The RRS Discovery floated in the calm waters east of the Mid-Atlantic Ridge on a Saturday morning in December of 2020.
The research vessel, mostly from the UK's National Oceanography Centre, used an acoustic signaling system to release a cable more than three miles long from its anchor on the sea floor.
Ben Moat, the expedition's chief scientist, and others walked up to the bridge to see the first floats. The technicians on deck were wearing hard hats and harnesses. They stopped the winch every few minutes to disconnected the floats and the data used to calculate the pressure, current speed, and volume of water flowing past.
The scientists and technicians are part of an international research collaboration, known as RAPID, that is collecting readings from hundreds of sensors at more than a dozen moorings along the Atlantic line.
They are looking for clues about the Atlantic Meridional Overturning Circulation, a network of ocean currents. They want to better understand how global warming is changing it, and how much more it could shift in the future.
Moat says that measuring the ocean system is vital to understanding the climate.
The world's mightiest river is the Atlantic circulation. It goes from the Southern Ocean to the southern coast of Africa and back again, ping-ponging between the southwestern coast of Africa, the southeastern US and Western Europe.
The system carries warm, shallow, salty water northward, transporting about 1.2 million gigawatts of heat energy across the array of moorings. It is equivalent to about 160 times the energy capacity of the world's electricity system. The currents, which heat up the surrounding air as they travel northward, are a major factor in why Western Europe is warmer than eastern Canada.
The waters become cooler and denser as they get to the high latitudes, causing the currents to dive miles below the surface. The system is propelled by the sinking of the water into the ocean.
The circulation in the Atlantic seems to be weakened, transporting less water and heat. The ocean is getting fresh water because of the melting ice sheets, and the surface waters are getting more heat because of the climate change. One of the currents' core driving forces may be undermined by the fact that warmer and fresher waters are less prone to sink.
Measuring the ocean system is important to understanding the climate.
The weather in the Northern Hemisphere is influenced by the currents, particularly around the coastal Atlantic but also as far away as Thailand. The weather and temperature will change if the currents change.
Climate models predict that the currents will decline by 45% this century. Evidence from the last ice age shows that the system can eventually switch off or go into a weak mode under certain conditions.
It would be a climate disaster if that happened. It could cause winter temperatures in the far north of Europe to fall by more than 10 C. Sea levels could rise as much as a foot on the Eastern Seaboard, flooding homes and businesses up and down the coast. The summer monsoons in Africa and Asia could weaken, raising the risk of famines and droughts that could leave untold numbers without adequate food or water.
It would be a global catastrophe according to the researcher.
A collapse of the currents is a remote possibility this century, but even a steep slowdown would have significant impacts, potentially cooling and reducing rainfall around the North Atlantic while increasing precipitation across parts of the tropics. It could raise the sea level by five inches.
Scientists have only a coarse comprehension of the currents behavior, the balance of the forces that drive them, or their susceptibility to shifting climate conditions. Moat and others are interested in the Atlantic circulation.
The circulation of the Atlantic is more variable and unpredictable than previously thought.
The current of Florida.
The Atlantic Oceanographic and Meteorological Laboratory is located on Virginia Key, a barrier island just a few miles from downtown Miami.
The Florida Current is a warm upper layer of the Atlantic circulation that runs past the island. It is an ideal place to observe one of the most powerful stretches of the system, because the currents can span hundreds of miles, down to dozens. The Florida Current is part of the Gulf Stream, a stretch of the Atlantic circulation that goes from the southeastern US to Europe.
The Florida Straits have been monitored by the scientists of the National Oceanographic and Atmospheric Administration since 1982. The phone lines along the ocean provide a cheap way to observe the circulation.
Researchers at the National Oceanographic and Atmospheric Administration found they could reliably measure the voltages along the sides of the cables. They get daily readings from the instruments in the trunk room. They are able to translate those measurements into estimates of how much water flows across the line of latitude.
The AMOC works.
The shallow upper leg of the Overturning Circulation carries warm, salty water northward. The warm currents help to create weather in Western Europe. The surface waters become cooler and denser as they get closer to the Arctic, driving the currents deep below the surface and helping to propel the system. The waters are deep and cool.
The oceanographers at the University of Miami have been using moorings and other instruments to study the currents east of the Bahamas since the 1980s. They have observed the boundary current flowing south and a stretch of the northward limb that flows around the islands.
The cable program was developed as part of a broader push to improve scientific understanding of how the oceans work and interact with the climate.
Concerns about the effects global warming could have on the Atlantic circulation and the impact that could have on the climate have taken on added importance as the ongoing cable measurement and historical records have. The ocean moves around heat. You need to understand climate change.
There were many attempts to measure the parts of the currents using different methods through the 1990s. Oceanographers realized that the snapshot observations weren't enough to fully capture the system's behavior. They needed ways to continuously monitor the currents across the ocean in order to distinguish short-term fluctuations from long-term trends.
The UK's National Oceanography Centre established the RAPID effort in 2004 to anchor cables across the Atlantic. It made sense to work with the University of Miami and the National Oceanographic and Atmospheric Administration as well.
