Extreme flooding events triggered by hurricanes are projected to become much more frequent along the Eastern and Southern United States’ coasts as a result of sea level rise and storm intensification.

The findings, published in a new study from Princeton University, reveal that the two sources of water can combine to create compound flooding events, which have far greater consequences than those caused by surge or rainfall alone.

The research, published in the journal Nature Climate Change, is one of the first to look at how climate change can affect the frequency of extreme rainfall events. The researchers expect that their findings will better inform policymakers and strengthen overall readiness for future catastrophic storms.

“Most studies have looked at the hazards of rainfall and storm surge separately, but in this study, we looked at the joint hazards from the Gulf Coast up through New England,” says Ning Lin, senior author of the study.

The new study projects a dramatic increase in the frequency of joint 100-year events—that is, events with a 1% chance of occurring in any given year for both rainfall and storm surge in the historical period—by the year 2100.

Extreme rainfall-surge episodes have historically occurred every 200-500 years along the Gulf of Mexico. However, towards the end of the twenty-first century, extreme occurrences may occur every 10-30 years on average, according to the study’s conclusions. The situation is considerably worse in New England, where extreme combined episodes are extremely unusual in the historical climate, occurring fewer than once every thousand years on average. By the end of the century, however, these catastrophic occurrences may occur once every five years on average.

To get at these forecasts, the researchers used eight distinct global climate models—mathematically complicated, data-rich simulations of how the world’s climate is changing as a result of greenhouse gas levels in the atmosphere. The models were then downscaled to generate tens of thousands of synthetic storms for the study region along the western Atlantic and Gulf of Mexico. Comparing their forecasts of past hurricane development and strength to historical simulations yielded a valid averaging of the models, which weigh climate component factors in different ways.

“This way, by comparing the models’ predictions of the past, we know we have a reasonable basis for the models’ predictions of the future,” adds Lin.

The researchers then created models of the hurricanes’ tremendous winds and how far the winds spread outward from the storm. These strong winds are responsible for storm surges, which occur when large amounts of water are pushed toward shore. The storm winds also play a part in simulating the rainfall from a storm. Along with wind speed, the translation speed of a storm, or how quickly it moves across land and water, has an effect on rainfall and storm surges. Slower storm translation speeds result in increased rainfall since the storm has more time to dump precipitation over a region. Increased rainfall can exacerbate local storm surge flooding, while slower storms give gale-force winds more time to pile up bigger, more devastating surges, indicating their intertwinement as joint risks.

The study allowed the researchers to estimate the proportional contributions of sea level rise and storm changes to the combined rainfall-surge dangers, both of which have been well-documented over the last decade. Given the rapid increase of about 0.13 inches per year over the previous thirty years, sea level rise has long been assumed to be a main driver of more frequent floods. However, the new analysis suggests that growing storm intensity and slowing translation speeds are the key reasons why joint extreme rainfall-surge events are likely to increase throughout the eastern seaboard of the United States.

“It’s a surprising finding that says we should not neglect changes in storm climatology, with enhanced rainfall projected to play a bigger and bigger role in driving compound flooding,” said Lin.

“Based on the results presented in this study and others,” comments professor Thomas Wahl from the University of Central Florida, who was not involved in the study, adding, “it is paramount for decision-makers to properly account for compound flooding events in their coastal management and adaptation plans.”

Source: 10.1038/s41558-021-01272-7

Image Credit: Getty

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