, J.P. and Ingram, B.L. 1999. Marine evidence for episodic Holocene megafloods in North America and the northern Gulf of Mexico. Paleoceanography 14: 498-510.
Campbell, C. 2002. Late Holocene lake sedimentology and climate change in southern Alberta, Canada. Quaternary Research 49: 96-101.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
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.
Haque, C.E. 2000. Risk assessment, emergency preparedness and response to hazards: The case of the 1997 Red River Valley flood, Canada. Natural Hazards 21: 225-245.
Hirsch, R.M. and K.R. Ryberg. Has the magnitude of floods across the USA changed with global CO2 levels? Hydrolological Sciences Journal, doi: 10.1080/02626667.2011.621895.
Kennett, D.J. and Kennett, J.P. 2000. Competitive and cooperative responses to climatic instability in coastal southern California. American Antiquity 65: 379-395.
Knox, J.C. 1999. Sensitivity of modern and Holocene floods to climate change. Quaternary Science Reviews 19: 439-457.
Knox, J.C. 2001. Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley. Catena 42: 193-224.
Kreutz, K.J., Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S.I. and Pittalwala, I.I. 1997. Bipolar changes in atmospheric circulation during the Little Ice Age. Science 277: 1294-1296.
Lamb, H.H. 1979. Variation and changes in the wind and ocean circulation: the Little Ice Age in the northeast Atlantic. Quaternary Research 11: 1-20.
Lins, H.F. and Slack, J.R. 1999. Streamflow trends in the United States. Geophysical Research Letters 26: 227-230.
Liu, K.B. and Fearn, M.L. 2000. Reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern Florida from lake sediment records. Quaternary Research 54: 238-245.
Meeker, L.D. and Mayewski, P.A. 2002. A 1400-year high-resolution record of atmospheric circulation over the North Atlantic and Asia. The Holocene 12: 257-266.
Molnar, P. and Ramirez, J.A. 2001. Recent trends in precipitation and streamflow in the Rio Puerco Basin. Journal of Climate 14: 2317-2328.
Ni, F., Cavazos, T., Hughes, M.K., Comrie, A.C. and Funkhouser, G. 2002. Cool-season precipitation in the southwestern USA since AD 1000: Comparison of linear and nonlinear techniques for reconstruction. International Journal of Climatology 22: 1645-1662.
Noren, A.J., Bierman, P.R., Steig, E.J., Lini, A. and Southon, J. 2002. Millennial-scale storminess variability in the northeastern Unites States during the Holocene epoch. Nature 419: 821-824.
Olsen, J.R., Stedinger, J.R., Matalas, N.C. and Stakhiv, E.Z. 1999. Climate variability and flood frequency estimation for the Upper Mississippi and Lower Missouri Rivers. Journal of the American Water Resources Association 35: 1509-1523.
Schimmelmann, A., Lange, C.B. and Meggers, B.J. 2003. Palaeoclimatic and archaeological evidence for a 200-yr recurrence of floods and droughts linking California, Mesoamerica and South America over the past 2000 years. The Holocene 13: 763-778.
St. George, S. and Nielsen, E. 2002. Hydroclimatic change in southern Manitoba since A.D. 1409 inferred from tree rings. Quaternary Research 58: 103-111.
Zong, Y. and Tooley, M.J. 1999. Evidence of mid-Holocene storm-surge deposits from Morecambe Bay, northwest England: A biostratigraphical approach. Quaternary International 55: 43-50.
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