Journal of Hydrology
Volume 552, September 2017, Pages 704-717
“Climate-driven variability in the occurrence of major floods across North America and Europe”
Trends in major-floods from 1204 sites in North America and Europe are assessed.
Trends based on counting exceedances of flood thresholds for groups of gauges.
The number of significant trends was about the number expected due to chance alone.
Changes in the frequency of major floods are dominated by multidecadal variability.
Concern over the potential impact of anthropogenic climate change on flooding has led to a proliferation of studies examining past flood trends. Many studies have analysed annual-maximum flow trends but few have quantified changes in major (25–100 year return period) floods, i.e. those that have the greatest societal impacts. Existing major-flood studies used a limited number of very large catchments affected to varying degrees by alterations such as reservoirs and urbanisation. In the current study, trends in major-flood occurrence from 1961 to 2010 and from 1931 to 2010 were assessed using a very large dataset (>1200 gauges) of diverse catchments from North America and Europe; only minimally altered catchments were used, to focus on climate-driven changes rather than changes due to catchment alterations. Trend testing of major floods was based on counting the number of exceedances of a given flood threshold within a group of gauges. Evidence for significant trends varied between groups of gauges that were defined by catchment size, location, climate, flood threshold and period of record, indicating that generalizations about flood trends across large domains or a diversity of catchment types are ungrounded. Overall, the number of significant trends in major-flood occurrence across North America and Europe was approximately the number expected due to chance alone. Changes over time in the occurrence of major floods were dominated by multidecadal variability rather than by long-term trends. There were more than three times as many significant relationships between major-flood occurrence and the Atlantic Multidecadal Oscillation than significant long-term trends.
Moreover, many studies have not separated the effects of human catchment alterations from those of climate (Burn et al., 2012; Merz et al., 2012; Harrigan et al., 2014); alterations such as urbanisation have been shown to impact observed flood trends (Vogel et al., 2011). To be informative about climate-driven flood trends, catchments should be relatively free of confounding human influences such as land-use change, diversions, abstractions and reservoir regulation. Thus, networks of minimally altered catchments—so called reference hydrologic networks (RHNs)—have been advocated (Whitfield et al., 2012).
The results of this study, for North America and Europe, provide a firmer foundation and support the conclusion of the IPCC (Hartmann et al., 2013) that compelling evidence for increased flooding at a global scale is lacking. Generalizations about climate-driven changes in floods across large domains or diverse catchment types that are based upon small samples of catchments or short periods of record are ungrounded.
Floods: ‘Floods are not increasing’: Dr. Roger Pielke Jr. slams ‘global warming’ link to floods & extreme weather – How does media ‘get away with this?’ – Pielke Jr. on how extreme weather is NOT getting worse: ‘Flood disasters are sharply down. U.S. floods not increasing either.’ “Floods suck when they occur. The good news is U.S. flood damage is sharply down over 70 years,” Pielke explained.
Scientist: ‘There Is No Such Thing As A 1000-Year Flood’ – Climate Statistician Dr. Matt Briggs: Phrases like “100 year rainfalls” or floods or whatever for whatever period of time are awful. They convey an improper idea of uncertainty. The phrase “X year event” is based on inverting the probability of the event; call that probability p. Thus “X year event” is equal to “1/p year event”, where p is the probability the event happens per year. That means a “100 year event” has a probability of 1%, and so on. A “1000 year event” sounds stupendous, and, to most ears, rarer than a 0.1% chance. Anyway, these are all wrong…It’s perfectly correct to make the statements like this: “The last time a flood this size occurred was in 1945.” That statement is not, however, equivalent to (in 2015) “That was a 70 year flood.”
Climate Depot Note: The media and climate activists love to hype so-called 1 in 100/1000 year extreme weather events. What they do not not explain is that your chance of the winning the lottery is very low, but the chance of someone, somewhere winning the lottery are very high. So the activists essentially hype “lottery winners” of extreme weather events and try to imply these events are increasing and happening everywhere. Lottery and casino ads do the same by showing all the winners and implying you are just one ticket or spin away from joining the lucky winners. Climate activists are trying to scare the public into believing that they are one bad weather event away from doom and only EPA and the UN Paris climate pact can save them!