Moat says the researchers are trying to shed light on how variable the currents are, how much heat they deliver, how much carbon they pull down from the air, and how much local winds influence the system.
Out at sea.
On a sunny day in early November, I followed a pair of researchers down a pier at the school of marine and atmospheric science.
The University of Miami owns the F.G. Walton Smith, a 96-foot-long catamaran with dark green hull and white deckhouse.
In the pre-pandemic times, researchers from both institutions have boarded the vessel for 30-hour sprints out and back to the Bahamas. They use an A-frame and a winch on the stern to lower what are known asCTDs into the waters at nine stations along the way.
The carousel of tubes that capture water samples is one of the features of theCTDs.
The instruments are lowered into the ocean.
The package of bottles and sensors is used to measure the properties of the Atlantic currents.
The voyages allow the researchers to determine how much heat and salt are moving through the straits and how fast the currents are.
The research teams go out on long voyages every 18 months to remove and replace the sensors on the eastern side of the Bahamas. The UK does the same job on the eastern side of the ocean.
Other groups have set up moorings in different parts of the Atlantic to better understand how different components work, how tightly the system is connected, and whether changes in one part are rippling throughout.
OSNAP is led by Susan Lozier, an oceanographer at the Georgia Institute of Technology. It has anchored cables in the Labrador Sea and from the southeastern edge of Greenland to the coast of Scotland.
The goal of the international research effort was to get a better understanding of the mechanisms that drive change in the AMOC, which is largely responsible for propelling the currents in the Atlantic.
She says that the monitoring programs have found that the Atlantic circulation is more variable than previously thought.
Its strength and speed change from month to month, year to year, and region to region. Most of the deep-water sinking in the North Atlantic seems to be occurring in the basins to the east of the island. The limbs are more independent than thought. Local wind patterns seem to have a bigger role than expected. Some findings are befuddling.
The circulation of the Atlantic is likely to have weakened. It may be the weakest it has been in more than 1,000 years, according to studies by the Potsdam Institute. The findings are based on long-term reconstructions of its behavior using records like Atlantic Ocean temperatures and the size of grains on the ocean floor, which can reflect changes in deep-sea currents.
If greenhouse-gas emissions continue, models agree that the currents will weaken this century.
There is uncertainty about what state the system is in at the moment, and whether the direct observations are aligning with the models.
The F.G. Walton Smith is visited by researchers from the National Oceanographic and Atmospheric Administration.
Afonso Dubran is from Mexico.
The data from the moorings showed a decline in the Atlantic circulation from 2004 to 2012. That is likely to have contributed to an especially cold winter in northwest Europe in 2012 as well as rapid sea-level rise along the northeastern US coast. The global climate models underestimated the size of the slowdown.
In the years that followed, the currents rebounded substantially. The circulation's strength is still below where it was when the measurement began. It has decreased more than predicted.
The data suggests that the system is already weakened. It showed that the ocean currents can vary over a decade, rather than being linked to global warming.
At this point, it is unclear. He said during an interview in his office overlooking the Florida Straits that they can't be sure if it's a long-term trend related to climate change or a natural one.
The Florida Current has only declined a small amount since 1982, and not quite a statistically significant amount at that, according to the findings. The place you would most expect to see a weakening trend is the powerful concentrated flow. He says the data is showing two different stories.
It is likely to take years to decades before the currents reveal how climate change is affecting them.
Scientists worry that the circulation in the Atlantic could weaken because it has done so before.
The climate across the North Atlantic region suddenly began cooling as the Earth emerged from the last ice age. The Younger Dryas were named for a wildflower that flourished in the cold conditions of Europe in the 1000-year period.
The leading theory is that the ice sheet caused it. As temperatures rose, it quickly melted and flowed into the ocean through the Mississippi River.
At some point, ice damming a massive lake on the southern edge of the glacier may have given way, unleashing a flood that may have diverted the drainage to the St. Lawrence River. It would have brought fresh water into the North Atlantic.
Jean Lynch-Stieglitz, a paleoclimate researcher at the Georgia Institute, says that the influx of fresh water could have had a negative effect on the Atlantic circulation.
By the late 1980s, some scientists wondered if the effects of global warming could halt the currents as they did in the case of the Laurentide.
The UN's Intergovernmental Panel on Climate Change has called a shutdown of the Atlantic circulation very unlikely. The climate models don't incorporate the increase in meltwater from the ice sheets in the area, which could lead to overestimation of the stability of the current.
The UN report released in August says that the models are negligent and that a collapse won't occur before 2200.
The researchers ran scenarios on a model developed at the university, turning the knobs on the levels, rates, and time frames of the water flowing from the ice sheets.
The Florida Current is monitored by the F.G. Walton Smith. The dropsonde floats a telephone cable.
Afonso Dubran is from Mexico.
A tipping point is a fixed physical threshold beyond which the system goes into a different state. They found that a phenomenon known as a rate-induced tipping point, triggered by a sudden increase in the system's rate of change, could halt the currents. Too much change could cause the system to break down.
