When major ice sheets thaw, they release enough fresh water to disrupt ocean currents world-wide and make the planet wobble with the uneven weight of so much meltwater on the move. Studying these effects more closely, scientists are discovering local variations in rising sea levels — and some signs pointing to higher seas around metropolitan New York.

Sea level may rise faster near New York than at most other densely populated ports due to local effects of gravity, water density and ocean currents, according to four new forecasts of melting ice sheets. The forecasts are the work of international research teams that included the University of Toronto, the National Center for Atmospheric Research in Boulder, Colo., Florida State University and the University of Bristol in the U.K., among others.

Scientists are laboring to make their predictions more reliable. While they do, New York has become an urban experiment in the ways that seaboard cities can adapt to climate change over the next century. For their part, the city’s long-term planners are taking action but are trying to balance the cost of re-engineering the largest city in the U.S. against the uncertainties of climate forecasts.

“We can’t make multibillion-dollar decisions based on the hypothetical,” says Rohit Aggarwala, the city’s director of long-term planning and sustainability.

Still, prompted by a possibility of floods from higher seas, some university-based marine researchers and civil engineers are debating whether New York ought to protect its low-lying financial district, port, power grid and subways with storm surge barriers like the mobile bulwarks that safeguard London, Rotterdam, Netherlands, and St. Petersburg, Russia. Engineering concepts for multibillion-dollar barriers around New York harbor were discussed here this week during the H209 Water Forum, an international conference on coastal cities and climate change, held by the Henry Hudson 400 Foundation at the Liberty Science Center.

World-wide, cities in 40 countries depend on dikes or seawalls. The seaside of the Netherlands is protected by storm surge barriers big enough to be seen from space. In Venice, Italy, engineers are completing a $7 billion barrier to block high tides that flood the city 100 times a year. In New Orleans, construction crews have started a $700 million barrier to help prevent hurricane floods. In California, it could cost $14 billion to protect 1,100 miles of vulnerable urban coastline with reinforced sea walls and $1.4 billion a year to maintain them, the Pacific Institute reported in March.

Unlike New York, though, those urban areas were built at or below sea level. Moreover, most of them also are on land that is sinking, adding to the danger posed by higher oceans.

While most of New York is above sea level, its subways, telecommunications cables, fiber-optics networks, plumbing and power mains aren’t. “There is so much underground,” says urban water management consultant Piet Dircke at Arcadis, one of four engineering firms that recently developed concepts for a storm surge barrier here. “The economic impact of flooding could be huge.”

Indeed, some civil engineers argue the city already risks catastrophic storm flooding. “A storm surge is not really a global warming issue” for New York, says senior engineer Dennis Padron at Halcrow Inc., which helped design a 15-mile-long storm barrier in St. Petersburg. “It could happen tomorrow.”

Under certain conditions, a hurricane now could generate a 30-foot-high storm surge and flood 100 square miles of New York. If ice melts and sea level rises, that risk increases. “If you have 20 inches of sea level rise, the edges of lower Manhattan would flood 20 times a year,” says Douglas Hill, a consulting engineer at the School of Marine and Atmospheric Sciences at Stony Brook University. “It would look like Venice.”

To be sure, the city that never sleeps is rarely dry even now. Every day, transit crews pump 14 million gallons of water from city subways. Authorities recently installed $400 million of more powerful pumps. Last year, they started installing higher sidewalk grates — disguised as street art, bike racks and benches — to help keep storm water away from subway rails.

Since last summer, city planners have been persuading federal, state and regional agencies as well as private concerns to gradually upgrade vulnerable facilities here as part of routine capital upkeep. They are reassessing building codes, raising key equipment in flood zones and taking inventory of infrastructure at risk. But it can be hard to get some landlords to even move a fuse box from a damp basement to a more protected place. Any talk of storm surge barriers is premature, they say.

“Our burden of proof is somewhat higher,” says the city’s Mr. Aggarwala. “We have to be very clear that we do all the low-cost stuff first.”

In their efforts, Mr. Aggarwala and his colleagues have been guided by a panel of city-appointed climate experts from NASA and Columbia University, whose report predicts that by 2080 New York will have the climate Raleigh, N.C., has today. By their estimate, it will be about seven degrees Fahrenheit warmer and sea level may be two feet higher, unless polar ice sheets do melt.

But such forecasts can be overtaken by new data. “You have to continually update plans as the models get better and the knowledge gets better and the unknowns become known,” Mr. Aggarwala says.

Scientists are still trying to gauge how much of the Greenland ice sheet may melt and how quickly the West Antarctic Ice Sheet might respond to rising temperatures due to greenhouse-gas emissions. “When an ice sheet melts, sea level change is not uniform,” says climatologist Jonathan Bamber at the U.K.’s University of Bristol who studies Antarctic ice sheets.

Generally, sea level today varies from place to place. The North Atlantic normally is two feet lower than the northern Pacific, because Atlantic sea water is colder, denser and saltier. This summer, weakened currents and persistent winds, for instance, caused sea level along the U.S. eastern seaboard to be two feet higher than normal, the National Oceanic and Atmospheric Administration reported last week.

By taking such factors into account, researchers earlier this year calculated that melting Greenland glaciers could shift ocean currents enough to make sea level along New York’s 570 miles of shoreline an additional 20 inches higher than seas elsewhere. “It will cause the sea level along the coastal region of the Northeast U.S. to rise faster,” says climate modeler Aixue Hu at the National Center for Atmospheric Research in Boulder, Colo.

So far, city planners are biding their time. “We are not planning for the worst case yet, but we are thinking about what happens if Greenland melts more quickly,” says Adam Freed, the city’s deputy director of long-term planning.

For Mr. Aggarwala, any changes in climate are best countered by incremental adjustments as science and circumstances demand. “If we have to shut the stock exchange for a day because water is running down Wall Street, that’s not unprecedented,” Mr. Aggarwala says. “A major snowstorm can do that. The key challenge is how quickly we can recover.” By Robert Lee Hotz, The Wall Street Journal.