On the morning of 22 September 2017, the small Caribbean territory of Puerto Rico lay battered and broken. During the preceding 48 hours, the island had been besieged by Hurricane Maria, its 250km/h (155mph) winds tearing through everything in its path, leaving behind a trail of devastation and despair.
As Maria departed, the island’s people emerged from their places of shelter to find homes reduced to rubble and entire communities – once vibrant and energetic – decimated. In the space of just two days, Maria had taken the lives of 2,975 Puerto Ricans, and caused an estimated $94.4 billion (approx £75.5bn) in damages.
The island’s infrastructure, already beleaguered following decades of neglect and economic struggle, simply could not withstand the ferocious power of Maria. Crops were destroyed, the electrical grid was crippled for 11 months and communications networks collapsed.
Roads, bridges and entire neighbourhoods were flooded, making rescue operations more challenging and leaving thousands without access to sufficient medical care, clean water or food.
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Maria was the most powerful hurricane to hit Puerto Rico in 89 years. The island, despite being in a hurricane hotspot, was underprepared and under-resourced. It was still coming to terms with the ramifications of Hurricane Irma, which had hit the island two weeks earlier, causing $1bn (approx £800m) in damages. When Maria arrived, Puerto Rico was already on its knees.
Maria is one of the deadliest hurricanes of the 21st century to date. Though in some ways anomalous (Puerto Rico hadn’t been hit with anything as powerful since the Okeechobee hurricane of 1928, which claimed the lives of an estimated 3,000 people) the arrival of a hurricane on such a scale didn’t surprise many in the scientific community.
In the years leading up to Maria’s rampage, experts from around the globe had been sounding the alarm, consistently arguing that, with global temperatures rising, hurricanes were not only liable to become more frequent, but more destructive.
The era of the mega-hurricane, they warned, was upon us.
The next generation of hurricane
Developed in 1971 by civil engineer Herbert Saffir and meteorologist Robert Simpson, the Saffir-Simpson hurricane wind scale is used to classify hurricanes into five categories based on the intensity of their sustained winds.
A hurricane is deemed to be Category 5 if it has wind speeds greater than 251km/h (156mph), meaning that when Maria hit Puerto Rico, it was technically Category 4. Yet this was far from Maria at her most powerful.
In the days prior to making landfall, Maria’s winds peaked at 282km/h (175 mph), placing the hurricane firmly in Category 5 territory. But even though Maria attained staggering wind speeds, it wasn’t even that month’s most powerful hurricane. Irma was recorded as having sustained winds of 298km/h (185mph) for 37 hours, making it one of the most powerful hurricanes ever observed.
In the 53 years since the Saffir-Simpson scale was introduced, there have been 24 recorded Category 5 Atlantic hurricanes. The fact that eight of these have occurred in the last eight years has, in part, led eminent names in meteorology to propose the scale be updated to include a sixth category.
They argue that, with the planet continuing to warm, Category 5 hurricanes are likely to become more prevalent and more powerful, with wind speeds well in excess of 253km/h (157 mph).
A leading voice in this camp belongs to James Kossin, an independent consultant specialising in climate risk and extreme weather events. In early 2024, Kossin, alongside Dr Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory in California, published a paper in the journal Proceedings of the National Academy of Sciences suggesting that the Saffir-Simpson scale be amended, and a sixth category introduced.
“Hurricane formation and intensification are governed by their environment, which in turn is governed by climate variability and change,” Kossin says. “When a hurricane forms, greenhouse gas warming makes it more likely to become a major hurricane.”
In their paper, Kossin and Wehner said that adding a sixth category would “communicate that climate change has caused the winds of the most intense [tropical cyclones] to become significantly higher,” which could help at-risk regions better comprehend, and better prepare for, the impacts of these more powerful hurricanes.
“The potential for damage increases exponentially with wind intensity,” Kossin adds. “If wind intensity doubles, the damage potential doesn’t double, but rather increases by eight times or more. The major hurricanes are responsible for the most damage and mortality, by far.”
The Saffir-Simpson scale
As it stands, the Saffir-Simpson scale is a five-point ranking system of hurricane intensity. Some experts argue that the scale needs to be expanded, but for now, the categories are as follows:
Category 1 - Wind speeds: 119-153km/h (74-95mph). Winds strong enough to cause minimal amounts of damage. Coastal flooding possible.
Category 2 - Wind speeds: 154-177km/h (96-110mph). Winds capable of causing extensive damage to vehicles and trees. Coastal flooding and roof damage likely.
Category 3 - Wind speeds: 178-208km/h (111-129mph). Winds likely to cause significant, potentially devastating damage to buildings. Major coastal flooding and extensive infrastructural damage to be expected.
Category 4 - Wind speeds: 209-251km/h (130-156mph). Winds liable to cause catastrophic damage. Severe coastal flooding and the complete destruction of small buildings very likely.
Category 5 - Wind speeds: 253km/h (157mph) or greater. Winds likely to cause catastrophic damage to homes, trees, electrical grids and entire communities. Extensive coastal flooding almost guaranteed.
