ONE OF THE UNCERTAINTIES UNDERLYING HOW MUCH SEA LEVELS WILL RISE along the Chesapeake Bay comes down to a nagging question: how is the Gulf Stream going to behave?

This famous ocean current is a giant. More than 3,000 feet deep in places and as much as 90 miles wide, the Gulf Stream begins near the Bahama Islands and curls up the East Coast of the United States from Florida to North Carolina. There, it takes a dogleg to the northeast, carrying warm, tropical waters into the deeper parts of the northern Atlantic Ocean.

Because of how much water it moves, the Gulf Stream has a small but noticeable effect on water levels along the Mid-Atlantic, starting at Cape Hatteras in North Carolina. Through a mix of ocean and planetary physics, the current lowers sea levels at sites like the Chesapeake Bay by about three to five feet. If the current weren't there, many shoreline neighborhoods in Maryland and Virginia would be underwater.

This relationship has put the Gulf Stream at the center of a scientific debate — one that has large implications for towns across the Chesapeake region. Up and down the Mid-Altantic coastline, data collected by tide gauges are showing that sea levels aren't just climbing. They're rising at what seems to be an accelerating pace. This rapid uptick goes beyond what researchers have found along the rest of North America's coasts.

Some scientists are putting the blame, at least in part, on the Gulf Stream. The current, they argue, could be losing speed because of manmade climate change — like warming temperatures caused by the greenhouse gases emitted by cars and other human sources. This slowdown, in turn, might be helping to drive up sea levels across a long stretch of the Atlantic Coast of North America. Other scientists, however, contend that it's too early to say whether the Gulf Stream will become a major player in long-term sea level rise on the Chesapeake Bay.

Researchers want to know more about what's happening to the Gulf Stream. A growing body of research indicates that as climate change alters the world's oceans, some of those changes will be felt close to home. "If you are worried about the Chesapeake Bay, you cannot just look at the Bay by itself but its connections to the large-scale climate change in the Atlantic Ocean," says Tal Ezer, an oceanographer at Old Dominion University. "You have to look much larger."

Ezer keeps his eye out for those larger connections during his morning jogs. The scientist lives and works in Norfolk, Virginia. Most days, he gets up early to run, picking a route that takes him through town. Sometimes during these runs, he encounters flooded streets and stops to snap a few photos.

These opportunities come often. Norfolk has struggled with flooding over its history, and the situation is only getting worse. Consider one of the city's low-lying neighborhoods, called The Hague. In 2009, a year that saw a big nor'easter hit coastal Virginia, streets in this part of town were flooded for the equivalent of 12 full days. Before the 1970s, the city rarely saw more than two days of flooding in a single year. In some parts of Norfolk today, all it takes is a spring high tide to overflow the city's shoreline.

One of many photos of flooding in Norfolk that Tal Ezer has taken with his smart phone. High water events along the city's waterfront neighborhoods, like the Ghent, shown here, have become increasingly common since the 1980s. Photograph: Tal Ezer

Ezer presents his photos of Norfolk flooding in talks he gives about his research, which delves into sea level rise. These high-water events, he says, are part of a larger pattern of sea level rise on the Mid-Atlantic coast.

The oceanographer was intrigued in 2012 when a team of scientists at the U.S. Geological Survey (USGS) published a landmark paper exploring that pattern in the journal Nature Climate Change. In it, the researchers analyzed sea level rise data taken along the Atlantic coastline by tide gauges. These instruments, often installed off docks, measure changes in water height over time.

When the USGS team dug deep into these records, they found something surprising: on a 600-mile stretch of coast beginning off of North Carolina and ending in Canada, sea levels were not only rising, they were rising faster and faster over time.

Between 1950 and 2009, the group found, sea levels in this region rose at about two millimeters per year on average. If you looked at the same sites between 1970 and 2009, however, those rates were much higher, or around four millimeters each year. The shift indicated that at some point in the last few decades, sea level rise on the Mid-Atlantic had kicked into a higher gear.

None of the traditional causes of sea level rise, including melting ice sheets or sinking land surfaces, could explain such a sudden shift.

