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.

All the while, an “international partnership” funded in part by the U.S. government (through grants to climate change cheerleader Climate Central), called World Weather Attribution (“international effort designed to sharpen and accelerate the scientific community’s ability to analyze and communicate the possible influence of climate change on extreme-weather events such as storms, floods, heat waves and droughts”) and was fervently working to formally (i.e., through a scientific journal publication) “attribute” the Louisiana rains to climate change.

The results of their efforts were made public a couple of weeks ago in parallel with the submission (we’ll note: not acceptance) of their article to the journal Hydrology and Earth System Science Discussions.

Their “attribution” can well, be attributed, to two factors. First, their finding that there has been a large increase in the observed probability of extreme rainfall along the central Gulf Coast—an increase that they claim can be directly related to the rise in the global (!) average temperature. And second, their finding that basically the single (!) climate model they examined also projects an increase in the probably of heavy rainfall in the region as a result of human-induced climate changes. Add the two together, throw in a splashy press release from a well-funded climate change propaganda machine and headlines like the AP’s “Global warming increased odds for Louisiana downpour” are the result.

As you have probably guessed since you are reading this under our “Spin Cycle” tag, a closer look finds some major shortcomings to this conclusion.

For example, big rains are part of the region’s history—and most (but not all) are result from meandering tropical weather systems whose progress has been slowed by mid-latitude circulation features. In most cases, the intensity of the tropical system itself (as measured by central pressure or maximum wind speed) is not all that great, but rather the abundant feed of moisture feed from the Gulf of Mexico and slow progress of the storm combine to produce some eye-popping, or rather boot-soaking, precipitation totals.  Here is a table of the top 10 rainfall event totals from the passage of tropical systems through the contiguous U.S. since 1921 (note that all are in the Gulf Coast region). Bear in mind that the further you go back in time, the sparser the observed record becomes (which means an increased chance that the highest rainfall amounts are missed). The August 2016 Louisiana event cracks the top 10 as number 10. A truly impressive event—but hardly atypical during the past 100 years. 

 

As the table shows, big events occurred throughout the record. But due to the rare nature of the events as well as the spotty (and changing) observational coverage, doing a formal statistical analysis of frequency changes over time is very challenging. One way to approach it is to use only the stations with the longest period of record—this suffers from missing the biggest totals from the biggest events, but at least it provides some consistency in observational coverage.  Using the same set of long-term stations analyzed by the World Weather Attribution group, we plotted the annual maximum precipitation in the station group as a function of time (rather than global average temperature). Figure 1 is our result. We’ll point out that there is not a statistically significant change over time—in other words, the intensity of the most extreme precipitation event each year has not systematically changed in a robust way since 1930. It’s a hard sell link this non-change to human-caused global warming.

Figure 1. Annual maximum 3-day rainfall total for stations with at least 80 years of record in the region 29-31N, 95-85W.

Figure 1. Annual maximum 3-day rainfall total for stations with at least 80 years of record in the region 29-31N, 95-85W.

Admittedly, there is a positive correlation in these data with the global average surface temperature, but correlation does not imply causation. There is a world of distance between local weather phenomena and global average temperature. In the central Gulf Coast, influential denizens of the climate space, as we’ve discussed, are tropical cyclones—events whose details (frequency, intensity, speed, track, etc.) are highly variable from year to year (decade to decade, century to century) for reasons related to many facets of natural variability. How the complex interplay of these natural influencers may change in a climate warmed by human greenhouse gas emissions is far from certain and can be barely even be speculated upon. For example, the El Niño/La Niña cycle in the central Pacific has been shown to influence Gulf Coast tropical cyclone events, yet the future characteristics of this important factor vary considerably from climate model to climate model and confidence in climate model expectations of future impacts is low according to the U.N. Intergovernmental Panel on Climate Change (IPCC).

Which means that using a single climate model family in an “attribution” study of extreme Gulf Coast rainfall events is a recipe for distortion—at best, a too limited analysis, at worse, a misrepresentation of the bigger picture. 

So, instead of the widely advertised combination in which climate models and observations are in strong agreement as to the role of global warming, what we really have is a situation in which the observational analysis and the model analysis are both extremely limited and possibly (probably) unrepresentative of the actual state of affairs.

Therefore, for their overly optimistic view of the validity, applicability, and robustness of their findings that global warming has increased the frequency of extreme precipitation events in central Louisiana, we rate Climate Central’s World Weather Attribution’s degree of spin as “Slightly Soiled” and award them two Spin Cycles.

 

Slightly Soiled.  Over-the-top rhetoric. An example is the common meme that some obnoxious weather element is new, thanks to anthropogenic global warming, when it’s in fact as old as the earth. An example would the president’s science advisor John Holdren’s claim the “polar vortex,” a circumpolar westerly wind that separates polar cold from tropical warmth, is a man-made phenomenon. It waves and wiggles all over the place, sometimes over your head, thanks to the fact that the atmosphere behaves like a fluid, complete with waves, eddies, and stalls. It’s been around since the earth first acquired an atmosphere and rotation, somewhere around the beginning of the Book of Genesis. Two spin cycles.