Tag: 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.

U.S. Floods, Droughts and Global Warming: Another Wardrobe Failure

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.”

It is the current rage in the mainstream media and the government to tie almost everything into human-caused global warming—from a sluggish economy to Ebola,  and everything in between (and then some).

In fact, virtually none of these claims are supported by a consensus of evidentiary science. Here is (yet) another example, debunking the popular notion floods are being worsened by dreaded climate change caused by pernicious economic activity.

Clinically speaking, a “flood” is actually an extreme excursion in streamflow. So, if changes in streamflow are related to long-term changes in climate, and we accept that the majority of those latter changes are caused by said economic activity (we don’t), then our activities should increase streamflow and therefore the frequency of floods (or their opposite, droughts).

Two scientists from the U.S. Geological Survey (USGS), Gregory McCabe and David Wolock, recently examined historical (1951-2009) streamflow records from 516 rivers and streams that they considered to be only minimally impacted by human development. They first sorted the data into regional patterns, and then compared the temporal behavior of these patterns to  common historical climate indices—such as well-known patterns of atmospheric circulation, sea surface temperatures, or even large-scale warming.

It turns out that there weren’t any relationships between streamflow and the larger atmospheric phenomena.  Or at least, so very few that they are hardly worth mentioning.

Here is how McCabe and Wolock describe what they (didn’t) find:

Comparing time series of climate indices…with the time series of mean [stream] flow for the 14 clusters [patterns] indicates weak correlations that are statistically significant for only a few clusters. These results indicate that most of the temporal variability in streamflow in the conterminous U.S. is unpredictable in terms of relations to well-known climate indices. [emphasis added]

In other words, trends and/or variability in larger-scale features of the climate (including rising temperature from global warming) are not very strongly (if at all) related to regional and temporal characteristics of streamflows across the U.S.

And before anyone starts to argue that we have left out the direct (i.e., local) effect of global warming—that warmer air holds more moisture and thus it can rain more frequently and harder—McCabe and Wolock report very few long-term trends that would be indicative of steadily rising moisture levels. Instead, the find the historical records dominated by periods of multidecadal variability. In their own words:

Analyses of the annual mean streamflow time series for the 14 streamflow clusters indicated periods of extended wet and dry periods, but did not indicate any strong monotonic trends. Thus, the mean cluster streamflow time series indicate nearly random variability with some periods of persistence.

The bottom line is that McCabe and Wolock do not identify any behavior in historical U.S. streamflow records that is suggestive of an influence from human-caused global warming.

So next time you hear that there are increasing droughts or floods in the U.S. and that they are, through some convoluted explanation, “consistent with” global warming, remember two things: 1) “consistent with” is not the same as “caused by” and, 2) the consensus science linking global warming to changing streamflow characteristics across the U.S. is lacking.

Reference:

McCabe, G. J., and D. M. Wolock, 2014. Spatial and Temporal Patterns in Conterminous United States Streamflow Characteristics. Geophysical Research Letters, doi:10.1002/2014GL061980