Last week, a story drawing attention to the surprisingly cold anomaly in the North Atlantic Ocean that has emerged recently — featuring record cold temperatures from January through August for a substantial area. This is happening despite the fact that the globe as a whole is likely en route to its warmest year on record.

I also quoted two prominent researchers who think this pattern reflects a much feared slowdown in Atlantic ocean circulation, a scenario made famous by the film The Day After Tomorrow. Granted, even if they’re right, what’s happening here will be nothing like the movie. At most, the circulation may be slowing, not stopping abruptly. And with a warming globe overall, there will definitely be no new ice age.

Still, if the circulation is really slowing we need to weigh what the impacts might be. So let’s probe a little bit deeper here to figure out what’s happening, and what it means.

What explains the cold blob in the North Atlantic?

Michael Mann of Penn State and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research say that to see a pattern like this, in an otherwise record hot year, is a sign that the Atlantic ocean’s so-called “meridional overturning circulation” or AMOC — which is driven by differences in ocean temperature and salinity in the North Atlantic — may be slowing down.

Indeed, they say this fits nicely with a study they published earlier this year, which found an “exceptional” slowdown in the circulation over the course of the last century, and suggested that the dramatic melting of Greenland, by injecting large volumes of freshwater into the ocean, may be the cause.

So are they right? First, let’s consider in more depth what the circulation is and how it works.

In the Atlantic ocean, warm surface water flows northward off the U.S.’s east coast — a current known as the Gulf Stream — and then continues into the North Atlantic. Here the current branches into different segments and eventually reaches regions where colder, salty water sinks beneath the surface, because of its greater density. It is this sinking that keeps the warmer waters flowing northward — they’re basically filling the gap that’s left behind by the sinking waters.

The problem is that a freshening of the North Atlantic — due to large amounts of melting from Greenland — might reduce the density of cold surface waters and prevent sinking. And that, in turn, would slow down northward heat transport. That’s what Mann, Rahmstorf, and their colleagues think is happening — and that the “blob” is a telling sign.

However, this is a vast and complicated system, and not every researcher is yet convinced they’re right about this. The trouble is that in the North Atlantic, much as with the global climate in general, there are human-caused factors, and then there are wobbles in the system that just happen on their own — what researchers often call “natural variability.” In the North Atlantic in particular, variability in surface winds can shape ocean temperatures, and researchers have accordingly identified a variety of different “modes” of variability in the Atlantic involving winds and ocean temperature patterns.

The disagreement turns on how much weight to give to the different factors in explaining what we’re currently seeing. “My assessment is the component due to natural variability is much larger than what we’re seeing from global warming,” says Tom Delworth, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, NJ, at least on the time scale of one or more decades. But Delworth agrees that in the long term, global warming should slow the circulation.

In particular, Delworth mentioned two related sources of natural variability — the so-called “North Atlantic Oscillation,” which refers to variability in winds and pressures over the Atlantic, and the “Atlantic Multidecadal Oscillation,” or AMO, which refers to changes in sea-surface temperatures.

But Mann disagrees that the AMO can account for what we’re seeing right now. Noting that he originally coined the term, he adds by email:

The AMO displays a completely different pattern of sea surface temperatures in the Atlantic as my collaborators and I have shown in past work, and the AMO doesn’t exhibit a long-term decline over the past century, which this particular pattern does.

And another team of researchers also would appear to agree with Mann and Rahmstorf.

Last year in Nature Geoscience, Jon Robson and his colleagues from the University of Reading in the UK published research suggesting that deep ocean waters in the Labrador Sea to the southwest of Greenland are becoming less dense, and that this evidence, combined with other measurements, “suggest that a substantial change in the AMOC is unfolding now.”

Who’s measuring the circulation?

You might think this could be definitively cleared up by direct oceanic measurements of the gigantic current — but in fact, scientists are only beginning to get a handle on this.

One research project, the so-called RAPID program, has been monitoring the ocean circulation at 26.5 degrees north Latitude — a west-to-east line that cuts through Florida — so as to determine the total strength of heat flux northward. This work began in 2004.

So far, says Meric Srokosz of University of Southampton, who runs the UK side of the program (in collaboration with researchers at the University of Miami and NOAA), RAPID has detected a “slow decline in the circulation over 10 years we have been measuring, and that decline is faster than the climate models predict.”

That includes a very “dramatic” decline from 2009-2010 that was associated with a sudden rise of sea level along the U.S. east coast, Srokosz says.

“The ocean has been delivering about 20 percent less heat to the North Atlantic” over the last decade, adds William Johns, an oceanographer at the University of Miami who also collaborates on the RAPID program.

However, with only about 10 years of data, Srokosz and Johns say they can’t definitively say whether global warming is driving the slowdown or it’s part of a natural cycle — although neither denies a possible climatic role.

More RAPID data is about to come in after researchers go out to collect information from the last 18 months from moorings across the ocean. “Whether something dramatic has happened in the circulation this year, we have not recovered the measurements yet,” Srokosz says.

RAPID monitors the circulation at a latitude line that cuts across Florida, but a new international research program, called O-SNAP (“Overturning in the Subpolar North Atlantic Program”) has just begun to monitor it in the chilly North Atlantic, where the current dramatic cooling is actually happening. However, there aren’t any results yet with respect to the strength of the circulation.

“We deployed the first instruments in 2014, and the first recovery of part of the array was this summer, in 2015,” says Amy Bower of the Woods Hole Oceanographic Institution, one of the principal researchers with the program. “Literally, the instruments were just pulled out of the water in July.”

What really happens if the circulation slows down?

The Day After Tomorrow was based on a sudden Atlantic circulation shutdown scenario, but scientists have long said that nothing like what was depicted in the movie would actually happen. And right now, we’re only talking about a possible slowdown, not a total cessation of the current.

“The things that aren’t likely to result are just about anything that’s portrayed in the movie, The Day After Tomorrow, which is really a caricature of the science,” says Mann.

That said, there would definitely be impacts. Sea level rise along the East Coast of the U.S. would be expected, because the Gulf Stream features warmer waters — which take up more space — on its European side. That keeps sea level higher away from our coast, but if the circulation weakens, the ocean might even out — delivering rising seas to cities like New York and Boston.

Indeed, during the sharp circulation slowdown measured in 2009-2010, sea levels suddenly rose 4 inches on the U.S. east coast.

Major temperature changes in the ocean would also have a dramatic effect on marine life. “This circulation pattern is tied to the productivity of the North Atlantic, one of the most productive regions from a fisheries standpoint,” says Mann. “So if the AMOC were literally to shut down, you could see sharp decreases in marine productivity in this region.”

One would also expect changes in weather — one recent climate change modeling study of what would happen with a slowdown of the AMOC found a “strengthening of the North Atlantic storm track.” Less northward heat transport by the ocean could also partly offset the rising temperature trend expected due to global warming. Whether the result would actually be a net cooling is not clear — but it certainly would not be a new ice age.

So will it all happen?

For now, we can only say that there is a mystery to be solved in the form of very cold North Atlantic temperatures — and some strong hypotheses about what the cause might be, with some researchers already directly blaming it on the AMOC, and few seeming to deny that this is at least a possible explanation.

All eyes now turn to the scientists doing the hard work of taking direct measurements of the circulation, at different points in the Atlantic, on long ocean voyages. Expect a great deal more discussion — and concern — if their results confirm the slowdown that some scientists already think is happening.

Clarification: An earlier version of this article stated that Rahmstorf, Mann and their colleagues had found a slowdown of the AMOC since the “turn of the century.” The actual finding was of a slowdown across the 20th century, especially since 1970.