Atlantic Ocean circulation is the weakest in at least 1,600 years, study finds – here's what that means for the climate

An influential current system in the Atlantic Ocean, which plays a vital role in redistributing heat throughout our planet's climate system, is now moving more slowly than it has in at least 1,600 years. That's the conclusion of a new study published in the journal Nature Geoscience from some of the world's leading experts in this field.

Scientists believe that part of this slowing is directly related to our warming climate, as melting ice alters the balance in northern waters. Its impact may be seen in storms, heat waves and sea-level rise. And it bolsters concerns that if humans are not able to limit warming, the system could eventually reach a tipping point, throwing global climate patterns into disarray.

The Gulf Stream along the U.S. East Coast is an integral part of this system, which is known as the Atlantic Meridional Overturning Circulation, or AMOC. It was made famous in the 2004 film "The Day After Tomorrow," in which the ocean current abruptly stops, causing immense killer storms to spin up around the globe, like a super-charged tornado in Los Angeles and a wall of water smashing into New York City.

As is the case with many sci-fi movies, the plot is based on a real concept but the impacts are taken to a dramatic extreme. Fortunately, an abrupt halting of the current is not expected anytime soon — if ever. Even if the current were to eventually stop — and that is heavily debated — the result would not be instant larger-than-life storms, but over years and decades the impacts would certainly be devastating for our planet.

Recent research has shown that the circulation has slowed down by at least 15% since 1950. Scientists in the new study say the weakening of the current is "unprecedented in the past millennium."

Because everything is connected, the slowdown is undoubtedly already having an impact on Earth systems, and by the end of the century it is estimated the circulation may slow by 34% to 45% if we continue to heat the planet. Scientists fear that kind of slowdown would put us dangerously close to tipping points.

Importance of the Global Ocean Conveyor Belt

Because the equator receives a lot more direct sunlight than the colder poles, heat builds up in the tropics. In an effort to reach balance, the Earth sends this heat northward from the tropics and sends cold south from the poles. This is what causes the wind to blow and storms to form.

The majority of that heat is redistributed by the atmosphere. But the rest is more slowly moved by the oceans in what is called the Global Ocean Conveyor Belt — a worldwide system of currents connecting the world's oceans, moving in all different directions horizontally and vertically. 

Through years of scientific research it has become clear that the Atlantic portion of the conveyor belt — the AMOC — is the engine that drives its operation. It moves water at 100 times the flow of the Amazon river. Here's how it works. 

A narrow band of warm, salty water in the tropics near Florida, called the Gulf Stream, is carried northward near the surface into the North Atlantic. When it reaches the Greenland region, it cools sufficiently enough to become more dense and heavier than the surrounding waters, at which point it sinks. That cold water is then carried southward in deep water currents.

Through proxy records like ocean sediment cores, which allow scientists to reconstruct the distant past going back millions of years, scientists know that this current has the capacity to slow and stop, and when it does the climate in the Northern Hemisphere can change quickly. 

One important mechanism through the ages, which acts as a lever of sorts controlling the speed of the AMOC, is the melting of glacial ice and resulting influx of fresh water into the North Atlantic. That's because fresh water is less salty, and therefore less dense, than sea water, and it does not sink as readily. Too much fresh water means the conveyor belt loses the sinking part of its engine and thus loses its momentum.

That's what scientists believe is happening now as ice in the Arctic, in places like Greenland, melts at an accelerating pace due to human-caused climate change. 

Recently scientists have noticed a cold blob, also known as the North Atlantic warming hole, in a patch of the North Atlantic around southern Greenland — one of the only places that's actually cooling on the planet.

 The fact that climate models predicted this lends more evidence that it is indicative of excess Greenland ice melting, more rainfall and a consequent slowdown of heat transport northward from the tropics.

In order to ascertain just how unprecedented the recent slowing of the AMOC is, the research team compiled proxy data taken mainly from nature's archives like ocean sediments and ice cores, reaching back over 1,000 years. This helped them reconstruct the flow history of the AMOC. 

The team used a combination of three different types of data to obtain information about the history of the ocean currents: temperature patterns in the Atlantic Ocean, subsurface water mass properties, and deep-sea sediment grain sizes, dating back 1,600 years. 

While each individual piece of proxy data is not a perfect representation of the AMOC evolution, the combination of them revealed a robust picture of the overturning circulation, says lead author of the paper, Dr. Levke Caesar, a climate physicist at Maynooth University in Ireland. 

"The study results suggest that it has been relatively stable until the late 19th century," explains Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research in Germany. 

The first significant change in their records of ocean circulation happened in the mid 1800s, after a well-known regional cooling period called the Little Ice Age, which spanned from the 1400s to the 1800s. During this time, colder temperatures frequently froze rivers across Europe and destroyed crops. 

"With the end of the Little Ice Age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century," said Rahmstorf. That second decline in recent decades was likely due to global warming from the burning and emissions of fossil fuel pollution.

Nine of the 11 data-sets used in the study showed that the 20th century AMOC weakening is statistically significant, which provides evidence that the slowdown is unprecedented in the modern era.

Impact on storms, heat waves and sea-level rise

Caesar says this is already reverberating in the climate system on both sides of the Atlantic. "As the current slows down, more water can pile up at the U.S. East Coast, leading to an enhanced sea-level rise [in places like New York and Boston]," she explained.  

On the other side of the Atlantic, in Europe, evidence shows there are impacts to weather patterns, such as the track of storms coming off the Atlantic as well as heat waves. 

"Specifically, the European heat wave of summer 2015 has been linked to the record cold in the northern Atlantic in that year — this seemingly paradoxical effect occurs because a cold northern Atlantic promotes an air pressure pattern that funnels warm air from the south into Europe," she said.

According to Caesar, these impacts will likely continue to get worse as the Earth continues to warm and the AMOC slows down even further, with more extreme weather events like a change of the winter storm track coming off the Atlantic and potentially more intense storms.

CBS News asked Caesar the million-dollar question: If or when the AMOC may reach a tipping point leading to a complete shutdown? She replied: "Well, the problem is that we don't know yet at how many degrees of global warming to hit the tipping point of the AMOC. But the more it slows down the more likely it is that we do." 

Moreover, she explained, "Tipping does not mean that this happens instantaneously but rather that due to feedback mechanisms the continued slow down cannot be stopped once the tipping point has been crossed, even if we managed to reduce global temperatures again."

Caesar believes if we stay below 2 degrees Celsius of global warming it seems unlikely that the AMOC would tip, but if we hit 3 or 4 degrees of warming the chances for the tipping rise. Staying below 2 degrees Celsius (3.6 degrees Fahrenheit) is a goal of the Paris Agreement, which the U.S. just rejoined.

If the tipping point is crossed and the AMOC halts, it is likely the Northern Hemisphere would cool due to a significant decrease in tropical heat being pushed northward. But beyond that, Caesar says that science does not yet know exactly what would happen. "That is part of the risk."

But humans do have some agency in all this, and the decisions we make now in terms of how quickly we transition away from fossil fuels will determine the outcome. 

"Whether or not we cross the tipping point by the end this century depends on the amount of warming, i.e. the amount of greenhouse gases emitted to the atmosphere," explains Caesar.