Less than a year after that first study, two independent teams analyzed tide gauge data using different methods and reached similar results. "We both got to the same conclusion that along the Mid-Atlantic and Chesapeake Bay area, sea level rise is accelerating," says Ezer, who led one of the studies.

Many scientists suspected that the Gulf Stream was behind this shift.

Their suspicion came from their knowledge of the long history of research on Atlantic Ocean currents. The Gulf Stream is just one leg of a larger pattern of circulation in the northern Atlantic. This pattern, called the Atlantic Meridional Overturning Circulation (AMOC), is the ocean's great mixer: it brings warm water from the tropics north toward the Arctic. There, the water cools and sinks, eventually moving back toward the equator. Think of a hamster running around and around its exercise wheel.

The Gulf Stream is shown here — arrows showing the stream’s flow were added to a satellite image that depicts water temperatures in the Atlantic Ocean. The current carries warm tropical water (orange and yellow) from the Bahama Islands toward the northern reaches of the ocean, where water temperatures are much cooler (blue). The ocean current is one step in a larger circulation pattern called the Atlantic Meridional Overturning Circulation. Credit: National Aeronautics and Space Administration Visualization Studio with arrows added by Sandy Rodgers

Climate change, however, could throw off the dynamics of this system, including the flow of the Gulf Stream. That's because as the planet heats up and ice melts throughout the Arctic, the Atlantic Ocean's northernmost regions will become warmer and less salty. Theoretically, these conditions could make it harder for water carried north by the Gulf Stream to sink, forcing it and the entire circulatory pattern to slow down.

"We are still working out how [the Gulf Stream] is going to behave," as climate change continues, says Hali Kilbourne, a paleoclimatologist at the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science.

How the Gulf Stream behaves is a worry because a slowdown of the Gulf Stream could have consequences for towns on the Mid-Atlantic coast like Norfolk. That's largely because of the current's influence on local sea levels.

The effect exists because as the Gulf Stream flows away from the shore near North Carolina, it draws water away from the coast and moves it toward the middle of the ocean. It's the result of how the rotation of the planet distributes water in the Atlantic. This shifting, in turn, causes the surface of the ocean to tilt up off the coast of the Mid-Atlantic — sea levels dip close to shore and rise the farther you get out to sea, like a surf board with its front end slightly higher than its back end.

This tilt in the Atlantic Ocean is tied to the Gulf Stream's speed. The Gulf Stream moves at an average of about four miles an hour. But over periods of months and years, it can also gain speed or slow down for a variety of reasons, such as wind patterns. More than a decade ago, Ezer published research that showed that when the Gulf Stream slows down, sea levels along the Mid-Atlantic rise. That's because the current draws away less water from shore. That, in turn, lowers the tilt of the ocean's surface.

And that's what seemed to be happening along the Mid-Atlantic.

"Clearly the Gulf Stream has been doing something over the last 40 years," says Robert Kopp, a climate scientist at Rutgers University in New Jersey. "You need that to explain what you see in the tide gauges."

But scientists can't say for sure whether the Gulf Stream is slowing down.

The difficulty stems from the fact that the ocean is a complex place, Kopp says. So many factors, such as natural shifts in wind patterns over the Atlantic, may influence the flow of the Gulf Stream at any one time, he says. And that can make it difficult for scientists to tease out how manmade impacts on the world's climate are affecting the Gulf Stream.

Tal Ezer, however, wanted to get beyond those confounding factors. To begin, he joined up with other scientists at Old Dominion University to analyze data from tide gauges on the Mid-Atlantic. The researchers compared these data to records of how fast the Gulf Stream was moving as measured by an underwater cable off the coast of Florida. Next the scientists employed new statistical tools to try to remove from the data many of the natural influences on the speed of the Gulf Stream, to quantify the portion contributed by manmade climate change.

The tilt of the Gulf Stream, which lowers sea levels along the Mid-Atlantic coastline by about three to five feet, is connected to its speed. According to an analysis by Tal Ezer, that tilt may have begun to dip starting around 2004 (wide gray line) suggesting that the current could be slowing down. At roughly the same time, rates of sea level rise accelerated at sites around the Chesapeake Bay (narrow colored lines). Source: Updating Maryland's Sea-level Rise Projects report, p. 12

And the group discovered that "there are signs in the data that this slowdown has already started," Ezer says.

