Climate
If you’re looking for the most inhospitable region of the world’s oceans, Cape Horn is a strong — or rather, a cold — contender. For around 300 days a year, winds at the southern tip of South America blow at gale force. One low-pressure system after another races from the Pacific toward the Atlantic, bringing rain or snow and piling up extraordinarily high waves. Captains navigating here must fear the rugged cliffs of Patagonia to the north and the icebergs calving from Antarctica to the south.
It’s no wonder Cape Horn is considered the world’s largest ship graveyard today. Over the centuries, more than 800 ships have been wrecked here, and an estimated 10,000 sailors have lost their lives. “If you want to grow old,” goes an old seafarer’s saying, “avoid Cape Horn and reef your sails in time.”
But there’s another way to look at it. According to some marine researchers, the region surrounding Antarctica is not remote at all — it’s more like the center of the world.
The reason conditions are so harsh south of the American continent is that there are no landmasses to stand in the way of nature’s forces — the wind can sweep around the South Pole unhindered.
This has allowed the strongest current on the planet to form in the ocean: the Antarctic Circumpolar Current (ACC). It is the only ocean current that sweeps through the Atlantic, Pacific, and Indian Oceans at the same time. The current is the central component of a global system of oceanic circulation that distributes water and heat around the world — warm water at the surface, cooler and saltier water in the deeper layers and at the ocean floor.
But climate change could cause this engine to falter — and the world may look quite different afterward.
Chapter 1
The global heat engine
Before one can truly understand what is happening, it’s necessary to look at the bigger picture.
Germany and other parts of Europe owe their mild climate largely to the sea. A vast portion of the sun’s radiation strikes the Earth around the equator. The oceans vigorously redistribute this heat, making many regions habitable in the first place. Driven by winds, Earth’s rotation, and differences in salinity, this “global conveyor belt” pushes about three petawatts of energy toward the poles — more than 150 times the total energy consumption of all humankind.
At the heart of this system lies the Antarctic Circumpolar Current, which oceanographer Taimoor Sohail from the University of Melbourne describes as a “melting pot.” It absorbs characteristics from all ocean basins and mixes them together.
However, Sohail fears that this global circulation may lose momentum as climate change progresses. In a study published in Environmental Research Letters earlier this year, he and two colleagues calculated that the strength of the Antarctic Circumpolar Current could decrease by up to 20 percent by 2050.
The ACC is driven not only by the westerly winds but also by differences in temperature and salinity between Antarctic waters and the temperate zones. Together with Earth’s rotation, these south–north contrasts produce a steady west–east movement. For a long time, scientists believed that the higher sea temperatures around the equator would actually accelerate the Antarctic current.
According to Sohail’s simulations, however, this effect is increasingly being outweighed by Antarctic ice melt. Like a continent-scale conveyor belt, Antarctica’s glaciers transport ice toward the sea — faster and faster as the planet warms. This leads to more freshwater entering the Southern Ocean. “The water next to the ice sheets is being infiltrated by meltwater,” Sohail says. This disrupts the salinity-driven engine of the ACC “and leads to a weakening.”
That could first have direct consequences for Antarctica itself. The Antarctic Circumpolar Current acts as a physical barrier that keeps invasive species away from the continent. If it weakens, invasive species such as certain sea grasses could reach the continent more easily and disrupt its ecosystem. The current also keeps warmer water masses at bay. “If it slows down,” Sohail notes, “the heat could more easily reach the underside of the ice shelves, accelerating their melting.”
The debate over the strength of the Antarctic current, however, is far from settled. A study published in Nature last year found that about 400,000 years ago — when the planet was warmer than it is today — the current was significantly stronger.
It remains a remote and harsh region, with very few long-term data series on the ACC’s strength. Its future development also depends heavily on melt rates in different parts of Antarctica, Sohail explains. “There is considerable uncertainty concerning East Antarctica — how, where, and when it will melt, and whether that melting will be constant or periodic.” All of this, he says, will affect whether the Antarctic Circumpolar Current actually weakens.
Nevertheless, the simulation results align with findings from other ocean currents, the climate researcher adds. “These interconnected systems suggest that the decline of the ACC may be part of a broader slowdown in global ocean circulation.”
The behavior of the ACC could also influence how warm Europe becomes — because, like any roundabout, the Antarctic Circumpolar Current has exits, and one of them leads toward the Atlantic, feeding into the Atlantic Meridional Overturning Circulation, or “AMOC.”
Chapter 2
In the Atlantic
The Atlantic Meridional Overturning Circulation, or AMOC for short, runs virtually the entire length of the Atlantic from south to north. It transports only about one-tenth of the water circulating around Antarctica, yet it is crucial for Europe’s climate. Without the AMOC, average temperatures in Scandinavia and Western Europe would likely be six to eight degrees lower.
The current begins to gain momentum in the South Atlantic, where it is sometimes referred to as the “Southern Meridional Overturning Circulation.” In the Caribbean and the Gulf of Mexico, near the equator, the water then warms up significantly.