How to brew a hurricane
A number of key environmental conditions are needed for hurricanes (and mega-hurricanes) to materialise. These include sea surface temperatures of at least 26.5°C (79.7°F), high levels of atmospheric moisture, low wind shear [change in wind velocity/direction over a short distance], and a degree of atmospheric instability, which allows moist air to rise rapidly and create strong updrafts.
Climate variability can, and does, influence both the frequency and intensity of hurricanes. The warmer the ocean, the more likely it is that a hurricane will form.
“The most obvious climate-related change is the increase in ocean temperatures around the tropics,” Prof Jenni Evans, director of Pennsylvania State University’s Institute for Computational and Data Sciences, explains. “Since hurricanes draw their energy from the ocean, and the energy available increases as the ocean warms, these increasingly warm temperatures increase the possibility of stronger storms.”
Prof Ralf Toumi, co-director of the Grantham Research Institute on Climate Change and the Environment in London, adds that global warming is, essentially, leading to the creation of mega-hurricane breeding grounds. “The warming of the ocean has contributed to an increase in the mega-hurricane frequency and their intensity. The ocean supplies moisture and thus heating to the hurricane. A warmer ocean provides more fuel.”
In 2015, Evans, along with Dr Alex Kowaleski, a renowned catastrophe scientist and meteorologist, carried out a study designed to better comprehend the maximum Potential Intensity (PI) of hurricanes. The pair wanted to ascertain just how powerful a hurricane could become if everything in its environment supported intensification.
Unlike previous research in this area, much of which had focused on the difference between ocean and upper atmosphere temperatures, Evans and Kowaleski’s work focused on the conversion of thermal energy to mechanical energy, a process that occurs when air spirals into a hurricane's centre.
They concluded that, particularly in the context of climate change, hurricanes observed to reach their PI are only liable to become more intense – and more dangerous – as the ocean becomes warmer.
“Because the intense winds of a hurricane mix the ocean waters down and create large waves, the water needs to be anomalously warm through the ocean mixed layer [the top layer of the ocean that is ‘stirred’ due to wind, waves and heat] at least down to the thermocline [the boundary between warmer surface waters and colder, deeper waters]. If nothing else changes, the frequency of very intense hurricanes will increase,” Evans warns.
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Areas at risk
“Mega-hurricanes are most likely to form in the tropical western Pacific where the background ocean temperature is already high,” says Prof Pao-Shin Chu, State Climatologist at the University of Hawaii. “In the eastern Pacific, the coast of Mexico is another area with high sea surface temperature in the [northern hemisphere’s] summer and autumn.”
Coastal areas and islands – especially those already in hurricane hotspots – will, Chu suggests, continue to be most exposed, and will likely struggle even more than they do now. Due to a chronic lack of resources, many at-risk areas will find it difficult to contend with the increasingly lethal combination of powerful winds and heavy rainfall. And poorer regions seem set to suffer the most.
“The most vulnerable communities are developing countries, as they have the least resilience,” says Toumi. “The geographic impact could spread more poleward in general, but the strongest hurricanes are likely to remain in the areas already seeing them.”
Kossin agrees. “In addition to formation and intensification, climate affects hurricane tracks as well. Greenhouse gas warming causes the tropics to expand poleward, and with the tropics come tropical cyclones. This track shift is especially strong in the western North Pacific, and Southeast Asia and Japan are being impacted.”
Around 10,000 people die annually as a direct result of hurricanes and tropical storms, and they cause damage to the tune of around $100bn (£80bn) annually in the US alone. These numbers will likely soar as mega-hurricanes become more common. All of which begs an obvious question: what can be done?
“There’s still much that needs to improve in terms of the forecasting,” Toumi says. “The strongest hurricanes undergo rapid intensification and yet current weather prediction models don’t capture this well. This leads to surprises at landfall, which can add to the damage.
“Countries need to invest in early warning systems – weather forecasts coupled to evacuation plans. This will limit the loss of life. Physical damage is unavoidable for the strongest storms, but improved building design standards will help. Reducing fossil fuel emissions reduces the risk of damage by stabilising the climate.”
Damage limitation
Frank Lombardo, an affiliate professor of atmospheric science and civil and environmental engineering at the University of Illinois, is all too aware of the scale of the challenges that lie ahead, and recognises that there’s no magic bullet.
“A number of effective – both in terms of cost and outcome – and fairly simple strategies can be used [by individuals] to mitigate impacts. For winds, strengthening openings such as garage doors and windows to protect from debris and wind pressure. Elevating your structure above some certain level will allow water and associated waves from storm surge to pass under the main structure. Also, rainwater may be a major issue, and the use of improved water-resistant barriers on roofs and walls can keep your home from suffering significant losses.”
Small-scale defences such as these undoubtedly have a part to play, but when it comes to expansive, sweeping solutions designed to protect entire populations, Lombardo admits that things become more complex.
“As far as innovative solutions,” he continues, “there has been a lot of work lately on looking at nature-based solutions to mitigate losses or to strengthen infrastructure. One in particular is the use of mangroves to significantly reduce the impacts of storm surge in hurricanes. On the wind side, more effort, especially on the research side, needs to be made to investigate innovative mitigation solutions.”
