by Judith Curry

The hypothesized link between a warming climate and increased frequency and magnitude of floods goes something like this:  a warmer climate is associated with more water vapor in the atmosphere, which means more rainfall and more floods.  Is there any observational support for this link?

A series of papers on this subject have been published by Gabriele Villarini at Princeton University.  Two of the papers related to the U.S. are highlighted here.

Flood peak distributions for the eastern United States

Gabriele Villarini and James Smith

Abstract.  Annual maximum peak discharge time series from 572 stations with a record of at least 75 years in the eastern United States are used to examine flood peak distributions from a regional perspective. The central issues of this study are (1) “mixtures” of flood peak distributions, (2) upper tail properties of flood peaks, (3) scaling properties of flood peaks, (4) spatial heterogeneities of flood peak distributions, and (5) temporal nonstationarities of annual flood peaks. Landfalling tropical cyclones are an important element of flood peak distributions throughout the eastern United States, but their relative importance in the “mixture” of annual flood peaks varies widely, and abruptly, in space over the region. Winter-spring extratropical systems and warm season thunderstorm systems also introduce distinct flood peak populations, with spatially varying control of flood frequency distributions over the eastern United States. We examine abrupt changes in the mean and variance of flood peak distributions through change point analyses and temporal trends in the flood peak records through nonparametric tests. Abrupt changes, rather than slowly varying trends, are typically responsible for nonstationarities in annual flood peak records in the eastern United States, and detected change points are often linked to regulation of river basins. Trend analyses for the 572 eastern United States gaging stations provide little evidence at this point (2009) for increasing flood peak distributions associated with human-induced climate change. Estimates of the location, scale, and shape parameters of the generalized extreme value (GEV) distribution provide a framework for examining scaling properties of flood peaks and upper tail properties of flood distributions. It is shown that anomalously large values of the GEV shape parameter estimates are linked to the role of tropical cyclones in controlling the upper tail of flood distributions. Scaling analyses of flood peaks highlight the heterogeneities in flood magnitudes over the region with maxima in scaled flood magnitudes in the high-elevation Appalachian Mountains and minima in the low-gradient Coastal Plain.

(2010),  Water Resour. Res., 46, W06504, doi:10.1029/2009WR008395.  [Link]  to abstract  (paper not available online)

Pielke Jr. cites the following from the paper’s conclusions:

Only a small fraction of stations exhibited significant linear trends. For those stations with trends, there was a split between increasing and decreasing trends. No spatial structure was found for stations exhibiting trends. There is little indication that human‐induced climate change has resulted in increasing flood magnitudes for the eastern United States.

Examining Flood Frequency Distributions in the Midwest U.S.

Gabriele Villarini, James Smith, Mary Lynn Baeck, Wiltoid Krajewski

Abstract.  Annual maximum peak discharge time series from 196 stream gage stations with a record of at least 75 years from the Midwest United States is examined to study flood peak distributions from a regional point of view. The focus of this study is to evaluate: (1) “mixtures” of flood peak distributions, (2) upper tail and scaling properties of the flood peak distributions, and (3) presence of temporal nonstationarities in the flood peak records. Warm season convective systems are responsible for some of the largest floods in the area, in particular in Nebraska, Kansas, and Iowa. Spring events associated with snowmelt and rain-on-snow are common in the northern part of the study domain. Nonparametric tests are used to investigate the presence of abrupt and slowly varying changes. Change-points rather than monotonic trends are responsible for most violations of the stationarity assumption. The abrupt changes in flood peaks can be associated with anthropogenic changes, such as changes in land use/land cover, agricultural practice, and construction of dams. The trend analyses do not suggest an increase in the flood peak distribution due to anthropogenic climate change. Examination of the upper tail and scaling properties of the flood peak distributions are examined by means of the location, scale, and shape parameters of the Generalized Extreme Value distribution.

Paper No. JAWRA-10-0046-P of the Journal of the American Water Resources Association(JAWRA).  [Link]  to abstract  (paper not available online)

CO2Science provides the following summary of the paper:

What was learned
The four U.S. researchers report that in the vast majority of cases where streamflow changes were observed, they were “associated with change-points (both in mean and variance) rather than monotonic trends,” and they indicate that “these non-stationarities are often associated with anthropogenic effects.” But rather than increases in anthropogenic CO2 emissions, they cite such things as “changes in land use/land cover, changes in agricultural practice, and construction of dams and reservoirs.”

What it means
Based on their findings, and, as they note, “in agreement with previous studies” they conclude that “there is little indication that anthropogenic climate change has significantly affected the flood frequency distribution for the Midwest U.S.” And as they make doubly clear in the abstract of their paper, they say that “trend analyses do not suggest an increase in the flood peak distribution due to anthropogenic climate change.”

On the stationarity of flood peaks in the continental U.S. and Central Europe

While the journal articles disappointingly are not available online, i spotted an extensive pdf presentation by Villarini [link].  Some text excerpts:

Problem Statement and Objectives

• There is a large spatial variability in flood hazards over the continental United States and central Europe
• Profound anthropogenic changes (e.g., changes in land use / land cover, construction of dams, river engineering)
• Projected increasing frequency of extreme rain and flood with human induced climate change
• Rapidly changing flood hazards with urbanization

Central Issues:

• temporal non-stationarities of annual flood peaks
• changes in frequency of extreme flooding due to climate change
• flood hazard assessment in urban environment
• “Mixtures” of flood peaks and impact of tropical cyclones on flood peak distributions

Conclusions

• Change-points rather than monotonic trends are responsible for the violation of the stationarity assumption.
• Analyses of historical discharge records do not point at changes in the flood peak distribution due climate change.
• Non-stationary modeling of flood peaks in urban environment
• Under nonstationary conditions, alternative definitions of return period have to be sought.
• Spatial heterogeneity in “mixtures” of flood peaks.
• Tropical cyclones control extreme flooding in much of the eastern US. Analyses of tropical storm frequency in a warmer climate do not suggest a significant increase in tropical storms in the North Atlantic basin over the 21st century.