If the water flowing from ice sheets increases rapidly enough, the Atlantic circulation could be at risk.
It is only one model and one study, but it suggests that the climate system could be more fragile than thought.
The chaotic dynamics mean that we might not be able to predict with 100% confidence whether an element of the climate system will go into another state or not.
The August paper concluded that the currents might be closer to the standard tipping point.
The author, a professor of Earth system modeling at the Technical University of Munich, wrote that scientists have found signs of a collapse in models and geological records from the last ice age.
The only thing we can say is that the AMOC has moved towards its critical point over the last century.
The signs include a decrease in sea-surface temperatures in the North Atlantic, a rise in the salinity in the Southern Atlantic, and a shift in current patterns. Evidence of these warnings was found in eight different records.
The AMOC may have evolved from stable conditions to a point close to a critical transition in the last century.
How close is it?
It is difficult to define a threshold in terms of a specific global temperature or time, given the many layers of uncertainty, according to an email from Boers.
The only thing we can say is that in the last century the AMOC has moved toward its critical point, which had not been expected by many. With every ton of greenhouse gases we emit, we will likely push it further.
Reality vs. Hollywood.
What happens if the circulation in the Atlantic collapses?
The Day After Tomorrow, a 2004 disaster film in which a sudden halt of the currents shock-freezes the Northern Hemisphere over a few days, is a wild Hollywood exaggeration. If the network of ocean currents collapsed, there would be changes that would take years or decades to unfold.
The global climate system would be turned into a fundamentally different state if a shutdown happened.
A study by researchers at the Met Office Hadley Centre in the UK found that a lot of Europe could become a different place. The sea surface temperature drops as much as 15 C from the Barents to the Labrador Seas and 2 to 10 C from the rest of the North Atlantic within 50 to 80 years after fresh water stops the Atlantic circulation.
The sea ice reaches the northern tip of the United Kingdom in late winter.
The continent is very cold as well. Winter storms become more frequent. Most of Europe gets dry during the summer. More precipitation arrives in the form of snow.
The cooler and dry conditions have a negative effect on surface runoff, river flows, and plant growth.
The Garonne River in southern France has less water during the winter. The needleleaf forests of Northern Europe are growing at a slower rate. Crop production decreases dramatically in Spain, France, Germany, and other countries.
Laura Jackson, the lead author of the study, stresses that it was an idealized model, using a large amount of fresh water to quickly shut down the Atlantic circulation and shorten the length of the experiments. She said in an email that a more realistic scenario might show different magnitudes of change.
A collapse of the Atlantic circulation would have a large-scale effect on the world.
Some models think that parts of Asia and North America could be cooler. Slowing currents could disrupt the delivery of vital nutrients, devastating certain fish populations and altering the marine environment.
As the Gulf Stream flattens, ocean levels could rise eight to 12 inches in the southeastern US. The tropical rain belt could drift south, weaken rainfall patterns across parts of Africa and Asia and cause monsoons in the Southern Hemisphere.
A certain amount of weakening may act as a counter force against climate change, which may cause some degree of warming. How these competing forces balance out over time would depend on a number of variables, including how much the system weakens; whether it shuts down entirely; how much carbon dioxide the oceans, forests, and farms pull down; and how much warmer the planet gets.
The ocean is important.
Difficult questions have been raised about the potential for a collapse of the AMOC.
How worried should we be about the possibility of a shutdown this century? How can we properly evaluate the risks and take appropriate actions? How much should today's policy debates or climate actions be shaped by the danger of events that may not occur until the 2200s or even the century century?
Lozier believes that people and the press are obsessed with the catastrophe scenario of The Day After Tomorrow.
The Atlantic Oceanographic and Meteorological Laboratory was established by Molly Baringer, who is the deputy director.
This is a distraction that misses the point. There are a lot of serious consequences unfolding in the present, so we don't need any danger in the distant future.
Lozier has studied the AMOC for a long time. Ocean warming, sea-level rise, ocean acidification, hurricanes are what we should be concerned with. These are the things we know are happening. Huge impacts are what those are. I think we should always keep this in mind.
I asked Baringer how concerned she was about the climate models predicting a collapse of the Atlantic circulation when we met on a picnic table outside of the lab.
Baringer said she doesn't worry a lot about it. She thinks it is hard to account for all the feedbacks in a system that is roughly understood. She believes that the field is largely underestimating ocean acidification, wildfires, and sea-level rise.
I asked if it was important to study the Atlantic circulation.
She said that she doesn't like the question of why we study oceanography.
The ocean is important. The ocean has a lot of heat. It sequesters carbon. It moves food. If we didn't have the ocean circulation, you wouldn't have fish. The AMOC is a big part of what the ocean is doing.
That is the biggest reason to worry about how human actions could affect one of the planet's most complex systems. There are more imminent climate risks to worry about. perturbing the ocean currents could be the biggest climate risk the world is taking.
James Temple is an editor at MIT Technology Review.
