Hurricane Sandy was not an extreme Black Swan hurricane

I was in San Francisco last week for the annual Fall Meeting of the American Geophysical Union (AGU), the world's largest gathering of Earth Scientists. Over twenty thousand scientists from all over the world, including many of the world's top climate scientists and hurricane scientists, were in town to exchange ideas to advance the cause of Earth Science. One of the more intriguing talks was given by Ning Lin, a professor at Princeton University. She and Dr. Kerry Emanuel of MIT are studying "Black Swans"--tropical cyclones that are a surprise to the observer, and cannot be anticipated based on the 162-year historical record. Very rare extreme hurricanes that one might expect to occur naturally once every 10,000 years are possible, and “climate change has increased the probability of such storms,” Emanuel said at a press conference last week. In terms of storm surge, Sandy was not a black swan, since the 1821 hurricane that hit New York City had a higher storm surge. Historical records recount that the water rose thirteen feet in one hour at The Battery on Manhattan during the 1821 hurricane. The water level did not rise as high as during Sandy, though, since the 1821 hurricane hit at low tide.

Lin and Emanuel used a climate model in combination with a detailed hurricane model to generate a large number of hypothetical hurricanes in the future climate, and generated 10,000 years worth of storm surge events. They then used a detailed storm surge model (ADCIRC) to evaluate the storm surge risk these storms posed. Their three case studies:

1) The Persian Gulf. No tropical cyclones have ever been observed in the Persian Gulf, due to the Gulf's low humidity and high wind shear. However, the Gulf has some of the warmest water temperatures on the planet, and could theoretically support a strong tropical cyclone. The researchers' modeling predicted a 1-in-248,000 probability that a strong tropical cyclone originating in the western Persian Gulf could move eastwards and bring a 7 meter (16 foot) storm surge to Dubai in the United Arab Emirates. Obviously, such an event would be extremely disruptive to the global energy economy, which relies heavily on the infrastructure in the Persian Gulf.

2) Darwin, Australia. In 1974, tiny but powerful Tropical Cyclone Tracy brought a 1.6 meter (5.2') storm surge to the city. Lin and Emanuel's research showed that a black swam tropical cyclone with a probability of 1-in-70,000 is capable of bringing a devastating 11.5 meter (38') storm surge to the city.

3) Tampa Bay, Florida. Only two major Category 3 hurricanes have hit Tampa: an 1848 hurricane that raised waters levels by 4.6 meters (15'), and a 1921 storm with a storm surge of 3.2 meters (10.5'.) A black swan storm moving from south to north just offshore could set up a resonance in Tampa Bay and generate a 13 meter (43') storm surge. Such a storm has a 1-in-14,500 chance of occurring in a given year. This would likely do over $250 billion in damage, I expect.


Figure 1. Damage to Bayshore Boulevard after the 1921 Tampa Bay hurricane. The road leads to the Tampa Bay Convention Center from the south.


Figure 2. Track of the Tampa Bay Hurricane of 1921, one of only two major hurricanes ever to hit the city. This Category 3 storm with 115 mph winds brought a storm surge of 10.5 feet to Tampa Bay. A "Black Swan" hurricane capable of generating a 43' storm surge would not take a track like this, and instead would move from south to north just offshore.

Jeff Masters