The AMOC carries this heat to Europe’s coasts.
This explains why, for instance, Bilbao is warmer than Boston, even though the Spanish city lies slightly farther north.
The Gulf Stream and the AMOC follow a similar route — from the Caribbean across the Atlantic toward Greenland and Norway.
While the AMOC moves less water than the Gulf Stream, it transports far more heat. The reason lies in how the two currents operate.
The Gulf Stream is a purely surface current, guided by Earth’s rotation and the wind patterns it influences, flowing from Florida toward Europe.
The AMOC, on the other hand, is driven by differences in temperature and salinity — a far more delicate mechanism.
To better understand this mechanism, one can imagine the Atlantic as the cross-section of a gigantic bathtub.
The fuel driving the AMOC is the warm water that flows northward along the surface toward the Arctic.
As it travels north, more and more water evaporates, increasing the salinity of the ocean. Because saltier water is also denser, it sinks in the North Atlantic.
In massive underwater “waterfalls,” this salty water likely plunges as deep as three kilometers toward the ocean floor, forming what’s known as deep water. This occurs mainly in two regions — south of Greenland and north of Iceland.
This deep water then flows southward as a broad current along the bottom of the Atlantic. In the Southern Ocean, tides and winds complete the circulation by bringing the deep water back to the surface, warming it, and sending it north again.
New studies on how fragile this mechanism is are currently being published almost weekly. “That the AMOC can collapse is certain,” says Niklas Boers, Professor of Earth System Modeling at the Technical University of Munich and the Potsdam Institute for Climate Impact Research (PIK). This is known, for instance, from ice core data. The presumed trigger of past collapses in Earth’s history was the massive influx of freshwater when large ice sheets from Ice Age glaciers slid into the North Atlantic. The incoming freshwater reduced salinity, causing less water to sink in the North Atlantic—until eventually the AMOC’s circulation came to a halt.
Even today, more and more freshwater is entering the critical regions of the North Atlantic, primarily due to melting ice from Greenland and increased rainfall — both driven by climate change. “The question is whether this additional freshwater input is enough to trigger a collapse of the AMOC,” says Boers.
Scientists have been systematically measuring the strength of the AMOC for only about 20 years — too short a period to identify long-term trends. Nevertheless, Stefan Rahmstorf, head of the Earth System Analysis Department at PIK, asserts: “In my view, the evidence is very strong that the AMOC is already weakening.”
Rahmstorf points to the “cold blob” that has spread south of Greenland. Unlike the rest of the world’s oceans, this region has cooled in recent decades — a sign, he argues, that the AMOC is transporting less heat there. At the same time, a temperature anomaly off the North American coast may also be linked to a faltering AMOC. In a 2018 study, Rahmstorf and other PIK researchers estimated that the AMOC has weakened by about 15 percent since the mid-20th century.
Just how close the circulation is to a tipping point remains unclear. Many studies conclude that a collapse this century is unlikely. “It is likely that the AMOC has already slowed measurably and will continue to slow significantly over the next 100 years, even without crossing a tipping point,” says Boers.
Yet even a slowdown could have major consequences for Europe — such as more summer droughts, because the cold blob in the Atlantic could deflect low-pressure systems away from the continent. Along the coasts of America and Europe, sea levels may rise faster than they already are. And in winter, northwestern Europe could experience more frequent cold extremes.
A full-scale collapse would have global repercussions, as tropical rain belts could shift. “A complete collapse of the AMOC would be a massive catastrophe on a planetary scale,” writes Stefan Rahmstorf in an article in Oceanography. “We really want to prevent that.”
Chapter 3
To the North Pole and back
But part of the once-warm tropical water flows past Greenland and Iceland into the Arctic Ocean. It’s there, in autumn, that you reach Thorsten Kanzow from the Alfred Wegener Institute. On board the German research icebreaker Polarstern, the oceanographer is currently studying how ocean currents around the North Pole are changing, off the coast of northeast Greenland. During the interview, a storm is raging — “we’re riding it out a bit,” Kanzow says. Yet deep within the sea ice, there’s hardly any swell.
What is changing, however, is the ice itself. Its seasonal cycle is normal: starting in September, the sea ice expands, and from around March, it begins to retreat again.
But in a few decades — some researchers even say just a few years — the Arctic could become an open ocean for the first time in summer. The crew of the Polarstern is among those investigating how this development relates to ocean currents. “We cross the current bands, stop at regular stations, lower sensors into the water, and measure temperature, salinity, and current profiles,” Kanzow explains.
One reason for such interest is that the Arctic influences Europe’s climate quite directly. The Arctic Ocean is, in a sense, the Atlantic’s backyard — the two oceans exchange water through large passages, such as those between Greenland and Scandinavia. By contrast, the connection to the Pacific is limited to a tiny backdoor: the Bering Strait, only 80 kilometers wide and 50 meters deep.
