Brace for impact: The case for Category 6 hurricanes

With climate change supercharging tropical cyclones, should we extend the hurricane rating scale to reflect new risks?

A new study in Proceedings of the National Academy of Sciences makes the case that global warming is leading to tropical cyclones so intense they warrant a new designation: Category 6. Note that “tropical cyclone” is the general name for storms that are called hurricanes in the Atlantic and typhoons in the Pacific, and they are often simply referred to as TCs.

TC intensity is ranked on the Saffir-Simpson Hurricane Wind Scale from Category 1 to Category 5 based on their maximum sustained winds. The scale currently ends at Category 5 for storms with winds faster than 157 mph.

But in this new study, researchers argue the scale should be extended to include a Category 6 for exceptionally intense TCs. They make several arguments in favor of this.

First, recent major TCs have reached extreme wind speeds above 86 m/s (192 mph), which the researchers propose as the Category 6 threshold. This includes devastating storms like Patricia, Haiyan, and Goni.

Second, we have a theoretical understanding of what controls the maximum intensity of these TCs. Analysis shows that climate change is increasing this maximum intensity.

Consistent with this, we see trends in hurricane intensity¹ and high-resolution climate models show more frequent Category 6 TCs in warmer future climate scenarios, even with no change in overall number of storms.

Thus, the odds are that we will see more of these monster storms in the future. Should we change the TC scale to account for this?

The argument in favor is that more information is usually better. This principle holds especially true in contexts where decision-making is critical, such as in preparing for and responding to TCs. By having a more granular classification system that includes a Category 6 for tropical cyclones, emergency planners and the general public can have a clearer understanding of the expected severity of a storm.

On the other hand, there’s really no practical difference between a Category 5 and 6 TC — both will wipe your house off the foundation, leaving your dead body floating in a nearby water-filled ditch.

Another objection is a general challenge to the Saffir-Simpson scale’s reliance on wind speed as the primary metric for assessing the destructive potential of TCs. Wind is not the most destructive aspect of a TC — storm surge and rainfall-induced flooding usually account for the majority of damage and fatalities associated with these events.

By the way, if you think storm surge isn’t that dangerous, watch this video of the storm surge from Category 6 Typhoon Haiyan. You do not want to get caught in this:

Focusing on wind speed alone can lead to a misunderstanding of the danger, especially in cases where lower-category storms cause catastrophic flooding and storm surges. Hurricane Harvey, for example, was a Category 4 storm at landfall, but its most devastating effects came from the rainfall and flooding in the Houston area.

The reliance on wind speed also overlooks the size of a storm, which can significantly influence the area impacted and the extent of storm surge. A large, yet lower-category storm can have a more severe impact than a compact, higher-category storm.

Given the maxim “more information is better,” I think adding an additional category is a reasonable suggestion. But what should we call it? “Category 6” is derivative and boring, so I suggest we call it “Category 5 Max”. Like a iPhone Max is better than an iPhone, Category 5 Max would signal that this storm comes with upgraded features: more wind, extra rainfall, and boosted storm surge! Max also works in the same way that 737 Max is both bigger and more dangerous than a 737.

It’s the kind of storm that, upon hearing its name, makes you want to immediately invest in a boat, even if you live in the mountains. Plus, imagine the weather forecasters announcing, “Great news, folks: a Category 5 Max is on its way, and it's bringing its premium disaster package, sponsored by ExxonMobil!”

Update: several people have pointed out that the authors of the paper are not actually suggesting a revision to the scale. They were using their paper to “raise awareness that the wind-hazard risk from storms presently designated as category 5 has increased and will continue to increase under climate change.”

1. The IPCC’s latest report says, “there is substantial literature that finds positive trends in intensity-related metrics in the best-track during the ‘satellite period’, which is generally limited to around the past 40 years”