Is Flood Magnitude in the USA Correlated with Global CO2 Levels?

by Marlo Lewis on October 31, 2011

in Features

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No — or, more precisely, not  yet — conclude R.M. Hirsch and K.R. Ryberg of the U.S. Geological Survey in a recent study published in Hydrological Sciences Journal.

“One of the anticipated hydrological impacts of increases in greenhouse gas concentrations in the atmosphere is an increase in the magnitude of floods,” note Hirsch and Ryberg. Righto! Google “global warming” and “flood predictions,” and you’ll find more than 2.7 million sites where this hypothesis is affirmed or at least discussed. The researchers explain:

Greenhouse gases change the energy balance of the atmosphere and lead to atmospheric warming, which increases the water-holding capacity of the atmosphere, which in turn, potentially changes the amount of precipitable water.

Sounds plausible, but all weather is local or regional, and a lot more goes into making weather than average global temperature.  In addition, all flooding is local or regional, and a lot more goes into determining flood risk than local or regional weather patterns.

As Hirsch and Ryberg point out, “human influences associated with large numbers of very small impoundments and changes in land use also could play a role in changing flood magnitude,” and “at time scales on the order of a century it is difficult to make a quantitative assessment of the changes in these factors over time.”

That, however, did not stop good ol’ Al Gore from claiming that global warming is responsible for a decade-by-decade increase in the number of large floods around the world (An Inconvenient Truth, p. 106). Gore’s source was a chart from the Millennium Ecosystem Assessment (Figure 16.5, p. 448):

The chart does appear to show significant decadal increases in the number of floods. However, what the chart actually measures is the number of “damaging” floods, and whether or not a flood is classified as “damaging” is influenced by socio-economic and even political factors. As the MEA report explains:

Only events that are classified as disasters are reported in this database. (An event is classified as a disaster if it meets at least one of the following criteria: 10 or more people reported killed; 100 or more people reported affected; international assistance was called; or a state of emergency was declared . . .

Obviously, the database is going to be skewed towards more events in later decades simply because of better reporting, more declared states of emergency, and more calls for international assistance. As the MEA report observes, “although the number has been increasing, the actual reporting and recording of floods have also increased since 1940, due to the improvements in telecommunications and improved coverage of global information.”

The MEA report also identifies several non-climatic factors that influence flood damage risk: wetlands loss and deforestation, changes in engineering practices, irrigation, urbanization, and, perhaps most importantly, population growth and economic development in flood plains.

In short, teasing out a greenhouse warming “signal” from flood damages influenced by both natural climate variability and a host of societal factors is a daunting task. Yet Gore treats flood damage data as unambiguous evidence of a warming-ravaged planet.

Okay, let’s get back to the USGS scientists. Hirsch and Ryberg acknowledge they cannot entirely filter out “reservoir storage, urban development, or other human activities in the watersheds” without narrowing their study “almost entirely to very small watersheds, typically in remote and often mountainous areas.” As a reasonable compromise, they examined flood data from “200 streamgauges operated by the US Geological Survey (USGS) in the coterminous USA, of at least 85 years length through water year 2008, from basins with little or no reservoir storage or urban development (less than 150 persons per square kilometre in 2000).”

What did they find? From the paper’s abstract:

In none of the four regions [Northeast, Southeast, Northwest, Southwest] defined in this study is there strong statistical evidence for flood magnitudes increasing with increasing GMCO2 [global mean carbon dioxide concentration]. One region, the southwest, shows a statistically significant negative relationship between GMOC2 and flood magnitudes.

For further reading, the Center for the Study of Carbon Dioxide and Global Change summarizes the results of 21 peer-reviewed studies on flooding and climate variability in North America. The Center concludes:

Taken together, the research described in this Summary 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.  Hence, although there could also be exceptions to this rule in the case of future warming, on average, 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, which, of course, is just the opposite of what the world’s climate alarmists continue to claim would occur.


Hugh Mann October 31, 2011 at 10:39 pm

Trollololololllololololololololololol… olololololooooololololololo lolo

Clifford T. Shea November 8, 2011 at 1:16 pm

Does anyone care to guess what policies we’d be living under if Al Gore had won the 2000 Presidential election?

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