Has the Magnitude of Floods Across the USA Changed with Global CO2 Levels?
Hirsch, R.M. and Ryberg, K.R. 2011. Has the magnitude of floods across the USA changed with global CO2 levels? Hydrological Sciences Journal DOI:10.1080/02626667.2011.621895.
Specifically, Hirsch and Ryberg (2011) used stream flow data on annual flood series from 200 stream gauges operated by the US Geological Survey (USGS) which had a minimum of 85 years of data through water year 2008 from basins with little or no reservoir storage or urban development (urban development was defined as at least 150 persons per square kilometer in 2000) to look for trends in the U.S. as a whole, as well as four major U.S. regions: the northeast, northwest, southwest and southeast.
The results, as shown in the figure below, indicate that, except for the decreased flood magnitudes observed in the southwest, there is no strong empirical evidence for any trend in flood magnitudes for the entire U.S. or any of the other three regions despite a 32% increase in carbon dioxide concentration over the study period.
Figure 1: Boxplots of estimates of the slope for flood magnitude with respect to global mean CO2 concentration. On the right-hand scale the estimates are re-expressed in units of percent change per 10 ppm increase in GMCO2. Box width is proportional to the square root of the sample size. Source: Hirsch and Ryberg (2011).
The results of this study, therefore, throw cold water on claims that CO2-induced global warming is increasing flood magnitudes in the U.S.A.