Mangrove roots can certainly limit the impact of a storm surge by absorbing a portion of its energy. A study published in the journal Nature in 2020 determined that “mangroves can reduce up to 66 per cent of wave energy in the first 100m (approx 110 yards) of forest width,” and can also provide “adaptive defences” owing to the fact that, when allowed to develop unimpeded, they’re able to grow vertically at a speed that aligns with sea-level rise.
Worryingly, though, research carried out by the UN Environment Programme World Conservation Monitoring Centre discovered that between 1996–2020, global mangrove coverage decreased by around 3.4 per cent, though it’s thought this decline has now largely been halted.
Prof Tim Dixon, director of the University of South Florida’s Natural Hazards Network, is another mangrove advocate. “Communities in tropical and sub-tropical areas who have been smart enough to leave dense mangroves fringing their coastlines do get some flood relief, but too often these are cut down by developers and people who want a better ocean view.”
Raise or retreat
Mangroves aside, there is one flooding solution, albeit an incredibly costly one, that Dixon believes is more effective than any other: elevation. But he readily admits this is not a suitable option everywhere.
“For coastal cities, and a few inland ones, elevation is everything when it comes to flood hazard. Typical storm surges are 1–5m (3–16ft) above mean sea level these days, so if you want to be safe, build above that level, and elevate buildings if necessary.
“Formerly safe cities are now at risk of flooding. Any coastal city can get hit with a big storm, and if it has subsidence, you’ve increased your chances of both catastrophic and non-catastrophic flooding. For some coastal areas, in my opinion – and people may not want to hear this – the only real viable solution may be managed retreat from the most vulnerable areas.
“At this point, we’re committed to sea level rise over the next 50–100 years that will likely approach 1m (approx 3ft). So, for infrastructure supposed to be long-lasting, we need to start thinking long-term and take that into account for planning purposes.”
Dixon has reservations about the effectiveness of some coastal defences, and notes the importance of ensuring money is only invested in projects and solutions that will provide benefits long into the future. “The history of sea walls, for example, is similar to that of levees – there are only two types: those that failed and those that will fail. Nature-based solutions and natural shorelines are probably the lowest cost way to proceed.”
Lombardo is of a similar mind, adding that ensuring all new buildings and infrastructure in at-risk areas are built to the highest, most hurricane-resistant standards is critical.
“The risk from hurricanes due to climate change and increased exposure of coastal communities will only continue to rise,” he says. “Traditional and innovative solutions used for individual structures and communities, as well as the improvement of building codes in coastal communities, all have a role to play as we adapt to these events.”
Time for action
A current American Society of Civil Engineers initiative, which Lombardo is involved with, is looking at how the US will be able to handle future threats due to climate impacts. And, though Lombardo is adamant building codes help, he says they’ll have to be “adopted and enforced by local jurisdictions to make their way into construction practices and yield a quantifiable difference.”
For Chu, however, there’s only one thing that will bring about lasting, tangible security, and it’s something that, as things stand, looks far from attainable. “There’s nothing we can do to mitigate [mega-hurricanes] unless we cut greenhouse gas emissions drastically. The ocean is becoming warmer and that sets the stage for mega-hurricanes.”
Just like Puerto Rico, growing numbers of coastal regions and islands around the world are now facing a future that’s likely to bring with it death and destruction on a larger scale.
Nations that are able to invest heavily in defences and rehousing initiatives may, for a time, be able to limit the impact of mega-hurricanes, but countries without access to sufficient resources will, as is all too often the case, endure the greatest hardships.
Mega-hurricanes are already here, and it’s more essential than ever that policymakers, researchers, construction companies and communities band together, pool their knowledge and ensure they’re all working towards the same goal. Only then will it be possible to contemplate a more resilient future.
About our experts
James Kossin is an independent climate scientist and extreme weather events expert. His work has been published in Proceedings of the National Academies of Sciences, Nature, and Geophysical Research Letters.
Prof Jenni Evans is the director of Pennsylvania State University’s Institute for Computational and Data Sciences. Her work has been published by Bulletin of the American Meteorological Society, Weather and Forecasting, and Journal of Climate.
Prof Ralf Toumi is the co-director of the Grantham Research Institute on Climate Change and the Environment in London. He's also advisor to the WMO Asia-Pacific Typhoon Collaborative Research Center and is on the Japan typhoon dropsonde program board. You can find his work published in Scientific Data, Journal of Physical Oceanography, and Nature Communications.
Prof Pao-Shin Chu is a State Climatologist at the University of Hawaii. His work has been published in Nature, Environmental Research Letters, and Journal of Climate.
Frank Lombardo is an affiliate professor of atmospheric science and civil and environmental engineering at the University of Illinois. You'll find his published work in Journal of Wind Engineering and Industrial Aerodynamics, Journal of Risk and Uncertainty in Engineering Systems, and Journal of Structural Engineering.
Prof Tim Dixon is the director of the University of South Florida’s Natural Hazards Network. His work has been published in Geophysical Research Letters, International Geology Review, and Earth and Planetary Science Letters (to name a few journals).
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