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What is the AMOC? The ocean current climate scientists watch

The AMOC is a loop of Atlantic currents that carries warm water north at the surface and cold water south at depth. It is expected to weaken; whether it could collapse is argued.

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The AMOC, short for Atlantic meridional overturning circulation, is a system of currents that moves water around the Atlantic Ocean in a giant loop. Warm, salty water flows north near the surface, cools and sinks in the far North Atlantic, and returns south deep below. Along the way it carries a large amount of heat northward. Climate models expect it to weaken as the world warms; whether it could collapse, and when, is one of climate science's sharpest debates.

In short

  • The loop runs on differences in water's density: cold, salty water is heavy and sinks, and that sinking pulls warm water north behind it.
  • It moves about 1.2 petawatts of heat north across the subtropical Atlantic, roughly 60 times humanity's whole rate of energy use.
  • The IPCC judges it very likely to weaken this century and unlikely to collapse before 2100; some recent studies argue a collapse could come sooner.

How does the AMOC work?

The name says what it does. Meridional means north–south, along the lines of longitude; overturning means water at the top ends up at the bottom and back again. Picture a conveyor belt with two levels.

On the upper level, warm water from the tropics flows north. The Gulf Stream along the coast of North America is the best-known part, and its extension, the North Atlantic Current, carries warm water toward Europe. As it travels north, the water gives up its heat to the air and some of it evaporates, which leaves the remaining water saltier. Cold and salty, it becomes dense enough to sink, in a few places such as the seas around Greenland and the Labrador Sea.

On the lower level, that dense water flows south along the ocean floor, as North Atlantic Deep Water, all the way past the equator. It rises back toward the surface far away, mostly in the Southern Ocean around Antarctica, where strong winds pull deep water up, and then it slowly finds its way back north. According to NOAA's Ocean Service, a parcel of water takes an estimated 1,000 years to travel the whole global conveyor, of which the AMOC is the Atlantic part.

The engine depends on the sinking. If the northern surface water became fresher, from melting ice or more rain, it would be less dense, sink less, and the whole loop would slow. That is the worry.

How big is it, in numbers?

Ocean scientists measure flows in sverdrups (Sv): 1 Sv is a million cubic meters of water a second. Measurements across the Atlantic at 26°N since 2004 have found the AMOC moving about 17 Sv on average, with large swings from season to season and year to year.

Work out what 17 Sv means.

  1. Water. 17 Sv is 17 million cubic meters a second. The Amazon, the world's largest river, carries about 0.2 million. So the AMOC moves about 80 Amazons' worth of water.
  2. Heat. The RAPID project measures the heat it carries north at 26°N as about 1.2 petawatts, which is 1.2 million billion watts, and about a quarter of all the heat the ocean and atmosphere together carry toward the poles at that latitude.
  3. Comparison. Humanity uses energy, all of it together, at a rate of roughly 20 terawatts, or 0.02 petawatts. 1.2 divided by 0.02 is 60.

The AMOC carries heat north at about 60 times the rate the whole human economy uses energy. A change in a flow that size changes weather on the continents around it.

Does the AMOC keep Europe warm?

Partly, and less than the popular version says. Western Europe is mild for its latitude: London is farther north than Calgary. The AMOC's warm water releases heat into the air over the North Atlantic, and westerly winds carry it over Europe. But the winds themselves, which blow off a large ocean that stores summer heat, do much of the work too, and studies disagree on the exact share. What models agree on is that a much weaker AMOC would cool the North Atlantic and the lands around it, even as the rest of the planet warmed.

The effects would reach further. A slower AMOC is expected to raise sea level along the US East Coast, as NOAA notes, to shift tropical rain belts southward, which would change monsoons in West Africa and South Asia, and to reduce how much heat and carbon dioxide the ocean takes up.

It has happened before, in a different world. During the last ice age, great floods of meltwater and icebergs into the North Atlantic slowed or stopped the circulation several times. The best known is the Younger Dryas, about 12,800 to 11,700 years ago, when the far north swung back to near-glacial cold within decades. Today's ice sheets are smaller, so the setting isn't the same, but the episodes show the loop can switch states.

How is the AMOC measured?

Directly, with instruments strung across the ocean. Since April 2004, the RAPID array has kept moorings across the Atlantic at 26°N, from Florida to the Canary Islands. They measure temperature, salt and pressure from the surface to the seabed, and together with a cable that measures the Gulf Stream in the Florida Strait, they let scientists calculate the total flow every day. Other arrays now watch it farther north, between Canada, Greenland and Scotland.

Twenty years is short for a system that varies over decades, so for the longer past scientists use proxies: sea surface temperatures, sediment grains on the seabed, or the ocean's heat exchange with the air. The proxies are where much of the disagreement starts. RAPID recorded a major downturn in 2010 and 2011 and a lower average since, but whether that is a long-term decline or a swing is still argued.

Is the AMOC going to collapse?

The mainstream assessment is that it will weaken but probably not collapse this century. The Intergovernmental Panel on Climate Change (IPCC), in its 2021 report, judged the AMOC very likely to weaken over the 21st century under every emissions scenario, with medium confidence that it won't collapse abruptly before 2100. Several studies since have pushed in both directions.

Still argued

Collapse sooner. In July 2023, Peter and Susanne Ditlevsen used a statistical early-warning analysis of a sea-temperature proxy to estimate a collapse between 2025 and 2095 under current emissions; a correction in 2025 moved the estimate to between 2037 and 2109, most likely around 2065. In February 2024, René van Westen and colleagues produced a collapse in a full climate model and reported that reanalysis data show the present-day AMOC "on route to tipping", though their model reached that point only under a slowly rising freshwater input, far larger than today's. In October 2024, 44 climate scientists wrote to Nordic governments that the risk of collapse in the coming decades had been greatly underestimated.

Collapse unlikely this century. In January 2025, Jens Terhaar and colleagues inferred the AMOC from air–sea heat exchange and found that its decade-averaged strength at 26.5°N had not weakened from 1963 to 2017, arguing that the temperature proxies used for alarm don't track it reliably. In February 2025, Jonathan Baker and colleagues found the AMOC weakened but survived in all 34 climate models they pushed with an abrupt quadrupling of carbon dioxide, and in the seven that also added heavy meltwater, because Southern Ocean winds keep pulling deep water up.

What the evidence shows: weakening under warming is widely expected; the timing and likelihood of a collapse are not settled, the statistical early-warning methods are disputed, and models may be either too stable or about right. The risk is low-probability but very high-impact, which is why it is watched.

A collapse would not freeze Europe overnight, as in the film The Day After Tomorrow. Models that produce one show cooling over decades, strongest in winter and in the far northwest of Europe, on top of global warming elsewhere. It belongs with other possible tipping points of climate change, changes that, once past a threshold, would be hard to reverse. Unlike El Niño, which swings back within a year or two, a shift in the AMOC would last centuries.

Check yourself

  • Why does saltier, colder water sink, and why would fresher water slow the whole loop?
  • Without looking back: how big is 17 sverdrups, compared with the Amazon, and how much heat does the AMOC carry?
  • Why do two recent studies reach opposite conclusions about whether the AMOC has already weakened?

Sources

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