In the 2004 movie The Day After Tomorrow, a super-storm covers the Northern Hemisphere and a 100m-high storm surge floods New York City. And then the water freezes as the Earth enters a new ice age.
The film and the book it’s based on are purely fiction, but last year’s Hurricane Sandy hit New York and the rest of the eastern seaboard with a vengeance; a 4m-high storm surge flooded lower Manhattan at the same time as a deluge delivered 180mm of rain to the coast. Water poured into subways, power failed, cars floated down Wall St, people died.
In Hurricane Katrina in 2005, an 8m-high storm surge inundated the city of New Orleans and other low-lying areas; seawater flowed up to 20km inland from the beaches.
In other low-lying parts of the world, such as the Bay of Bengal, storm surges only a few metres high can be devastating. During 1970’s Bhola cyclone, the deadliest storm on record, a storm surge ranging in height from 4m to 10m flooded much of the low-lying islands of the Ganges Delta, killing an estimated 500,000 people in what is now Bangladesh.
But how can a storm suddenly raise the sea level by several metres?
The sea level is constantly responding to atmospheric pressure, says Professor Lionel Carter, an oceanographer at Victoria University. “It goes up by 1cm for every hectopascal drop in pressure.” A high-pressure air system presses down on the water and lowers the sea level. In an extreme low-pressure system, the sea level rises.
But another other key contributor to a storm surge is the wind. Extreme low-pressure systems create a large pressure differential, which generates strong winds. And if these winds are directed onshore, they push the water towards the land. If a storm surge coincides with a high tide and large waves, the effect is even more extreme.
Storm surges of the size seen in the tropics and hurricane-swept regions have not been observed in New Zealand, says Carter, but storm surges caused by wind and pressure changes can cause the sea level to rise 0.5-0.7m above the mean sea level. Although records are sketchy, says Carter, the largest surge in recent times is likely to have accompanied the severe storm of 1936, which had a storm surge estimated at 0.9m. “But if you add in a spring tide and wave run-up, you could be looking at something like about 1.3-1.5m above average sea level.”
Climate scientists are projecting an increase in the intensity of storms as the climate warms. The combination of a rising sea level and more big storms will lead to more damaging storm surges. “You only have to raise sea level by 10cm and you’ve potentially increased the frequency of flooding by about a factor of three,” says Carter.
Much of New Zealand is bordered not by coastal plains but by steep cliffs, where higher storm surges are likely to accelerate erosion. A rising sea level and an increase in storm surges will have a much greater impact on our Pacific neighbours in low-lying island groups, such as Tuvalu and Kiribati, where a 2m-high storm surge could inundate entire islands.
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