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

3.1.1 Flood Trends of the Past Century


The first step in evaluating claims that rising CO2 is causing more frequent and severe flooding begins with a rather simple analysis of flood events over the past few decades during which time the bulk of anthropogenic CO2 accumulated in the atmosphere. If the observational data show no trend in flood events, or if they are shown to decline over this period, the hypothesis that rising CO2 is increasing the frequency and/or magnitude of these events can be falsified, as such findings contradict the hypothesis. This section thus examines the results of several scientific studies that have performed this initial phase of flood uniqueness evaluation.

Starting 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 1,500 individual stream gauges, some of which had continuous data stretching all the way back to 1914. In the mean, they found "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, which findings are just the opposite of routine climate-alarmist claims.

In a 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 during the warming of the 20th-century.

With respect to the implications of these findings, increased precipitation in a semiarid region is a major benefit. Having most of the increase in the moderate rainfall intensity range is also 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 suggests the best of all worlds.

Knox (2001) identified an analogous phenomenon in the more mesic Upper Mississippi River Valley. 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, which again is just the opposite of climate-alarmist claims.

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. 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," as they put it. 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."

Writing as background for their work, Hirsch and Ryberg (2012) state that "one of the anticipated hydrological impacts of increases in greenhouse gas concentrations in the atmosphere is an increase in the magnitude of floods," citing Trenberth (1999), the IPCC (2007) and Gutowski et al. (2008); and they therefore set out to see if such might have occurred across the United States over the past century or so.

Working with the global mean carbon dioxide concentration (GMCO2) and a streamflow data set that consisted of long-term (85- to 127-year) annual flood series from 200 stream gauges that had been deployed by the U.S. Geological Survey in basins with little or no reservoir storage or urban development (less than 150 persons per square kilometer in AD 2000) throughout the coterminous United States-which they divided into four large regions-Hirsch and Ryberg employed a stationary bootstrapping technique to determine if the patterns of the statistical associations between the two parameters were significantly different from what would be expected under the null hypothesis that flood magnitudes are independent of GMCO2.

In describing their findings the two researchers report that "in none of the four regions defined in this study is there strong statistical evidence for flood magnitudes increasing with increasing GMCO2." In fact, they say that one region, the southwest, showed a statistically significant negative relationship between GMCO2 and flood magnitudes. As such, Hirsch and Ryberg conclude "it may be that the greenhouse forcing is not yet sufficiently large to produce changes in flood behavior that rise above the 'noise' in the flood-producing processes." On the other hand, a simpler conclusion is that the "anticipated hydrological impacts" envisioned by the IPCC and others are simply incorrect.

In another study, 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 (1) "only a small fraction of stations exhibited significant linear trends," (2) "for those stations with trends, there was a split between increasing and decreasing trends," and (3) "no spatial structure was found for stations exhibiting trends." Thus, they concluded, most importantly of all, that "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 has been reported for Canada. Cunderlik and Ouarda (2009) evaluated trends in the timing and magnitude of seasonal maximum flood events across that country, based on 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 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 "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 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 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."

Working in France, Renard et al. (2008) employed four different procedures for assessing field significance and regional consistency with respect to trend detection in both high-flow and low-flow hydrological regimes of French rivers, using daily discharge data obtained from 195 gauging stations having a minimum record length of 40 years. In doing so, they determined that "at the scale of the entire country, the search for a generalized change in extreme hydrological events through field significance assessment remained largely inconclusive." In addition, they discovered that at the smaller scale of hydro-climatic regions, there were also no significant results for most such areas.

According to Korhonen and Kuusisto (2010), "annual mean temperatures in Finland increased by about 0.7°C during the 20th century," citing Jylha et al. (2004) and while noting that under such a warming regime "both droughts and floods are expected to intensify." In a study designed to explore the soundness of this contention, the two Finnish researchers analyzed long-term trends and variability in the discharge regimes of both regulated and unregulated rivers and lake outlets in Finland up to the year 2004, using data supplied by the Finnish Environment Institute.

This analysis revealed that as "winters and springs became milder during the 20th century ... the peak of spring flow has become 1-8 days earlier per decade at over one-third of all studied sites." However, they say that "the magnitudes of spring high flow have not changed." On the other hand, low flows, in their words, "have increased at about half of the unregulated sites due to an increase in both winter and summer discharges." Nevertheless, they indicate that "statistically significant overall changes have not been observed in mean annual discharge." Thus, in contrast to typical global warming projections, at the high end where flooding may occur, there has been no change in the magnitude of flows that can lead to that unwelcome phenomenon for the region examined. And at the low end, where droughts may occur, there has actually been an increase in flow magnitude; and that increase either acts to prevent or leads to less frequent and/or less severe episodes of this other unwelcome phenomenon.

Finally, in regard to the IPCC's Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation, or SREX for short, Kundzewicz et al. (2014) conducted a follow-up study to assess "the literature included in the IPCC SREX report and new literature published since," while also examining "changes in flood risk in seven of the regions considered in the recent IPCC SREX report-Africa, Asia, Central and South America, Europe, North America, Oceania and Polar regions." Among the highlights of their work, the team of seventeen researchers hailing from eleven different countries report the following: (1) "no gauge-based evidence has been found for a climate-driven, globally widespread change in the magnitude/frequency of floods during the last decades," (2) "there is low confidence in projections of changes in fluvial floods, due to limited evidence and because the causes of regional changes are complex," (3) "considerable uncertainty remains in the projections of changes in flood magnitude and frequency," (4) increases in global flood disaster losses reported over the last few decades "may be attributed to improvements in reporting, population increase and urbanization in flood-prone areas, increase of property value and degraded awareness about natural risks (due to less natural lifestyle)," (5) "the linkages between enhanced greenhouse forcing and flood phenomena are highly complex and, up to the present, it has not been possible to describe the connections well, either by empirical analysis or by the use of models," and (6) "the problem of flood losses is mostly about what we do on or to the landscape," which they say "will be the case for decades to come."

In the concluding paragraph of their extensive study, Kundzewicz et al. state "although media reports of both floods and global flood damage are on the increase, there is still no Mauna-Loa-like record (see Vorosmarty, 2002) that shows a global increase in flood frequency or magnitude." Thus, they write "blaming climate change for flood losses makes flood losses a global issue that appears to be out of the control of regional or national institutions." And they therefore state "the scientific community needs to emphasize that the problem of flood losses is mostly about what we do on or to the landscape," which implies that individual, community, county and state responsibility "will be the case for decades to come."'

Taken together, the research described in the paragraphs above suggests that, if anything, 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 that any further warming of the globe would tend to further reduce both the frequency and severity of flooding, which is just the opposite of what climate models suggest should occur under such conditions.

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