By Tim Radford
New York City—hit by Superstorm Sandy five years ago at a cost of $50 billion—could be under water again soon. What 200 years ago would have been regarded as the kind of flood that happened only once in 500 years could, by 2030, bring superstorms every five years or so.
It won't be that 2.8 meter (approximately 9.2 feet) storm surges of the kind that delivered floodwater to the streets and subways of America's iconic city will be any more frequent. It will be the impact of sea level rise, as a consequence of global warming driven by ever higher greenhouse gas levels in the atmosphere, in response to the profligate global consumption of fossil fuels.
Although researchers have repeatedly warned that global warming could bring more hurricane and storm hazards to the US northeast, the latest study based on computer simulations predicts that the expected stronger storms of tomorrow are likely to shift offshore, in theory reducing the risk to New York City. But sea levels are rising rapidly, to increase the risk of flooding.
"If we cause large sea-level rise, that dominates future risks, but if we could prevent sea-level rise and just have the storm surge to worry about, our projections show little change in coastal risk from today during most years," said Michael Mann, distinguished professor of meteorology and atmospheric science and director of Penn State's Earth System Science Center, and one of the authors.
"While those storms that strike New York City might be bigger and stronger, there may be fewer of them as changing storm tracks increasingly steer the storms away from NYC and toward other regions."
It takes two things to flood a coastal city. The tide must be high, and ferocious winds must pile up the water to unusual heights at the same time: this is the storm surge.
In 2012 Superstorm Sandy piled the tide up to 2.8 meters above its average level off the coast of New York. Researchers report in the Proceedings of the National Academy of Sciences that they looked at the history of sea level and storm surges from the year 1800 to 2300, and then simulated the pattern of events in a world in which humans abandoned the vows made in Paris in 2015 and went on burning fossil fuels under the notorious "business as usual" scenario.
They found that what had in 1800 been the chance of the one-in-500-years flood event—2.25 meters (approximately 7.4 feet) above mean tidal height—increased with time and sea level. By the period 1975 to 2005, the chance of such flooding had fallen to every 25 years. By 2030 to 2045 it could happen every five years.
But sea level rise would continue as yet more glaciers melted and the Greenland and Antarctic ice packs became less stable. By 1970 to 2005, the one-in-500-years event would bring a flood of 3.4 meters (approximately 11.2 feet). By 2080 to 2100 this could reach from four to 5.1 meters (approximately 13.1 to 16.7 feet) and by 2300 the flood hazard could have increased to between 5 meters and 15.4 meters (approximately 16.4 to 50.5 feet), depending on what happened as global warming affected Antarctica.
"Sea level is rising and higher sea level increases the damages from coastal storms," said Richard B. Alley, professor of geosciences at Penn State.
"Human decisions about energy will be important in determining how much the sea rises and thus how much damage we face, and accurate projections of storms will help in minimizing the risks."
Reposted with permission from our media associate Climate News Network.