Tropical storms are hurricanes that develop over warm ocean waters and can move onto land, flooding coastal areas. The most intense storms can produce destructive storm surges — seawater elevated by tides and waves that inundate land, leading to disastrous flooding in coastal regions. A recent investigation by MIT researchers reveals that, as global temperatures rise, the frequency of harmful storm surges will rise tenfold for one of the most adversely affected areas across the globe.
In a research article published today in One Earth, the researchers indicate that, for the densely populated coastal nation of Bangladesh, an event that once occurred every century could now happen every 10 years — or even more frequently — by the century’s end.
In a scenario where fossil fuels continue to combust as they do at present, what was once deemed a catastrophic, once-in-a-century storm surge will strike Bangladesh, on average, every decade. Furthermore, storm surges that used to happen approximately every decade will likely assault the country’s coastline more regularly, every few years.
Bangladesh ranks among the most densely populated nations worldwide, with over 171 million individuals residing in an area roughly equivalent to the size of New York state. The nation has historically been susceptible to tropical storms, as it is a low-lying delta that is prone to flooding from storms and experiences a seasonal monsoon. Some of the most devastating floods globally have taken place in Bangladesh, where agricultural economies have increasingly struggled to recover.
The research also indicates that Bangladesh will likely encounter tropical storms that coincide with the prolonged monsoon season. Historically, storms and the monsoon occurred at distinct times of the year. However, as global temperatures rise, the researchers’ models indicate that storms will intrude into the monsoon period, leading to consecutive flooding occurrences throughout the nation.
“Bangladesh is actively engaged in preparing for climate-related hazards and threats, but the challenge lies in the fact that their efforts are largely based on current climatic conditions,” states study co-author Sai Ravela, principal research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “We are now witnessing an almost tenfold increase in the frequency of damaging storm surges almost everywhere in Bangladesh. This issue cannot be overlooked. Hence, we believe it is crucial for them to reassess how they safeguard against these storms.”
Ravela is joined by co-authors Jiangchao Qiu, a postdoctoral researcher in EAPS, and Kerry Emanuel, an emeritus professor of atmospheric science at MIT.
Magnitude of tides
In recent times, Bangladesh has made significant investments in storm readiness, such as enhancing its early warning systems, strengthening village embankments, and improving access to community shelters. However, such measures have typically been predicated on the existing storm frequency.
In this latest investigation, the MIT team aimed to deliver comprehensive forecasts of severe storm surge risks, which are flooding events where tidal influences amplify cyclone-driven storm surges, for Bangladesh under various climate warming scenarios and sea-level rise predictions.
“Many of these events occur at night, so tides play an extremely important role in determining the additional water levels, depending on the tide,” explains Ravela.
To assess the threat posed by storm surges, the team first utilized a physics-based downscaling method, which Emanuel’s team developed over two decades ago and has utilized since to explore hurricane activity in various regions globally. This approach involves a low-resolution model of the global ocean and atmosphere that is integrated with a finer-resolution model capable of simulating weather phenomena as intricate as an individual hurricane. The researchers then introduce hurricane “seeds” into a targeted area and run the model forward to track which seeds develop and make landfall over time.
To the downscaled model, the researchers added a hydrodynamic model that simulates the height of a storm surge, considering the wind patterns and intensities at the time of a particular storm. For each simulated storm, the team also monitored the tides, along with the effects of rising sea levels, and combined this data into a numerical model that computed the storm surge height, accounting for tidal impacts as a storm arrives onshore.
Intense overlap
Utilizing this framework, the scientists simulated tens of thousands of potential tropical storms near Bangladesh, under numerous future climate scenarios, varying from one that mirrors current conditions to one where the world sees further warming due to ongoing fossil fuel combustion. For each simulation, they documented the highest storm surges along the coast of Bangladesh and recorded the frequency of storm surges of differing heights in a given climate scenario.
“We can examine the entire set of simulations and ascertain how many storms correspond to a particular storm surge height, for instance, 3 meters, from which we can deduce the relative frequency of that type of storm,” says Qiu. “Subsequently, you can reverse that number to determine a return period.”
A return period refers to the interval it takes for a storm of a specific kind to strike land again. A storm categorized as a “100-year event” is typically more intense and destructive, producing more extreme storm surges and thus, more severe flooding compared to a storm with a 10-year return period.
From their models, Ravela and his collaborators found that under scenarios of escalating global warming, the storms previously regarded as 100-year events, yielding the highest storm surge levels, might recur every decade or even sooner by the end of the century. They also noted that, towards the conclusion of this century, tropical storms in Bangladesh will likely occur over a wider seasonal timeframe, potentially overlapping in certain years with the seasonal monsoon period.
“If the monsoon rains have saturated the soil before a cyclone arrives, it exacerbates the situation significantly,” Ravela remarks. “Communities won’t have any respite between the severe storm and the monsoon. There are numerous compounding and cascading impacts between the two. This situation only arises due to global warming.”
Ravela and his team are utilizing their modeling to assist experts in Bangladesh in better assessing and preparing for an increasingly storm-prone future. He notes that the climate reality for Bangladesh is, in some respects, not exclusive to that region.
“The narrative of climate change unfolding in Bangladesh in a particular manner will similarly manifest in different forms elsewhere,” Ravela observes. “Perhaps where you are, the narrative revolves around heat stress, worsening droughts, or wildfires. The risks differ. Nevertheless, the fundamental storyline of catastrophe is not vastly different.”
This research is partially funded by the MIT Climate Resilience Early Warning Systems Climate Grand Challenges project, the Jameel Observatory JO-CREWSNet project; MIT Weather and Climate Extremes Climate Grand Challenges project; and Schmidt Sciences, LLC.