October 15, 2020

Are Tropical Cyclones Becoming More Frequent and More Intense?

Are Tropical Cyclones Becoming More Frequent and More Intense?

With Hurricane Delta in our rearview mirror, the 2020 hurricane appears to finally be over.  Officially the season goes through the end of November, but storms beyond mid-October are rare.  True to form for 2020, this season has been the worst since 2005 and the second worst since we began keeping record.

A few weeks ago, I was listening to a Kinder Institute webinar on public health and urban planning.  However, a large part of the program turned out to be an infomercial on climate change.  The presenter, who was from California, mentioned in an almost off-handed fashion, that, of course, Houston had been and would continue to be facing more and stronger hurricanes because global warming.  This narrative has become a standard talking point for climate change activists.  But, it is true?

Trying to get a handle on whether storms are actually getting worse or more frequent is not as clear as you might expect.

First, exactly how you measure the intensity of a storm is somewhat problematic.  Hurricane Sandy in 2012 caused extensive damage to the East Coast and was frequently referred to in the media as a “superstorm.”  Yet it was only a Category 1 storm on the Saffir-Simpson scale.  That scale is exclusively determined by wind speed.  (See this link for discussion of that 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.

One complicating factor is that the farther we go back into historical records, the less reliable the information is about the incidence and the severity of the storms is.  Since the 1960s we have had reliable data from satellites.  But that is a blink of the eye on the scale of climate history.  Before satellites the data is much less reliable, relying frequently on incomplete eye witness 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.

When you graph the data, it looks like this:

Even if you assume that cyclones were underreported before 1960, the upward trend since the early 90s is pretty clear and appears to be unprecedented in the historical record.  It is noteworthy that trend is more pronounced in terms of the total number of storms rather than an increase in more severe storms.

But is this increase due to climate change and will it continue or even accelerate, as we so often hear?  Here is where things become much murkier.

The Geophysical Fluid Dynamics Laboratory, a NOAA agency, has recently updated its summary of the research on these questions on its website.  Its conclusions regarding storms in the Atlantic basin are:

  • Warmer temperatures will likely increase cyclonic rainfall by 10-15% over the next century.
  • Warmer temperatures will likely increase the intensity of tropical cyclones 1-10% over the next century.
  • It is premature to conclude with high confidence that increasing atmospheric greenhouse gas concentrations from human activities have had a detectable impact on Atlantic basin hurricane activity.

The report concludes as follows: “In summary, neither our model projections for the 21st century nor our analyses of trends in Atlantic hurricane and tropical storm activity support the notion that greenhouse gas-induced warming leads to large increases in either tropical storm or overall hurricane numbers in the Atlantic.”

In other words, “we think it is going to get marginally worse.”

Part of the problem in attempting to tease out the effect of warmer temperatures on tropical storms is that this is just one of many factors that affect these enormously complex phenomena.  One of the main drivers is the La Nina/El Nino weather patterns over the Pacific Ocean.  The effect of higher temperatures on that phenomenon is not well understood at this point.  In another note, I was fascinated to see the NOAA report mention that volcanic activity has some effect.

But one thing that is for certain: The cost of tropical storm landfalls in the U.S. is becoming astronomically higher.  However, the increased costs are not being driven by more intense or frequent storms – instead they are due to us putting more people and improvements in harm’s way.  In this Wall Street Journal article discussing the rising cost of storms,  meteorologist and risk manager expert Steve Bowen summarized the problem: “Coastal population and exposure growth is certainly the predominant driver of increased damage costs associated with hurricanes.”

The best example of this is Hurricane Sandy, which hit the New Jersey/New York area in 2012. It made landfall barely weighing in as a Category 1 storm.  Yet, it was one of the costliest and deadliest in U.S. history because of where it made landfall.  Had Sandy made landfall along a coastline with far less population, no one would have referred to it as a “Superstorm.”

Here is where I part ways with many climate change activists.  While pursuing policies to reduce carbon emissions is certainly a laudable goal, it is not the only problem we face.  It is also not the answer to every problem.  In this case, there is no evidence that policies to reduce greenhouse emissions will have any consequential effect on reducing the damage done by tropical cyclones, and especially not in the near term.

To tackle the problem of hurricane impact mitigation, we need to 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, not incentivizing risky behavior by socializing losses (e.g., subsidized federal flood insurance), and restoring natural features that dampen the impact of the storms, such as wetlands and oyster reefs.

Of course, it is not an “either-or” choice.  We can make our coast lines more resilient and work to reduce carbon emissions at the same time.  But if the goal is to reduce the impact of tropical cyclones, a discussion about climate change is a distraction.  Instead, we need policies primarily aimed at strengthening our coastal defenses and being smarter about coastal development.

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