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500-year floods could strike NYC every five years

Climate study says mean tidal level to rise

(CNN) - In the coming years, New York City might look less like a concrete jungle and more like a concrete swamp.

The Big Apple could see a surge in significant floods every five years between 2030 and 2045 as an impact of climate change, according to a study published this week in the Proceedings of the National Academy of Sciences.

The study suggests that as global sea levels rise and as a warming atmosphere drives tropical storms, the combination could leave the city facing more frequent and more dangerous flood events.

"We know that storm surge flooding tends to be the deadliest aspect of many tropical cyclones," said Andra Garner, a climate researcher at Rutgers University and lead author of the study.

"It is very important that we take action to mitigate future warming in order to avoid the worst-case scenarios of future sea-level rise, which would help to limit the increases we see in future flood risk," she said. "This work emphasizes the need for adaptation planning for the New York region in order to protect the city's coastal infrastructure in both the near future and in centuries to come."

Garner and her colleagues pointed in their study to Hurricane Sandy, which spawned devastating floods in New York City in 2012, as an example of extreme weather events the city could face.

Forecast models show that a big storm could hit the Northeast on Sunday -- the fifth anniversary of Hurricane Sandy's US landfall -- and into Monday morning. The storm could bring flooding rains from the mid-Atlantic through New England.

However, the storm system is not forecast to be as strong as Sandy, which was directly responsible for about 147 deaths in the United States, Canada and the Caribbean, according to the National Hurricane Center.

For the new study, scientists used computer models to project probable sea level rise and storm surges in New York City from the preindustrial era before 1800 to 2300.

The scientists took a close look at the frequency of troublesome flood heights of 2.25 meters, or 7.38 feet, above the average tidal level for the region; in contrast, Hurricane Sandy had a storm surge of 2.8 meters, 9.19 feet, above the mean tidal level, according to the study.

The models projected that flood heights of 2.25 meters above the mean tidal level will become more frequent over time.

During the preindustrial era, such flooding occurred only about every 500 years, the researchers found. By the modern period, such flooding occurred about every 25 years, and it could occur every five years in the future.

Some of the same researchers previously found that global sea levels rose faster in the 20th century than during any of the 27 previous centuries, according to a separate study published last year in the Proceedings of the National Academy of Sciences.

That study also suggests that less than about half of the observed rise in sea levels in the 20th century would have occurred in the absence of global warming.

New York, for instance, has experienced at least a foot of sea level rise since 1900, mostly due to the expansion of warming ocean waters, according to the state's Department of Environmental Conservation.

Additionally, scientists argue that the effects of climate change, including rising sea levels and warmer oceans, recently made Hurricanes Irma and Harvey more powerful than they otherwise would have been.

In the new study, even though the climate models showed more frequent flooding, they also projected that tropical cyclones could shift farther east and farther out to sea, the researchers found.

"The track shift was really not something we expected to find in this work and not something we had originally set out to investigate," Garner said. "The finding reveals the need for additional research into the way that future storm tracks may behave in the Atlantic basin, and it's something that we hope to look into more."

Even with a possible track shift, Kim Knowlton, senior scientist at the Natural Resources Defense Council in New York City who was not involved in the study, highlighted that rising sea levels would still have a significant impact on flooding in the future and public health, which "is an area of big concern," she said.

"It's quite likely that we're going to see that sea level contribution to coastal floods for some time, that we're going to have to contend with this," said Knowlton, who is also an assistant clinical professor at the Columbia University Mailman School of Public Health.

Frequent flooding, such as what was projected in the new study, could influence public health by posing the direct risk of potential drowning and injury, as well as other health consequences associated with evacuation efforts and power outages, and mental health impacts, such as post-traumatic stress disorder, she said.

There are numerous public health concerns related to flooding beyond the most obvious risks of drowning and trauma, said Jonathan Patz, a professor and director of the Global Health Institute at the University of Wisconsin-Madison, who was not involved in the new study.

"These range from contaminated drinking water and risk of infectious diseases simply from wading in floodwaters, to asthma and other respiratory illness from exposure to mold and fungus during post-event cleanup when people return to their homes," Patz said.

Patz added that prolonged power outages can further delay access to clean water and cause heat-related deaths due to a lack of air conditioning.

"The most important thing that we can do to avoid such devastating floods is to reduce our emissions of greenhouse gases by pursuing a clean energy economy," he said. "That pursuit in itself offers enormous and immediate health benefits, from improved air quality from clean energy, and improved fitness levels from urban design to enhance walking or biking opportunities."


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