If you live in the Midwest, you’re over, say, 50 years old, and you’ve had the impression that floods are happening more frequently than they used to, your memory is not playing tricks on you. A pair of researchers at the University of Iowa have studied the daily records collected at stream gauges in 14 states by the U.S. Geological Survey from 1962-2011. Four times as many stations (264, or 34 percent) showed an increase as showed a decrease during that time (66, or nine percent).
Iman Mallakpour and Gabriele Villarini published their findings, “The Changing Nature of Flooding across the Central United States,” in the February 9 advance online edition of Nature Climate Change, a scientific journal. Villarini is an assistant professor of civil and environmental engineering, and Mallakpour is a graduate student in the program who served as lead author on the paper. The U.S. Army Corps of Engineers Institute for Water Resources, the Iowa Flood Center, and IIHR-Hydroscience & Engineering supported their work, along with the National Science Foundation.
The two authors studied the data for both changes in peak flow—the magnitude of the events—and the frequency with which floods occurred. They did not find a statistically significant pattern of increased large events, but the data on increasing frequency of flooding was quite convincing. They also examined seasonal differences and found some differentiation between the central Midwest and its perimeters, where the pattern of increased flooding was generally less pronounced. They also found an “overall good match” when they overlaid the areas of increasing flood frequency with those experiencing heavy rainfall events. Although the two authors did not go so far as to relate these results to climate change, this does not mean there is no connection; they simply chose not to speculate beyond the information provided by the stream flow data they examined. As the article states, “a direct attribution of these changes in discharge, precipitation and temperature to human impacts on climate represents a much more complex problem that is very challenging to address using only observational records.”
However, as this blog has noted previously (“Iowa Faces Its Fluid Future”), this tracks well with the prevailing theory among climate scientists that, as the atmosphere becomes warmer, it can hold more moisture, resulting in more intense events of higher precipitation—offset, at other times, by an increased propensity for drought. The expected tendency is for a flattening of the bell curve of weather events—more on the extreme ends in both directions, lessening the dominance of more moderate events. The article by Mallakpour and Villarini is one more in a long string of indicators that change is afoot with regard to weather and flooding patterns. The science of climate change has not been and will not be built on one or two studies, but hundreds, if not thousands. That is why the Intergovernmental Panel on Climate Change involves thousands of researchers from 195 countries. It is important to pay attention to both individual pieces of evidence and its massive overall accumulation to understand how the case for the human impact was built over time.
Jim Schwab