Put simply, the warmer water from the Atlantic flows beneath the sea ice toward the Arctic Ocean, where it circulates in vast gyres like the Beaufort Gyre north of Canada and then sinks. The Arctic, in turn, exports colder, saltier deep water back to its main trading partner — the Atlantic.
Researchers suspect, however, that rising temperatures are disrupting this cycle, partly because of melting in Greenland. “More freshwater from Greenland is entering the ocean,” says Kanzow. In addition, more icebergs are calving from Greenland and drifting southward. A study published this year showed that a massive freshwater bubble has formed in the Beaufort Gyre. How exactly this freshwater is distributed across the Arctic still needs to be studied in detail, Kanzow notes, but “the freshwater issue potentially affects the thermohaline circulation” — the global conveyor belt. “In general, both ocean warming and increased freshwater input are expected to reduce deep-water formation rates,” says Kanzow.
What is certain is that the return flows from the Arctic also influence the AMOC. Between Greenland and Scandinavia, the deep water from the Arctic merges with the sinking water of the North Atlantic. Together, these two factors keep the AMOC running — at least for now.
Chapter 4
In the Indo-Pacific
Europe’s climate is also likely shaped by forces on the other side of the globe.
The boundary region between the Atlantic and Indian Oceans is considered one of the most energetic marine areas on Earth. There, the Agulhas Current carries more than 75 million cubic meters of warm, salty water per second from the tropics southward — roughly 350 times the flow of the Amazon River.
The Agulhas Current is one of the so-called “western boundary currents.” Driven by the westward-blowing trade winds and Earth’s rotation, water piles up along the western edges of many ocean basins, concentrating into a narrow jet. Along South Africa’s coast, this means towering waves and perfect surfing conditions.
Once the Agulhas Current shoots past Cape Agulhas, at Africa’s southern tip, the westerlies steer most of it back into the Indian Ocean. However, part of the water pinches off into eddies that drift into the Atlantic. With diameters of more than 200 kilometers, these “Agulhas rings” rank among the largest vortices in the world’s oceans.
The processes behind this dynamic are not yet fully understood. The oceans of the Southern Hemisphere are less well studied, notes Taimoor Sohail of the University of Melbourne. “Much of global research capacity is concentrated in the Northern Hemisphere,” he says — and those research institutions often focus primarily on their own regions, such as the AMOC.
What is clear, however, is that the Agulhas rings also play a key role in regulating the global climate. They carry large amounts of warm, salty water into the Atlantic, helping to drive the northward branch of the AMOC. It takes about seven years for water from the Indian Ocean to reach the Caribbean — and up to forty years to arrive in the North Atlantic.
Simulations suggest that water transport via the Agulhas rings could increase by around 13 percent over the course of the 21st century, thereby partially stabilizing the AMOC. The reason: higher greenhouse gas concentrations are strengthening the westerly winds in the Southern Hemisphere — the very winds that generate the Agulhas rings. In this way, global warming could, paradoxically, contribute to a stabilizing effect on Europe’s climate — though many questions about these processes remain unresolved.
The world’s largest ocean also presents its share of mysteries. Unlike the Atlantic, the Pacific has no large-scale overturning circulation transporting heat from the tropics northward; its existing currents barely extend beyond the equator.
Why is that? One hypothesis is that the Atlantic is saltier. This denser water is more prone to sinking, which helps sustain overturning circulation — whereas in the Pacific, the “engine” never really starts. This asymmetry between the Atlantic and the Pacific likely dates back two to three million years.
A simulation by two climate scientists at Yale University, however, suggests that this could change under conditions of very high global temperatures. According to their study published in the Journal of Climate, if atmospheric CO₂ concentrations became extremely high, the Atlantic overturning circulation would collapse within about 200 years. Roughly 2,500 years later, a new overturning circulation in the Pacific would begin — at first sputtering, then strengthening — moving heat northward. Off Japan’s northern coast and along Kamchatka, the water would then sink into the depths and flow southward again, much as it does today near Greenland.
The scenario assumes a CO₂ concentration more than twice today’s level — practically ruled out. Yet such a process might begin earlier. Paleoclimate data indicate that during the Pliocene warm period, around five to three million years ago, a Pacific overturning circulation (the PMOC) did exist. During that time, atmospheric CO₂ concentrations were about as high as they are today.
A 2024 study by researchers at the Potsdam Institute for Climate Impact Research also found that a collapse of the AMOC could have surprisingly far-reaching effects on the Pacific. More iron particles would likely be transported into the ocean, potentially stimulating phytoplankton growth in the eastern Pacific.
How quickly the oceans can change was already noted more than a hundred years ago by Norwegian polar explorer Fridtjof Nansen. In a lecture given in 1897, he concluded with these words: “Everything is drifting; the whole ocean moves ceaselessly, a link in nature’s never-ending cycle, just as shifting and transitory as the human theories.”