With the Atlantic hurricane season already underway, millions of people are anxiously eyeing the forecasts, scanning the horizon for signs of a brewing storm.
This year is expected to be another intense season. Ocean temperatures remain staggeringly high, and conditions in the Pacific are primed to boost Atlantic storm activity.
But beyond this season’s outlook, something more profound – and more alarming – is happening to tropical cyclones across the globe.
As global temperatures rise due to human activity, climate change is supercharging the storms that lash our coasts. They’re becoming wetter, more intense – and in some cases, so powerful that our current system for classifying them is no longer fit for purpose.
Yes, you read that right. Some experts now say that Category 5 hurricanes – the most severe storms on the Saffir–Simpson scale – are no longer the ceiling. The storms of the future may demand a new category entirely.
“It’s an argument that has been made several times, and I think it’s a good argument,” Dr Tom Matthews, a senior lecturer in environmental geography at King’s College London, tells BBC Science Focus.
“We do need a new category because we’re extending so far into Category 5 on the Saffir–Simpson scale that it’s misleading to call it a Category 5.”
How do we categorise hurricanes?
Hurricanes are currently classified using the Saffir–Simpson scale, which is based on sustained wind speeds:
- Category 1 – 74 to 95 mph (119 to 153 km/h). Very dangerous winds will produce some damage
- Category 2 – 96 to 110 mph(154 to 177 km/h). Extremely dangerous winds will cause extensive damage
- Category 3 – 111 to 129 mph (178 to 208 km/h). Devastating damage will occur
- Category 4 – 130 to 156 mph (209 to 251 km/h). Catastrophic damage will occur
- Category 5 – 157 mph or higher (252 km/h or higher). Catastrophic damage will occur
But climate change is pushing storms well beyond those upper limits. Hurricane Patricia in 2015 reached 215 mph. Hurricane Dorian in 2019 hovered over the Bahamas with sustained winds around 185 mph.
Meanwhile, Typhoon Haiyan, which Matthews points to as a prime example of these next-generation storms, slammed into the Philippines in 2013 with sustained winds of 195mph (314 km/h) and gusts up to 220mph (354 km/h).
These are not the kind of storms we’re used to.
How climate change is impacting hurricanes
You’d be forgiven for thinking that as the planet warms, we’ll get more and more hurricanes. In truth, the picture is more complicated.
“The reason for that,” Matthews explains, “is that as the atmosphere as a whole warms, the upper atmosphere warms faster than the lower atmosphere. That makes it more stable – more resistant to vertical motion, which hurricanes need to get going.”
Think of it like this: hurricanes rely on rising air. But as the upper atmosphere heats up, it becomes harder for warm air near the surface to punch through.
“It’s like trying to take a hot air balloon up when the atmosphere is warmer than your burner is making the inside of your balloon,” Matthews says.
“Another good analogy is that the lid on the upper atmosphere to convection – the upward motion of air that hurricanes need to get started – is becoming stronger, so it’s harder for a hurricane to get started.”
This means fewer hurricanes may form. But when they do, they’ll be explosive.
There’s also another way to think about it, Matthews says. "Hurricanes are a redistribution of heat from the ocean to the atmosphere. Essentially, you need more heat to be distributed before you can push through that lid and cause a hurricane.
“That means they might become less frequent, but when they go, they really go.”
At the same time, rising sea levels mean that even storms of the same strength can now push further inland, causing more widespread damage. “That’s just an inescapable reality, unfortunately,” Matthews says.
Why we may need a new category
Categorising tropical cyclones is about more than simply putting different storms into boxes for our own sakes. With storms growing this strong, the current five-level classification may no longer be enough to be meaningful.
Even within Category 5, there’s now such a wide range that it’s misleading and could hamper preparations.
“Especially with windspeeds, what can sound like a small change equates to huge changes in damage.”
This ramping up of effects is because the force of wind hitting an object is proportional to the square of its speed, while the power is proportional to its speed cubed. In other words, what sounds like a small bump in the numbers can make an enormous difference on the ground.
“What seems like apparently small changes can be catastrophic, or rather, they can lead to really counterintuitive increases in damage, especially if structures are only built to withstand winds up to a certain point.”
It’s a stark warning. As our climate continues to warm, the strongest storms on Earth are getting stronger – and the scale we’ve long used to measure them may no longer be up to the task.
Read more:
- 6 brutal hurricanes could soon hit the US. Here's what that means
- Deadly, unpredictable hurricanes are changing speed around the world. And scientists are worried
- The arrival of mega-hurricanes will show us nature's darkest side. Here's what that means
About our expert
Tom Matthews is a senior lecturer in environmental geography at King's College London in the UK. His research focuses on environments and events that are meteorologically extreme. The former has seen him work extensively in mountain regions, including in the Himalaya, where he co-led the installation of the highest-altitude weather stations in the world on Mt. Everest. For the latter, Tom has led research on severe extra-tropical cyclones and compound events, but he specialises in extreme humid heat events, including their drivers and projected changes under climate warming.