State and local officials are gathering today to discuss the impacts of rising sea levels on Massachusetts’ South Shore. But just what should they be planning for?

Sea level is expected to rise at least one to three feet this century. There’s concern that New England could see far more than that, but the science behind such predictions is rapidly evolving.

On a crisp fall afternoon, Magdalena Andres searched a dock in Woods Hole, on Cape Cod, for the source of some of the data at the heard of this debate. An oceanographer at Woods Hole Oceanographic Institution, Andres has worked with tide gauge data for years, but she’s never actually visited one … until now.

“Okay, so we found it. We found it! Okay,” she said, grinning and then settling herself. “This gauge has been measuring sea level since the early 1930s, and in that time, sea level here has risen something like 10 inches or so.”

Tide gauges around the world show that sea level is rising, and the pace is accelerating. At a minimum, sea levels are expected to rise another one to three feet this century. But here’s the catch: that’s a long-term, global average.

What property owners and local officials need to know is what’s going to happen right here, in the next ten or twenty years. That can be very different, and much harder to nail down. 

“From year to year, things vary. And spatially, things vary,” explained Andres. “That’s because the ocean isn’t just like a bathtub that is slowly filling up with water, or where the water is getting warmer so it’s expanding uniformly.”

The northeast coast is a prime example. In 2012, scientists reported something unusual happening between North Carolina’s Cape Hatteras and Newfoundland, Canada. Sea level rise along this stretch of coast was accelerating at three to four times the global average. They called it a hotspot.

Four years later, things have changed.

“I think it might have been a hotspot of sea level rise in 2012 or 2010,” Andres said. “But it feels like the rest of the world has caught up.”

What’s actually happened is that sea levels along the New England coast have dropped back to meet the global average. Like most things in the natural world, sea level rise doesn’t necessarily follow a straight line. It bounces up and down from year to year, some places more than others.

In Woods Hole, and across the northeast, those bounces can be as big as five centimeters – two inches – in a given year. It may not seem like much, but it dwarfs the three or so millimeters per year of global rise caused by warming oceans and melting glaciers.

It seems the northeast may not be a hotspot for sea level rise as much as it’s a hotspot for sea level variability. The question is: why?

Some oceanographers point the finger at changes in the Gulf Stream, caused by a “pool” of fresh water in the North Atlantic coming from Greenland’s melting ice sheets.

Imagine the Gulf Stream is the top of a conveyer belt. Typically, salty, tropical water flows north to Greenland, where it gets colder. And colder water is denser. When it gets dense enough, it sinks to the seafloor and flows back to the tropics.

“The problem comes when you start putting fresh water up in the North Atlantic,” explained Scott Rutherford of Roger Williams University. “That water doesn’t become dense enough to sink and so you sort of shut down one of the drains that, if you will, drains the gulf stream.”

In other words, Greenland’s melting glaciers could slow down the conveyer belt. That would allow water to pile up along the northeast coast of the U.S., raising sea level. That’s what many climate models show.

“When you see a story like that: A plus B equals C, and that’s what you expect to happen, chances are you’ve got the story straight,” said Rutherford.

But Tom Rossby of the University of Rhode Island has been measuring the Gulf Stream directly for over two decades, and he says the models are wrong.

“I’m quite confident that the Gulf Stream is not slowing down,” Rossby laughed. “I could almost say it’s that simple.”

Of course, it’s not that simple. How the Gulf Stream is impacted by climate change is not settled science.

For Christopher Piecuch of Atmospheric and Environmental Research, that’s beside the point. He’s not convinced the Gulf Stream has much effect on coastal sea levels here in New England. It’s just too far offshore. 

“What we see north of Cape Hatteras is there’s much more of a relationship – at least year to year, and decade to decade - with local meteorology,” Piecuch said. “things like the coastal currents, the winds along the shore, the atmospheric pressure.”

Barometric pressure – basically, the weight of the atmosphere – appears to account for about a quarter – in extreme cases, as much as half –  of year to year changes in sea level in New England. Piecuch’s favorite analogy is a water bed.

“If you sit on a water bed, you push down the water, it goes horizontally outwards," he explained. "The atmosphere works the same way. If you have more atmospheric pressure, there's more mass in the atmospheric column, so it sort of pushes the water out to the sides. Vice versa, if there's lower atmospheric pressure, the sea level could rise up."

The effect of wind is also pretty intuitive. Steady winds can pile water up at the coast, or push it offshore, depending on the direction.

If Piecush is right, and these are the biggest factors influencing sea level in the northeast, that presents a new challenge.

“Those are really hard to forecast, for the identical reason it’s hard to forecast the weather,” explained Piecush. “This is the weather we’re talking about.”

There’s no question climate is changing, and sea levels are rising. It’s unequivocal. But scientists can’t yet put their finger on exactly how high the water will be in Massachusetts twenty years from now, any more than they can give us a weather forecast for 2036.

That’s cause for concern if you’re trying to plan ahead. But for Magdalena Andres, it raises a different worry.

“I worry when people recognize that science doesn’t have all the answers yet, and therefore think that science can’t have any answers,” she said.

Andres says the most important thing we can do right now to prepare for future sea level rise is keep studying it.

Rising Tide: Boston Underwater is produced in partnership with The GroundTruth Project.