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Extreme Weather Events: Are they Influenced by Rising Atmospheric CO2?

3.1.3. Other Factors Driving Observed Flood Trends


Although the prior two subsections have convincingly demonstrated that the hypothesis that rising CO2 is causing an increase in floods is false, the present section provides additional evidence negating such claims. It also demonstrates the presence and importance of other natural and anthropogenic factors that influence flood records. These influences must be studied and factored out before any portion of a flooding trend could be attributed to CO2-induced global warming.

The first of these examples comes from a study testing for long-term changes in flood magnitudes and frequencies conducted in the Mississippi River system 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." 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 studied 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 IPCC. 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."

Writing as background for their work, Barredo et al. (2012) say that "economic impacts from flood disasters have been increasing over recent decades," but they add that "despite the fact that the underlying causes of such increase are often attributed to a changing climate, scientific evidence points to increasing exposure and vulnerability as the main factors responsible for the increase in losses," citing the studies of Pielke and Landsea (1998), Crompton and McAneney (2008), Pielke et al. (2008), Barredo (2009, 2010), and Neumayer and Barthel (2011). Ever curious, however-and possibly looking for exceptions-Barredo et al. set out to examine "the time history of insured losses from floods in Spain between 1971 and 2008," striving to see "whether any discernible residual signal remains after adjusting the data for the increase in the number and value of insured assets over this period of time."

The "most salient feature" of Barredo et al.'s findings, as they describe it, was "the absence of a significant positive trend in the adjusted insured flood losses in Spain," which suggests, in their words, that "the increasing trend in the original losses is explained by socio-economic factors, such as the increases in exposed insured properties, value of exposed assets and insurance penetration." And they add that "there is no residual signal that remains after adjusting for these factors," so that "the analysis rules out a discernible influence of anthropogenic climate change on insured losses," which they say "is consistent with the lack of a positive trend in hydrologic floods in Spain in the last 40 years."

In the introduction to their study of the hydrology of German rivers, Bormann et al. (2011) write that "following several severe floods in Germany during the past two decades, [the] mass media as well as scientists have debated the relative contributions of climate and/or anthropogenic processes to those floods." Driven by a desire to help resolve this climate-change impact debate, the three researchers utilized long time-series of stage and discharge data obtained from 78 river gauges in Germany, searching for trends in flood frequency, peak discharge, peak stage and stage-discharge relationships, where all variables investigated had to have a temporal history on the order of at least half a century.

In doing so, the three researchers first established the nature of Germany's temperature history, noting that Schonwiese (1999) identified a homogenous positive trend of 0.5-1.0C over the course of the 20th century, which was subsequently confirmed by Gerstengarbe and Werner (2008) and Bormann (2010). Then, in terms of land use change between 1951 and 1989, they report that "agricultural area in Germany decreased from 57.8% to 53.7%, while forested areas remained almost constant." During this same time period, they report "impervious areas increased sharply from 7.4% to 12.3%," and they say "this trend has continued since 1989," with impervious areas further increasing from 11.2% to 13.1%, forest areas increasing from 29.3% to 30.1%, and agricultural area decreasing from 54.7% to 52.5%. And as a consequence of the net increase in impervious surfaces, they say "runoff generation can be expected to increase and infiltration and groundwater recharge decrease," which would be expected to lead to increases in river flow and a potential for more frequent and extreme floods. However, they report "most stations analyzed on the German rivers did not show statistically significant trends in any of the metrics analyzed."

In light of these several observations-plus the fact that "most decadal-scale climate-change impacts on flooding (Petrow and Merz, 2009) are small compared to historic peaks in flood occurrence (Mudelsee et al., 2006)"-Bormann et al. concluded their report by stating that these significant facts "should be emphasized in the recent discussion on the effect of climate change on flooding." And if this is done, there is no other conclusion to be drawn but that the warming experienced in Germany over the past century has not led to unprecedented flooding throughout the country. In fact, it has not led to any increase in flooding.

The findings presented above clearly illustrate the fact that many other factors, natural and anthropogenic and unrelated to CO2-induced global warming, can influence records of flooding. A proper accounting of their influence must be conducted before assessing a potential role from rising CO2.

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