A decade after the storm, Hurricane Katrina remains the deadliest and costliest natural disaster in recorded U.S. history. While it's unclear whether the fallout could have been avoided, the tempest was definitely predicted: Based on the region’s long history of tropical storms, climate analysts had previously estimated that a devastating storm had a strong chance of striking the U.S. Gulf Coast. And in the days before Katrina made landfall, the National Weather Service issued warnings of the potential destruction that could be wrought due to the storm's strength combined with the flood levees' weaknesses.
But according to a new model, climate change may cause an increase in less predictable “grey swan” tropical storms around the world. If it bears out, the model suggests that Tampa, Florida; Cairns, Australia and the Persian Gulf are all at risk of highly damaging hurricanes sometime in the next century.
Statistically speaking, a black swan is an event, like a storm, that comes as a total surprise. By contrast, grey swan storms are harder to see coming based on historical information alone, but they could potentially be predicted using a combination of historical records and knowledge of an area's changing physical conditions, from the local climate to rising sea level.
For their work, Ning Lin of Princeton University and Kerry Emmanuel of MIT used a model that combines climate data and hydrodynamics to examine the statistical likelihood of hurricanes and their possible storm surges, the push of water above normal tide levels that can be one of the most fatal aspects of tropical cyclones.
As they report this week in Nature Climate Change, Lin and Emmanuel focused their work on Florida, Australia and the Persian Gulf because these areas have highly vulnerable coasts. They chose to study Tampa in particular because it sits on low-lying land surrounded by a bay of shallow water, putting it at high risk for damaging storm surges.
Their model predicts that Tampa could experience hurricane storm surges of about 20 feet today and 36 feet by the end of the century. Because Tampa has not experienced a major tropical storm since 1921, people may have forgotten that the city is vulnerable to such extremes, says Lin.
Cairns is at risk of grey swan storms because of its own history of tropical cyclones and its proximity to areas that have experienced some of the most destructive recorded storms in the Southern Hemisphere. Though Cairns itself has been hit by 53 tropical cyclones since it was founded in 1876, the storm surges so far have been less than three feet. But according to Lin and Emmanuel’s research, the next major storm could produce surges of almost 19 feet.
The hot, shallow water and high levels of saline in the Persian Gulf are the perfect ingredients for cooking up extreme tropical cyclones and high storm surges. The new model predicts that a strong storm could create a surge of up to 13 feet today and up to 23 feet by the end of the century, which could impact major cities such as Dubai, Abu Dabi and Doha. But any storm that hits this highly susceptible region would be considered a grey swan because there are no tropical cyclones in its historical record.
Lin and Emmanuel also predict that climate change will cause grey swan storm surges to increase in strength and frequency in the coming decades.
“So the message here," says Lin, "is [we] really should try to incorporate physical knowledge, indirect information and models together with historical records and experience to predict those extremes, and then prepare for them as much as we can rather than wait for the consequences.”