2/6/13: Paragraph 11 of this story has been updated to include more information from FEMA.
When the federal government released updated flood maps for the New York City region last week, residents were shocked to find that the number of houses and businesses in the region's flood zone had doubled since the maps were last revised, in 1986.
But it now appears that those maps might have underestimated the extent of New York's flood risk, because they don't factor in the effects of future climate change. Scientists say that by the 2080s, sea levels off the city's coast could rise by as much as five feet from melting glaciers, making storm surges more severe and causing floods much further inland than the new maps indicate.
The maps also don't incorporate data from Hurricane Sandy, which caused catastrophic flooding in the nation's financial capital. Many structures destroyed by the superstorm are not included in the newly drawn flood zones.
If future sea level rise had been taken into account, the flood zone would likely have been much larger, said Philip Orton, a physical oceanographer at the Stevens Institute of Technology in New Jersey, who served as a technical reviewer on the updated maps.
"The fear is that we'll get a meter [3.3 feet] of [sea level] rise by the end of the century, potentially more," Orton said. "People are rightfully concerned. ... The New York City area isn't ready for the storm surges of today, as we learned from Sandy, let alone what is possible in the future."
The omissions mean the maps already may be outdated—or will be very soon—some scientists said, with implications for Hurricane Sandy rebuilding efforts, as well as the city's plans for adapting to long-term climate change. The flood maps, which are produced by the Federal Emergency Management Agency (FEMA), are used to set insurance requirements and building codes. If the New York maps are too conservative, property owners might be wasting their money by rebuilding in especially vulnerable areas or by adapting structures to meet standards that will have to be revised in a few years.
"Old statistics on flood risk are obsolete," said Kevin Trenberth, a distinguished senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research (NCAR). "Increasingly, [FEMA] should be looking ahead."
FEMA officials who spoke on background to InsideClimate News said future sea level rise wasn't included because the agency has traditionally used historical storm information to determine where flood zones should be set. Incorporating impending climate change simply wasn't part of the process.
In the summer of 2012, however, Congress passed the Biggert-Waters Flood Insurance Reform Act, which requires FEMA to at least consider using the "best available science regarding future changes in sea levels, precipitation, and intensity of hurricanes" in its flood zone maps. But because the New York City region's FEMA maps had been underway since 2010, they were exempt from the law.
FEMA officials said data from Hurricane Sandy wasn't factored into the maps because the agency had completed most of its modeling before the storm struck in October 2012. FEMA considered incorporating Sandy into its models, but decided against it after researchers scoured their storm data and found that it included a synthesized storm with similar intensity and direction.
FEMA said adding data from Sandy likely wouldn't have affected the flood maps. Many scientists still consider a superstorm of that magnitude a 500-year storm—meaning there's only a 0.2 percent chance of it hitting the region in any given year. FEMA's maps show areas vulnerable to what have traditionally been thought of as 100-year floods, which have a 1 percent chance of striking annually.
Trenberth said it is risky to assume that storms like Sandy are extremely rare events. He believes they have become the new 100-year storms due to global warming—or at least are in the process of becoming so. "This has been proven to be the case now many times, especially in the Pacific Northwest and along the Mississippi where there have been three 500-year storms in the past 20 years."
But Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology who specializes in hurricanes, said FEMA's decision to leave Sandy out of the maps was likely a good one, because it is too soon to tell just what the superstorm represents.
"The problem is there has been just one storm of Sandy's character," he said. "We do not yet know if we are looking at a once in 100-year event or a once-in-a-1,000-year event."
Climate change is warming the oceans and causing more moisture to accumulate in the atmosphere, fueling more and stronger storms.
The maps FEMA released last week covered parts of Queens, Brooklyn, Staten Island, New Jersey and Westchester County. They included 35,000 structures that were never considered a flood risk before. Maps for Manhattan and areas of the Bronx will be released later this month and are expected to further increase the number of structures in high-risk zones.
Some of the additional flood height shown on the maps is the result of sea level rise since the 1980s, according to FEMA officials. Sea levels along New York's coast have increased between 9 and 11 inches over the past century.