Whether or not a coastal city floods during a hurricane depends on the storm, tide and sea level, and now a team of climate scientists show that the risk of New York City flooding has increased dramatically during the industrial era as a result of human-caused climate change.

"We wanted to look at the impact of climate change on sea level and storm characteristics to see how that has affected the storm surge on the Atlantic coast, specifically in New York City," said Andra Reed, graduate student in meteorology, Penn State. "Hurricane Sandy was the motivating factor."

During Hurricane Sandy in the fall of 2012 most of New York City's transportation tunnels flooded and the storm surge breached the sea walls on the southern tip of Manhattan Island at Battery Park, flooding subway tunnels. The high storm surge was the result of rising sea level, high tide and the storm's force.

"Unfortunately, the storm surge record only goes back to the 1850s and we don't think that the record is reliable before the end of World War I," said Reed. "That's less than 100 years."

The researchers wanted to compare the levels of storm surge before human induced climate change - before 1800 - and in the time since, so they had to look elsewhere for a reliable record.

"Actual storm surge records don't go back far enough to establish a pre-industrial baseline," said Michael Mann, distinguished professor of meteorology, Penn State. "So we used a combination of models and paleoclimate data to describe the longer-term storm surge history."

The researchers made use of proxy sea level records of sediments and foraminifera - tiny ocean organisms - developed by Ben Horton, professor, department of marine and coastal sciences, Rutgers University and Andrew Kemp, assistant professor of coastal processes and climate change, Tufts University, to characterize past changes in sea level.

Then they used simulated tropical cyclone histories spanning the past 1,000 years produced by Kerry A. Emanuel, professor of atmospheric science, Massachusetts Institute of Technology based on driving a model of tropical cyclone behavior with long-term climate model simulations. Finally, the tropical cyclone information was fed into a model of storm surge by Ning Lin, assistant professor of civil and environmental engineering, Princeton University.

"In the pre-anthropogenic era, the return period for a storm producing a surge of 2.81 meters (9 feet) or greater like Sandy at the Battery would have been about 3,000 years," said Reed. "We found that, in the anthropogenic era, the return period for this same storm surge height has been reduced to about 130 years."

Prior to 2012, the largest recorded surge in New York City's Battery Park area was in 1938 when a nearly 10 foot surge flooded Long Island, N.Y., but only a half inch of water breached the sea wall at the Battery.

There are several factors behind increased storm surge and flooding from land-falling hurricanes. The strength of the surge is not just dependent on the storm's force, but also on the size of the storm, the state of the tides and sea level. While most homeowners listen to hear the category of the hurricane, in the case of Sandy, it was the overall size of the storm that caused the surge.

"Sea level is rising because of climate change," said Mann. "But climate change also appears to be leading to larger and more intense tropical storms."

The combination of more intense and larger hurricanes is what the researchers find to be leading to larger storm surges. Storms are likely to both cause flooding in low-lying areas like Long Island, Staten Island and beachfronts and to breach existing seawalls in areas like lower Manhattan.

Also working on this project was Jeffrey P. Donnelly, associate scientist geology and geophysics, Woods Hole Oceanographic Institute.