Five years ago, a report called "Nation Under Siege " illustrated the vulnerability of 31 U.S. coastal cities to flooding. But not just to any kind of flooding—to the flooding of a permanent kind from sea level rise.
What will happen to these cities, the report asked, as sea levels continue to increase from global warming?
The study provided answers in a series of 3-D maps constructed using data from federal science agencies and the United Nations' climate panel. The maps provide an uncanny prediction of what transpired Monday night when superstorm Sandy engulfed 1,000 miles of Atlantic coastline.
Most striking is the 3-D map of New York (pictured above), which shows what could happen to the city with a 3-meter (9.8-foot) rise in sea level: Lower Manhattan, the East Village neighborhood and the FDR Drive underwater. That's exactly what Sandy's 3-meter storm surge delivered.
Was superstorm Sandy a preview of what sea level rise will bring—permanently—to New York and other coastal cities by century's end?
"This was essentially what we thought of as the once-in-a-100-years event," said Radley Horton , a NASA scientist specializing in climate projections at the Center for Climate Systems Research at Columbia University . "As sea levels rise, we'll see these types of events more often."
The storm—which brought the nation's financial capital to its knees—showed just how unprepared New York City and other coastal cities are for flooding from weather disasters, which are expected to increase as climate change intensifies and oceans rise.
According to figures from the UN Intergovernmental Panel on Climate Change (IPCC), the world's oceans have risen on average by about 7 inches since 1900. The rise has been even higher along the East Coast. A report by the New York State Sea Level Rise Task Force  said sea levels along New York's coast have increased between 9 and 11 inches over the past century.
It's just the beginning, scientists warn. In a June report , the U.S. Geological Survey (USGS) predicted the world's oceans would rise between 0.6 and 1.8 meters (2 and 5.9 feet) by 2100, plus another 1.5 meters (4.9 feet) during violent storms.
Katharine Hayhoe , an associate professor of atmospheric sciences at Texas Tech University, said it's impossible to tease out precise effects that each climate variable, such as sea level rise, storm surge and high tide, had on the size and strength of Sandy. However, she said, the sea level rise of 7 inches this century (the IPCC estimate) made the megastorm worse.
"I don't know where, and I don't know how much, but I do know that due to those 7 inches there is at least some added damage," Hayhoe said in an interview. "In some places, the coastline was high enough or the sea wall was built up enough, but in other places maybe that 7 inches did make a difference."
In a series of tweets this week , Hayhoe gave three reasons why climate change exacerbated superstorm Sandy. One, already higher sea levels made the storm surge more severe. Two, higher sea surface temperatures from warming provided more energy for Sandy—with about 15 percent of the temperature increase attributed to climate change, she said. And three, record loss of sea ice in the Arctic this year may have steered Sandy towards the coast.
Hayhoe told InsideClimate News that as sea levels continue to rise, it won't take a hurricane of Sandy's strength to cause the amount of flooding wreaking havoc on the East Coast.
Sandy has already been branded as one of the most destructive and expensive storms in U.S. history, with damages estimated as high as $20 billion. Nationwide, at least 74 people were killed in the storm —22 in New York City alone. More than 5.6 million homes and businesses are still without power, down from 8.5 million after landfall, with outages stretching as far inland as Ohio and Michigan.
The storm left New York City crippled, mainly due to flooding and high winds. The biggest hit was to the subway system, which carries some 5 million passengers a day. Seven subway tunnels under the East River were flooded out, making it the worst disaster the system has seen in its 108 years . Fourteen of 23 lines will resume limited service today. A massive fire in Queens destroyed roughly 100 homes in a beachside community, an unexpected disaster linked to the storm.
The storm's effects were felt across the East Coast. Pieces of the boardwalk in Atlantic City were swept away, and city streets were flooded with upwards of 2 feet of water. The Potomac River rose by as much as 9 feet, far exceeding the flood stage of 6 feet. Cities from North Carolina to Maine were inundated with several feet of water.
It's a similar picture to the one painted in the report, which was published by Architecture 2030 , an advocacy group working to reduce greenhouse gas emissions from the building sector. Using elevation data from USGS, sea level rise and other global warming projections from the National Oceanic and Atmospheric Administration (NOAA), NASA and the IPCC—and maps from Google—the group projected flooding and outages in 31 coastal cities from Seattle to Miami. The effects would be "calamitous, having the potential to destabilize many areas of the country," the report said.
Consequences of rising seas would reach far beyond those 31 cities, climate experts have warned. More than 50 percent of the U.S. population lives in coastal areas. A 2011 University of Arizona study  identified 180 American cities that would be at risk from rising sea levels, including 20 that have more than 300,000 residents. A 1-meter (3-foot) rise in sea levels by 2100 would put an average of 9 percent of land in those cities at risk of flooding.
Nine large cities, including Boston and New York, would see damage on more than 10 percent of their cities in that scenario.
Columbia's Horton called Sandy a wake-up call for New York City and other areas vulnerable to flooding. "I hate to say there's a silver lining, but if there's any positive it's that we can use this to learn and get a better opportunity to prepare for the future," he said.
The state Sea Level Rise Task Force found that 11 percent of the city was at risk of flooding from higher sea levels.
That report predicted "dramatic implications" from rising seas—from flooding to coastal erosion—and said the state should begin updating flood maps, evaluating risk areas and preparing at-risk communities for coming floods. Essential infrastructure should be built at higher elevations and transportation and utility networks should be protected from salt water, the report said.
Columbia's Horton, who served on a separate New York City Panel on Climate Change, said Mayor Michael Bloomberg's administration has been leading the pack among mayors in preparing for climate change—both studying the impacts and using policies to reduce the city's carbon footprint. However, "there's no denying how vulnerable New York City and its infrastructure is right now," Horton said.
"Clearly, there are issues that need to be investigated and with the storm there will be a fast-tracking of those investigations," Horton noted. Among those issues are determining the best way to heighten sea walls or whether to construct surge barriers to protect flood-prone areas.
Scientists have discussed the possibility of building such barriers around New York City, which could include using existing bridges as barriers. (London and the Netherlands have installed similar protections). But because it would take time to study and design the barriers, and because construction costs would be in the billions, little progress has been made.
Ultimately, Texas Tech's Hayhoe said, Sandy highlights the pressing need now for flood prevention.
"Even if you don't think climate change is real, the steps you take to prevent flooding are the same. You're building sea walls and preparing your systems," she said. "We all agree coastal cities are vulnerable. What matters is how prepared they are."
Clarification: Architecture 2030 originally illustrated the impact of sea level rise on 31 cities in its 2007 report. It has since updated its website site to include maps of 108 cities.