The scientists' analysis suggested that the Gulf Stream had slowed down beginning around 2004, although by how much wasn't clear. The loss of speed also seemed to be closely tied to a recent acceleration in sea level rise in the Mid-Atlantic. The team published its results in 2013 in the Journal of Geophysical Research: Oceans.

But some scientists pushed back. Thomas Rossby is a veteran oceanographer and a professor emeritus at the University of Rhode Island. He's also one of the leaders of a scientific study called the Oleander Project, a long-running effort to directly measure the speed and other attributes of the Gulf Stream.

The project relies on an unusual tool: a cargo ship. This commercial vessel, called the Oleander, makes weekly trips from New Jersey to Bermuda, crossing the Gulf Stream as it goes. More than 20 years ago, Rossby and his colleagues installed a set of specialized instruments on the ship. These instruments use sound waves to track the motion of water below the boat's hull. It's a bit like how dolphins use sonar to track objects underwater. With every voyage that the Oleander makes, Rossby gets a snapshot of how fast the Gulf Stream is moving.

"Here with the Oleander there are no ifs, buts, or maybes," Rossby says. "We are measuring the Gulf Stream."

And in a paper published in 2014, the oceanographer and his colleagues summarized their analysis of about two decades' worth of data: a big, fat nothing. Based on their observations, the Gulf Stream didn't seem to be slowing down or speeding up. "That's not to say that it won't change at some point in the future," Rossby says. But so far, the Gulf Stream's speed has been "as solid as it can be." The team's results appeared in the journal Geophysical Research Letters.

Those results may sound contradictory, but Ezer and Rossby agree on more than they disagree. Neither scientist disputes the fact that sea level rise is accelerating on the Mid-Atlantic. Nor do they shed doubt on the science underlying climate change. How the Gulf Stream will behave in a warmer world, however, is still a new field of study.

"It's a natural progression of science," says Benjamin Horton, a paleoclimatologist at Rutgers who has studied change in sea levels along the Mid-Atlantic coast. "Someone proposes a hypothesis, and we go out and test it."

The differences in Ezer's and Rossby's conclusions may stem largely from differences in how they tested the hypothesis that the Gulf Stream was losing speed. In his research, Ezer used statistics to try to single out what climate change alone was doing to the current. Rossby didn't. Instead, his research focused on how the Gulf Stream's speed as a whole had changed over two decades.

Today, it's common for researchers to turn to math, as Ezer did, to try to find patterns in complex arenas like the oceans, a method sometimes dubbed "big data," says Robert Kopp of Rutgers University. But even the best statistical tools have their limits, he notes. "My preferred approach is [to gather the best] statistics as you can, and then go out and get more data," he says.

Kopp says that there's an alternative explanation for what may have happened to the Gulf Stream. Speed may have nothing to do with it. Instead, the answer may lie in the Gulf Stream's location. Scientists know that the path of the current's flow can change for natural reasons over months or years, shifting a little to the north or the south. Such wiggles can, in theory, change the effect of the Gulf Stream on sea levels along the East Coast of the United States.

Scientists have gotten hints that the current's path shifted to the north in recent years. Kopp explains that such a move could create higher sea level rise patterns in the Mid-Atlantic similar to those scientists were seeing. The seas might look like they were rising at an accelerating pace — even if the Gulf Stream's speed remained unchanged.

In the end, it will take more time to get to the bottom of the Gulf Stream's behavior, Kopp says. He released his own research study in 2013 on sea level rise in states like Maryland and Virginia. In that paper, Kopp argued that it could take 20 more years of tide gauge data for scientists to know for sure what's happening to the current: is it slowing down, or did it just take a temporary excursion to the north?

For now, the role that the Gulf Stream plays in sea level rise up and down the Mid-Atlantic remains unclear. Ezer feels confident that his finding that the Gulf Stream appears to be slowing will be confirmed. "I think, eventually, that if we have long enough records that all the data will show similar trends."

Until then, there may be a lot more photos of flooding for him to take around Norfolk.