Harvey has rekindled the debate between climate change advocates and deniers over whether tropical cyclones are becoming more frequent and intense.  Both sides tend to overstate their respective cases.

Trying to get a handle on whether these storms are getting worse is not as easy as it might seem.  First, exactly how you measure the intensity of a storm is somewhat problematic.  Hurricane Sandy, which caused extensive damage to the East Coast and was frequently referred to in the media as a “superstorm,” was only a Category 1 storm on the Saffir-Simpson scale.

The Saffir-Simpson scale is exclusively determined by wind speed.  [Click here for discussion of the scale.]  But we have learned that wind speed is just one factor in determining the severity of a storm and the resulting damage.  For example, the physical size of the storm is another important indicator of its tidal surge.  And, of course, the resulting damage from any storm is largely determined by where it makes landfall and the population and property in its path.  Had Sandy made landfall along a coastline with far less population, no one would have referred to it as a “superstorm.”

Another problem is that the farther we go back into historical records, the less accurate the information is about the incidence and the severity of the storms.  Since the 1960s we have had reliable data from satellites.  But that is the blink of the eye on the scale of climate history.  Before that, the data is much less accurate, relying frequently on incomplete narrative accounts and spotty meteorological observations.

Nonetheless, the National Weather Service has compiled a list of tropical cyclones that have occurred in the Atlantic basin since 1850, based on the best information they have.  They have categorized the storms into three categories: tropical storms, hurricanes and major hurricanes.  Click here to see the table.

When you graph this table, it looks like this:

The upward trend is pretty clear, at least, on this most global level of the data.  And the 2017 data will tilt the trend lines a bit higher since this has been such an active season.

If we look only at all the storms that reached tropical storm status since 1970, the trend line flattens out significantly, supporting the notion that there is an observational bias before 1970 that under counted storms.  The trend is still up but it is considerably less dramatic than the hyperbole we frequently hear from climate change advocates.

Of course, history is not necessarily a predictor of the future.  But these trend lines significantly undermine those who argue that the current spike in storms is nothing more than a naturally recurring weather cycle.

Weather events, by their nature, are episodic and tend to be cyclical.  There are lots of highs and lows.  Climate change advocates cherry pick the highs to bolster their case and climate change deniers do the same with the lows.

I think there are two conclusions we can draw from the data.

First, tropical cyclones do appear to be getting somewhat worse.  You can attribute that to climate change or not, but the best bet going forward is that we need to prepare for more frequent and intense storms.  However, there is little in the historical record to support a prediction that the increase will be dramatic.[1]

Second, storms of relatively similar strengths are causing much more damage now than in the past because there are so many more people and so much more property in their paths.

Both of these conclusions support policies that would better prepare our coastlines to withstand storms and to mitigate their impact.  That means building structural protections, improving flood control, adopting better building codes, being more careful where we build things and restoring natural features that dampen the impact of the storms, e.g., wetlands and oyster reefs.

While pursuing policies which will reduce carbon emissions is a laudable goal, there is no evidence that policies to that end will have any short or medium term effect on reducing the frequency or severity of tropical cyclones.

Of course, it is not an “either-or” choice.  We can work to make our coast lines more resilient and to reduce carbon emissions at the same time.  But if the goal is to reduce the impact of tropical storms in the short or medium term, we need policies primarily aimed at strengthening our coastal defenses.

[1] The issue of whether tropical cyclones are getting worse and the effect of climate change on tropical storms is a topic that is hotly debated within the meteorological community.  Ryan Maue, a well-known climate change denier, has made the case that frequency and intensity of tropical cyclones has actually declined in recent years. [Click   here to see his data and argument.]  However, the conclusion that storms are gradually getting more frequent and intense represents the consensus opinion of the meteorological community.  [Click here.]

If you would like to be added to our distribution list, please send your email address to here and we will add you to our distribution list.