It is highly unlikely that the Gulf region will ever experience a storm so devastating that our coastal areas are submerged. Neverthless, it is the job of infrastructure experts to plan and prepare for the unthinkable.
Scientists have spelt out a future scenario for the Arabian Gulf that could include “high-consequence” weather events that could leave large parts of the region under water.
But before anyone starts heading for the high ground or piling up the sandbags, the chances of this happening are not likely in anyone’s lifetime.
In fact, the most serious storm, the scientists calculate, might be expected just once every 30,000 years – many times longer than recorded human history.
In a paper published last week in the journal Nature Climate Change, scientists described events called “grey swans”, “high-consequence events that are unobserved and unanticipated but which may nevertheless be predictable”.
The event in question is a “high-impact” tropical cyclone, bringing with it a storm surge that could leave vast areas of Gulf countries under between 5.6 and 9.5 metres of seawater.
To set that in context, the very worst case scenario seen as a consequence of climate change and melting ice caps would be a gradual nine-metre rise in global sea levels by 2100.
But given the almost inconceivable infrequency of such events, why the concern?
The reason, the authors of the report say, is that when decisions are made about buildings and infrastructure designed to serve dozens, and in some cases, possibly hundreds of years, even the most distant, unlikely problems should be considered.
The damage to property, trade and tourism would be enormous, not to mention to the Gulf’s fragile coastal ecological systems.
It’s a scenario that seems as outlandish as it is terrifying, given no such storm has been experienced in the Arabian Gulf since records began in 1945.
But researchers at Princeton University and Massachusetts Institute of Technology claim it is not out of the question.
They say that thanks to a complex computer modelling system that draws on a wide range of global meteorological data, it is predictable.
There is, the authors admit, a very low probability of such an extreme, grey-swan tropical cyclone striking the region.
They estimate the chances of “the big one” happening only once in 30,000 or more years, although these odds decrease for lower levels of inundation. A four-metre surge, for example, might come round once every 10,000 years.
The prognosis, says Ning Lin, a professor of civil and environmental engineering at Princeton, is much more than an academic exercise in estimating “non-negligible probabilities”, and should be taken very seriously.
“You might think such events are just too unlikely,” she says. “But think about engineering design. For important facilities the design actually requires you to design for these very long return periods.”
The paper presents a series of grey-swan scenarios for the Gulf, including a maximum surge of 9.5 metres, depending on local geophysical features.
The decision to study the risk of a grey swan tropical cyclone striking in the Gulf grew out of earlier research to assess the risk of storm-surge damage to New York City. This was calculated using complex computer modelling based on a wide range of meteorological and historical data, including detailed worldwide observations, dating back to 1948, compiled by the US National Centres for Environmental Prediction and Atmospheric Research.
As the pioneering models could be applied to any coastal city, the authors decided to run the numbers for three other locations – Tampa in Florida, Cairns in Australia and the Gulf.
They picked these places, says Prof Lin, because scientifically they were “very special locations: x have geophysical features that make them vulnerable but on the other hand they have never experienced extreme events”.
Although no tropical cyclones have been recorded in the Arabian Gulf, there was a very near miss in June 2007, and it was this that drew scientific attention to the issue.
Cyclone Gonu, born as a mere storm in the Arabian Sea, was the first in the region to achieve Category 5 hurricane status – “the very peak of possible storm strengths”, as Nasa reported, with winds in excess of 252kph.
Heading north-west, by the time it smashed into the coast of Oman it had lost some of its strength, but still managed to kill more than 70 people and cause damage estimated at US$4.4 billion (Dh16.16bn).
Gonu went on to hit Iran, but for a while it was touch and go whether it would be funnelled through the Strait of Hormuz. It would, thought the authors of the new paper, be “scientifically interesting and socially important to ask if such a strong tropical cyclone can travel into the Gulf”.
But when they used global climate data from 1980 to 2010 to simulate 3,100 “synthetic surge events” in the Gulf, “we got a surprise”, says Prof Lin.
Although, as expected, a small number of the cyclones did originate in the Arabian Sea and make their way through the Strait of Hormuz, they found that most originated in the Gulf. Moreover, these were the ones that caused the most extreme surges.
And the computer modelling showed that, thanks partly to the nature of the Gulf’s hot, shallow seas, any tropical cyclone born there would be likely to outgun anything previously seen anywhere else in the world.
The worst surge predicted was the result of a “synthesised” tropical cyclone “with extremely high intensity” – a wind speed of an incredible 410kph. That’s 60 per cent greater than a Category 5 hurricane or tropical cyclone, the highest category there is, achieved when winds exceed 252kph.
The authors noted that it would be “far beyond the highest observed tropical cyclone intensity worldwide” – Typhoon Haiyan, which in 2013 generated winds in excess of 315kph and devastated parts of the Philippines.
The freakishly high winds anticipated in the Gulf would be made possible by “the area’s high sea surface temperature and the deep dry temperature profiles characteristic of desert regions”, compounded by recorded reductions in “vertical wind shear”, a meteorological phenomenon that normally holds tropical cyclones in check but appears to be on the decline in the region.
Under these circumstances, any cyclone born in the north-west of the Gulf would be unimpeded and free to build up speed.
So what should be done? That, says Prof Lin, depends on any given society’s appetite for risk.
“Do you tolerate a probability of 0.01 of getting flooded in a year, or only 0.001?”
Clearly, she says, you don’t need to build seven-metre-high levees to protect against the seven metres of surge predicted by this study. That would be impractical and vastly expensive.
“But if a particular facility or infrastructure is very important, you will want to consider the extremes we are providing you with, carry out a cost-benefit analysis and find a balance that works for your situation.”
Either way, as Prof Lin and her colleagues made clear in a complex cost-benefit analysis they carried out for New York City last year, the important thing is not to take comfort in the improbability of a grey goose taking flight.
“Uncertainty is inherent to such estimations,” they wrote. “But it should not be used to justify not doing anything.”
