floods

Heavy Rains Increasing, but Not Disproportionately So

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

A new paper has been published in the journal Geophysical Research Letters that examines trends in heavy rainfall amounts across the U.S. The paper is authored by Newcastle University’s Renaud Barbero and colleagues, and, to summarize, finds that the heaviest rainfall events of the year have been increasing in magnitude since 1951 when averaged across nearly 500 stations distributed across the U.S. (note: results from individual stations may differ from the general finding).

Someone with a critical eye might ask the real question, which is “how much?” That such a number does not jump out of this paper—a cynic would say—probably means it is very small. Read on and you will find the answer.

That rainfall on the rainiest day of the year is increasing is, of itself, hardly surprising considering that the total annual rainfall amount averaged across the U.S. has also been increasing during this same period (again, results from individual locations/regions may (and do) depart from this generality).

Changes in heavy rainfall like this are often luridly described as a “disproportionate increase” in extreme events, or that extreme precipitation increases are “worse than expected.”

Floods Not Increasing Across the U.S.

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our “Current Wisdom.”


In our continuing examination of U.S. flood events, largely prompted by the big flood in Louisiana last month and the inevitable (and unjustified) global-warming-did-it stories that followed, we highlight a just-published paper by a research team led by Dr. Stacey Archfield of the U.S. Geological Survey examining trends in flood characteristics across the U.S. over the past 70 years.

Previous studies we’ve highlighted have shown that a) there is no general increase in the magnitude of heavy rainfall events across the U.S., and thus, b) unsurprisingly, “no evidence was found for changes in extreme precipitation attributable to climate change in the available observed record.”  But since heavy rainfall is not always synonymous with floods, the new Archfield paper provides further perspective.

The authors investigated changes in flood frequency, duration, magnitude and volume at 345 stream gauges spread across the country. They also looked to see if there were any regional consistencies in the changes and whether or not any of the observed changes could be linked to large-scale climate indices, like El Niño.

What they found could best be described largely as a “negative” result—basically, few departures from the a priori expectation (often called the null hypothesis) that there are no coherent changes in flood characteristics occurring across the U.S.  Here’s their summary of their research findings:

Spin Cycle: Attributing Louisiana Floods to Global Warming

The Spin Cycle is a reoccurring feature based upon just how much the latest weather or climate story, policy pronouncement, or simply poobah blather spins the truth. Statements are given a rating between 1-5 spin cycles, with fewer cycles meaning less spin. For a more in-depth description, visit the inaugural edition.

In mid-August a slow moving unnamed tropical system dumped copious amounts of precipitation in the Baton Rouge region of Louisiana. Reports were of some locations receiving over 30 inches of rain during the event. Louisiana’s governor John Bel Edwards called the resultant floods “historic” and “unprecedented.”

Some elements in the media were quick to link in human-caused climate change (just as they are to seemingly every extreme weather event). The New York Times, for example, ran a piece titled “Flooding in the South Looks a Lot Like Climate Change.”

We were equally quick to point out that there was no need to invoke global warming in that the central Gulf Coast is prime country for big rain events and that similar, and even larger, rainfall totals have been racked up there during times when there were far fewer greenhouse gases in the atmosphere—like in 1979 when 45 inches of precipitation fell over Alvin, TX from the slow passage of tropical storm Claudette, or in 1940 when 37.5 in. fell on Miller Island, LA from another stalled unnamed tropical system.

But we suspected that this wouldn’t be the end of it, and we were right.

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