An increased likelihood of extreme tropical storms, unpredictable from the historical record alone, is expected in certain regions, according to a study published online in Nature Climate Change. The increased risk of such ‘grey swan’ storms will impact Cairns, Australia; Tampa, Florida and cities on the Persian Gulf.
Infrequent and high-impact storms that cannot be anticipated have been described as ‘black swans'. In contrast, grey swans are extreme tropical cyclones that can induce devastating storm surge impacts beyond what might be considered plausible based on the historical record alone, but that may be foreseeable using physical knowledge together with historical data.
Ning Lin and Kerry Emanuel modelled the risk of storm surge from grey swan events for three highly vulnerable coastline regions. They identify large potential risks in the Persian Gulf, where tropical storms have never been recorded, as well as larger than expected threats in Tampa and Cairns. The analysis shows that surges generated by simulated grey swan storms striking Tampa, Cairns, and Dubai today could reach as high as 6, 5.7 and 4 metres, respectively. Moreover, the simulations indicate the possibility of storm surges up to 7 and 11 metres for Dubai and Tampa, respectively, by the end of the century.
Grey swan storm surges are not only projected to become more powerful over coming decades, but the risk of them occurring is expected to increase due to climate change. Thus, the return time of a grey swan storm with 6-metre storm surge for Tampa is estimated to fall from around 10,000 years today to 3,100-1,100 years by mid-century, with a further fall to 2,500-700 years towards the end of the century. This means that the likelihood of such a storm in any given year will be between around 4 and 14 times higher at the end of the century than it is today.