Scientists are warning of a new class of devastating tropical storms that have been ominously named “gray swans.”
In a new study published in the journal Nature Climate Change, researchers simulated thousands of hurricanes that could hit sometime in the next century. The results reveal that future storms could devastate places previously believed to be at low risk for hurricanes, and the destruction from some of these hurricanes would be unlike anything we’ve ever seen, the researchers said.
Gray swan cyclones are extremely rare tropical storms that are impossible to anticipate from historical record alone. They will become more frequent in the next century for parts of Florida, Australia, and cities along the Persian Gulf, researchers are warning. The shallow and warm waters of the Persian Gulf, where cyclones have never been recorded, might generate the storms in the future as a result of global warming. Tampa, Florida and Cairns, Australia, two places where cyclones already happen, would be increasingly vulnerable to extreme storms this century, according to the report.
“You can’t always rely on history” to predict the future, lead author Ning Lin of Princeton University told Reuters of the findings she reached with Kerry Emanuel of the Massachusetts Institute of Technology. “These are all locations where either no one’s anticipated a hurricane at all, such as in the Persian Gulf, or they’re simply not aware of the magnitude of disaster that could occur,” Emanuel said.
Emanuel concedes that the probability of cyclones in the Persian Gulf “is very low but … if you build a nuclear power plant you have to consider these things.”
These extreme tropical cyclones have been dubbed gray swans as a metaphor inspired by black swans. Black swans were considered to be impossible by Europeans until they were found in Australia. These gray swan storms cannot be predicted from history alone.
Past studies have also pointed to risks of abrupt changes in the climate system linked to global warming, including that the Arctic Ocean could be ice-free in the summer or that monsoon rains could veer off track. Jean-Pascal van Ypersele, a vice chair of the U.N.’s Intergovernmental Panel on Climate Change, said the build-up of greenhouse gases from human activities means more energy accumulates in the climate system. “Bad climate surprises may happen,” he told Reuters at U.N. talks in Bonn on a deal to slow climate change.
According to the study, extreme hurricanes that are currently likely to hit Tampa only once every 1,000 years, would occur every 60 to 450 years by the late 21st century. Cairns, Australia was also pointed out as being more vulnerable to worsening storms.
Thus far, a gray swan megastorm has not occurred. Every hurricane that has ever occurred in recorded history could have been predicted by the previous pattern of storm activity. “In the realm of storms, I can’t really think of an example in the last five or six decades that anybody could call a black swan,” Emanuel said. “For example, Hurricane Katrina was anticipated on the timescale of many years. Everybody knew New Orleans was going to get hammered. Katrina was not meteorologically unusual at all.”
However, scientists are anticipating that as global warming significantly alters the Earth’s atmosphere and oceans, the track and magnitude of hurricanes may skew widely from historical patterns.
The researchers explained that the storm surges from gray swan storms could be devastating. “A storm surge of 5 meters is about 17 feet, which would put most of Tampa underwater, even before the sea level rises there,” Emanuel says. “Tampa needs to have a good evacuation plan, and I don’t know if they’re really that aware of the risks they actually face.”
Emanuel went on to explain that Dubai, and the rest of the Persian Gulf, have never experienced a hurricane in recorded history. Therefore, any hurricane, of any magnitude, would be an unprecedented event. “Dubai is a city that’s undergone a really rapid expansion in recent years, and people who have been building it up have been completely unaware that that city might someday have a severe hurricane,” Emanuel said. “Now they may want to think about elevating buildings or houses, or building a seawall to somehow protect them, just in case.”
The team also found that as storms grow more powerful in the coming century, with climate change, the most extreme storms will become more frequent. “Hurricanes, unlike earthquakes, are like a roll of the die,” Emanuel said. “Just because you had a big hurricane last year doesn’t make it more or less likely that you’d have a big hurricane next year.”
But in 100 years, Tampa’s odds of a 6-meter storm surge will be 14 times higher, as the world’s climate shifts. “What that really translates to is, you’re going to see an increased frequency of the most extreme events,” Emanuel explained. “Whereas the upper limit of hurricane wind speeds today might be 200 mph, 100 years from now it might be 220 mph. That means you’re going to start seeing hurricanes that you’ve never seen before.”
Though the likelihood of gray swan storms is small, climate change will significantly raise the likelihood of gray swan storms hitting. And understanding this is important when planning future city infrastructures.