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‘Storm of the Century’ to become ‘Storm of the Decade’

Posted on 12 March 2012 by John Hartz

This is a reprint of a news release posted by the Massachusetts Institute of Technology (MIT) on Feb 14, 2012 

With climate change, today’s ‘100-year floods’ may happen every three to 20 years, according to new research.

NASA Satellite Photo of Hurricane Irene

Last August, Hurricane Irene spun through the Caribbean and parts of the eastern United States, leaving widespread wreckage in its wake. The Category 3 storm whipped up water levels, generating storm surges that swept over seawalls and flooded seaside and inland communities. Many hurricane analysts suggested, based on the wide extent of flooding, that Irene was a “100-year event”: a storm that only comes around once in a century.

However, researchers from MIT and Princeton University have found that with climate change, such storms could make landfall far more frequently, causing powerful, devastating storm surges every three to 20 years. The group simulated tens of thousands of storms under different climate conditions, finding that today’s “500-year floods” could, with climate change, occur once every 25 to 240 years. The researchers published their results in the current issue of Nature Climate Change.

MIT postdoc Ning Lin, lead author of the study, says knowing the frequency of storm surges may help urban and coastal planners design seawalls and other protective structures.

“When you design your buildings or dams or structures on the coast, you have to know how high your seawall has to be,” Lin says. “You have to decide whether to build a seawall to prevent being flooded every 20 years.”

Lin collaborated with Kerry Emanuel, the Cecil and Ida Green Professor of Atmospheric Science at MIT, as well as with Michael Oppenheimer and Erik Vanmarcke at Princeton. The group looked at the impact of climate change on storm surges, using New York City as a case study.

To simulate present and future storm activity in the region, the researchers combined four climate models with a specific hurricane model. The combined models generated 45,000 synthetic storms within a 200-kilometer radius of Battery Park, at the southern tip of Manhattan.

They studied each climate model under two scenarios: a “current climate” condition representing 1981 to 2000 and a “future climate” condition reflecting the years 2081 to 2100, a prediction based on the Intergovernmental Panel on Climate Change’s projections of future moderate carbon dioxide output. While there was some variability among the models, the team generally found that the frequency of intense storms would increase due to climate change.

Once they simulated storms in the region, the researchers then simulated the resulting storm surges using three different models, including one used by the National Hurricane Center (NHC). In the days or hours before a hurricane hits land, the NHC uses a storm-surge model to predict the risk and extent of flooding from the impending storm. Such models, however, have not been used to evaluate multiple simulated storms under a scenario of climate change.

Again, the group compared results from multiple models: one from the NHC which simulates storm surges quickly, though coarsely; another model that generates more accurate storm surges, though less efficiently; and a model in between, developed by Lin and her colleagues, that estimates relatively accurate surge floods, relatively quickly.

Today, a “100-year storm” means a surge flood of about two meters, on average, in New York. Roughly every 500 years, the region experiences towering, three-meter-high surge floods. Both scenarios, Lin notes, would easily top Manhattan’s seawalls, which stand 1.5 meters high.

But with added greenhouse gas emissions, the models found that a two-meter surge flood would instead occur once every three to 20 years; a three-meter flood would occur every 25 to 240 years.

“The highest [surge flood] was 3.2 meters, and this happened in 1821,” Lin says. “That’s the highest water level observed in New York City’s history, which is like a present 500-year event.”

Carol Friedland, an assistant professor of construction management and industrial engineering at Louisiana State University, sees the group’s results as a useful tool to inform coastal design — particularly, she notes, as most buildings are designed with a 60- to 120-year “usable lifespan.”

“The physical damage and economic loss that result from storm surge can be devastating to individuals, businesses, infrastructure and communities,” Friedland says. “For current coastal community planning and design projects, it is essential that the effects of climate change be included in storm-surge predictions.”

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Comments 1 to 9:

  1. If you include a meter (or two) of sea level rise the surge becomes even higher. How many times can a city be flooded before people abandon the low lying areas? Much of New Orleans has not been rebuilt, and seems unlikely to ever be rebuilt, because people are unwilling to take the risk. Parts of southern Florida have also not been rebuilt after the bad hurricanes 5 years ago. As the OP says, it is very expensive to repair the damage when the sea wall is overtopped.
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  2. Coastal, estuarine and riverside conurbations are likely to become more scarce (or at least more sparsely populated) once insurers refuse to cover damage caused by such frequent events.
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  3. # Michael Sweet. The paper indicates that part of the reason for increased frequency of high storm surges is due to rising sea level and part is due to the increasing frequency/intensity of the storms. Both effects have a similar impact. A 1 m sea level rise was used. Only when all these factors are included is the figure of 3 to 20 years arrived at.
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  4. There has been (in the US at least) an interesting redistribution of property over the last 30-40 years. A huge amount of investment has been made in litoral (coastline) areas, with a corresponding increase in damages when larger storms or surges occur. The insurance industry is starting to adjust: according to fairly recent Federal Emergency Management Agency (FEMA) work, flood insurance rates may rise by 70% by 2100, greatly discouraging coastline development.
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  5. An interesting effort by the UN World Disaster Reduction Campaign is called 'Making cities resilient.' Their map shows the 982 local governments around the world that have signed on to this campaign. There are exactly 2 in the US (and neither of those are coastal cities). I seem to recall a fable about an ant and a grasshopper. The one that survived was the one that prepared for survival. Oh well.
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  6. muoncounter--most nations seem to have just a few; only a handful have double digits-worth of 'resilient cities.' The top five, if I'm not mistaken: 5) Serbia (49) 4) Lebanon (56) 3) Phillipines (107) 2) India (118) 1) Austria (272!) Seems the future may speak German.
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  7. Today, a “100-year storm” means a surge flood of about two meters, on average, in New York. Roughly every 500 years, the region experiences towering, three-meter-high surge floods. Both scenarios, Lin notes, would easily top Manhattan’s seawalls, which stand 1.5 meters high. But with added greenhouse gas emissions, the models found that a two-meter surge flood would instead occur once every three to 20 years; a three-meter flood would occur every 25 to 240 years.
    I raised an almost identical point at a local council public forum on climate change about six years ago. The engineers had prepared quite a good plan for future council policies and strategies to avoid or to ameliorate the consequences of sea level rise for low-lying suburbs. However, they presented their calculations, graphs and maps based on 1-in-100 year storm events. At the end of the presentation I stood and asked why they used a current 1-in-100 year storm event definition, rather than estimating current and future changes in frequency of flooding to various levels above the current Australian sea level datum, given that 1-in-100 year event frequencies would change over time, and given that such changes would impact on Council responses to sea level rise. The engineers stared at each other for a while, and then one of them admitted that they hadn't thought of that. The embarrassed silence that followed was palpable.
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  8. Structures such as a port have to be built on the coast (obviously) but wouldn't it be more sensible to pull every other construction back from the coast rather than trying to build bigger sea walls. The same goes for flood plains. Turn areas that will be inundated into parks, wild life reserves, even sports grounds and simply avoid the problem. If you have been to Hawaii, for instance, you will see hotels with the front door right on the beach. That is just asking for trouble and spoiling what should be a beautiful area for recreation.
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  9. Adding to Bernard's comment - engineering design and costs are based on century extremes. They do not allow for changes to those extremes, until the event occurs (a disaster is the penalty). The key to the extreme-event consequence with AGW is the combination of speed and extent at which those extremes have, and will, change. It's basically the Achilles' Heel of the "adaptation" theory. If you need to understand why the century-rule won't be abandoned ... go talk to the cost engineer.
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