Background
The authors write 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.

What was done
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 streamgauges 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.

What was learned
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.

What it means
In discussing the meaning of their findings, Hirsch and Ryberg state that "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, it could mean that the "anticipated hydrological impacts" envisioned by the IPCC and others are simply incorrect.

References
Gutowski Jr., W.J., Hegerl, G.C., Holland, G.J., Knutson, T.R., Mearns, L.O., Stouffer, R.J., Webster, P.J., Wehner, M.F., Zwiers, F.W., Brooks, H,.E., Emanuel, K.A., Kormar, P.D., Kossin, J.P., Kunkel, K.E., McDnald, R., Meehl, G.A. and Trapp, R.J. 2008. Causes of observed changes in extremes and projections of future changes. In: Karl, T.R., Meehl, G.A., Miller, C.D., Hassol, S.J., Waple, A.M. and Murray, W.L. (Eds.). Weather and Climate Extremes in a Changing Climate-Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands. U.S. Climate Change Science, Washington, DC, USA.

IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physical Science Basis. Solomon, S., Qin, D., Manniing, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.). Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

Trenberth, K.E. 1999. Conceptual framework for changes of extremes of the hydrological cycle with climate change. Climatic Change 42: 327-339.