May 26, 2011

No Long-term Trend in Atlantic Hurricane Numbers

Filed under: Climate Extremes, Hurricanes

Long-debated has been whether or not there is a long-term trend in the number of Atlantic tropical storms and hurricanes.

The answer to this seemingly straightforward question turns out to be complicated because there have been changes in the observing practices over time—including changes in the spatial coverage of observing systems as well as the technologies employed. Therefore, teasing out the real climate signal from the noise induced by the changing nature of the observations has proved challenging and lends itself to a variety of methodologies producing a variety of results.

Of top of this less than perfect solution is the desire (for some at least) to want to try to involve anthropogenic global warming, hoping to find that anthropogenic climate change is leading to more tropical storms and hurricanes. But thus far, the evidence for this is scant, to say the least.

And now, it just got scanter. (We know the word is “scantier” but the one we concocted rhymes with our pugilistic friend in climate hyperbole, Ben Santer).

A just-published paper in the Journal of Geophysical Research authored by a team of leading hurricane researchers has once again examined the historical record of tropical storm observations from the Atlantic Ocean this time focusing on the number of tropical storms whose entire lifetime was less than two days. The authors’ termed these very minor storms “shorties.” The identification of shorties is one element of the tropical cyclone record that could be very impacted by changing observational methodologies and technologies. Short-duration storms are presently identified much more readily than they were, say, prior to the satellite era.

If the Atlantic tropical cyclone history is divided up into “shorties” and, we guess, “longies,” something very interesting pops out. Over the entire record, there is a big upward trend in the number of “shorties” but there is no trend in the annual number of “longies” (Figure 1).

Figure 1. Time series of the number of North Atlantic tropical storms lasting (top) 2 days or less (shorties) and (bottom) more than 2 days (longies) (from Villarini et al., 2011).

Obviously, lumping the two together would produce an apparent upward trend in the total annual number of tropical storms and hurricanes—and give fuel for the fire which burns for those trying to develop a link to anthropogenic global warming.

This situation is akin to the observed record of tornadoes in the U.S.—the number of weak tornadoes has increased markedly in the last half century, while the number of strong tornadoes shows no such behavior. For tornadoes, this is because better observing technologies (and a lot more people looking) have increasingly identified small storms which were previously overlooked. But the big storms cause such damage that they can’t go unnoticed. The positive trend in total annual number of tornadoes is driven not by climate change (as some would have you believe), but instead by changing observational methods.

The authors of the current study, Gabriele Villarini, Gabriel Vecchi, Thomas Knutson, and James Smith, wanted to more closely examine the record of shorties to see if they could determine the reasons behind the large upward trend in the number of shorties.

They did this through combining statistical methods together with their understanding of the physical processes involved in tropical storm formation in the Atlantic basin (a topic that the authors are well versed in).

What they found was a lack of evidence for a detectable climate change signal in the century-long record of shorties. Instead, they concluded that non-climatic signals (i.e. data quality issues) were contaminating the record and making it impossible to isolate a climate signal from the raw data, if such a signal even existed at all (a possibility which the authors think as unlikely, at least as far as there being a significant positive trend over the 20th century).

What’s more, the authors warn that all approaches to identifying a secular trend in Atlantic tropical cyclone counts which do not explicitly account for the non-climatic influences of shorties are likely to be in error, as are statistical models of Atlantic tropical cyclone activity that include shorties in their datasets. We add that there are quite a few published studies that do just what Villarini et al. have warned against.

Here is how Villarini et al. conclude their paper:

[B]ased on our results, it appears that the long-term record of the basin-wide shorties is sufficiently contaminated by spurious components to mask any climatically induced variation within the raw data. Moreover, based on these results and those of Vecchi and Knutson [2008] it is unlikely that a homogeneous record of Atlantic tropical storm counts would contain a statistically significant positive trend since the late 1800s. Our results provide a context for interpreting studies exploring trend behavior in the North Atlantic tropical storm activity starting prior to the 1940s. In particular, the conclusions of certain studies reporting large secular increases in North Atlantic tropical storm activity in which shorties are included [e.g., Holland and Webster, 2007; Mann et al., 2007] could be affected by what we interpret as likely spurious nonphysical trends unless an alternative physical explanation can be uncovered for the pronounced increase in shorties starting from the middle of the 20th century. Further, statistical models of tropical storm activity built using century‐scale records that include shorties [e.g., Mann et al., 2007; Sabbatelli and Mann, 2007; Mann et al., 2009] likely include an element reflecting the spurious shorties in the record.

[Yes, there are more references to Michael Mann here than you might find in a UVa FOIA!—eds.]

Villarini et al.’s findings will require a reassessment of the existing assessments concerning the long-term temporal behavior of Atlantic tropical cyclones and their potential linkages to anthropogenic climate change.


Reference: Villarini, G., et al., 2011. Is the recorded increase in short-duration North Atlantic tropical storms spurious? Journal of Geophysical Research, 116, D10114, doi:10.1029/2010JD015493.

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