Why the slowdown of ocean currents is dangerous for the planet

Climate change does not only appear as heatwaves, droughts or storms. Some of its most important effects are hidden below the ocean surface. One of them is the weakening of ocean circulation – the planetary system of currents that moves heat, oxygen, carbon and nutrients around the globe.

What is the trend?

Global warming is the long-term rise in Earth's average temperature, driven mainly by human activity. It has been observed since the late 19th century, and its pace has accelerated strongly in recent decades. If emissions are not reduced quickly, the consequences will include rising seas, more extreme heat, damage to food systems, freshwater stress and greater pressure on societies and infrastructure, especially in coastal and densely populated regions.

Deep in the cold ocean, dense, dark "rivers" of water move continuously, often only a few centimetres per second. These currents may be slow, but they are one of the planet's main life-support systems. They distribute heat, carry oxygen into the deep sea, store carbon for centuries, and bring nutrients back toward the surface, where marine food webs begin.

In recent decades, parts of this circulation have shown signs of weakening, and scientists are increasingly concerned about the consequences.

A problem that was once underestimated

When the Intergovernmental Panel on Climate Change published its first assessment report in 1990, the interaction between climate and ocean circulation was far less developed in climate models than it is today. Early projections often represented the ocean in a simplified way: as a large heat reservoir rather than a dynamic, three-dimensional circulation system.

Since then, ocean science has changed dramatically. Researchers now understand that the ocean is not a passive background to climate change. It is an active regulator of the climate system.

How currents move

Ocean water moves in three dimensions. Some currents flow horizontally across ocean basins; others move vertically, carrying water from the surface to the deep ocean and back again. Horizontal currents are largely shaped by winds, Earth's rotation and the shape of ocean basins. Vertical movement depends heavily on density, which is controlled by temperature and salinity.

Near the poles, sea ice formation leaves salt behind in the surrounding seawater. This makes the remaining water saltier, colder and denser. Dense water then sinks toward the deep ocean. In the Southern Ocean around Antarctica, this process forms Antarctic Bottom Water – one of the most important water masses on Earth.

Trillions of tonnes of cold, dense water sink in polar regions and begin a long journey through the deep ocean. Eventually, this water rises elsewhere, interacts with warmer surface currents and returns poleward. Together, these processes form a global overturning system sometimes described as an ocean "conveyor belt".

The Atlantic part of this system is the Atlantic Meridional Overturning Circulation, or AMOC. It includes warm northward flow near the surface and cold southward flow at depth. The Gulf Stream is related to this wider system, but it is not identical to it. For Europe – especially the UK, Ireland, Scandinavia and the North Sea region – this heat transport helps moderate the climate.

The slowdown

Antarctica is one of the most important engines of deep-ocean circulation. But evidence suggests that the formation and export of Antarctic Bottom Water is weakening in some regions. Observations in the Australian Antarctic Basin and near the Ross Sea show that the transport of cold, salty, oxygen-rich bottom water has declined over recent decades, while oxygen levels in deep water have also fallen.

The mechanism is straightforward. As the planet warms, Antarctic ice melts more rapidly. This adds fresh water to the ocean. Fresher water is less dense and therefore less likely to sink. When less dense water sinks, the deep overturning circulation slows.

This does not mean that the ocean suddenly stops. The real danger is gradual but systemic: less oxygen reaches the abyss, less heat and carbon are stored in the deep ocean, and fewer nutrients return to the surface.

Sea-level rise and coastal risk

The 2004 disaster film The Day After Tomorrow turned ocean circulation into a dramatic story: the AMOC shuts down, and the Northern Hemisphere is plunged almost overnight into a new ice age. That is not how scientists expect climate change to unfold.

The more realistic concern is that weaker circulation changes how heat reaches polar ice shelves. If warmer water penetrates the edges of Antarctica, it can melt ice shelves from below. These shelves act like buttresses, slowing the flow of land ice into the sea. If they thin or collapse, glaciers behind them can accelerate, increasing sea-level rise.

For Europe, the risk is not abstract. Ports, deltas and coastal cities around the North Sea, the Baltic, the Atlantic coast and the Mediterranean are already planning for higher sea levels, stronger storm surges and more expensive coastal protection. A slowdown in major ocean circulation systems could amplify regional sea-level differences and complicate adaptation planning.

Faster climate change

Ocean circulation also matters because the ocean is the world's largest climate buffer. It absorbs the vast majority of excess heat trapped by greenhouse gases and stores large amounts of carbon. The deep-ocean "conveyor belt" helps move heat and carbon away from the surface and keep them locked away for decades to centuries.

If overturning circulation weakens, this buffer can become less effective. More heat may remain near the surface, affecting weather patterns, marine ecosystems and the rate at which the atmosphere continues to warm.

In the North Atlantic, a weaker AMOC could mean less heat transport toward north-western Europe. This does not cancel global warming. Instead, it can create a more unstable regional climate: altered storm tracks, changed rainfall patterns, colder winter extremes in some scenarios and stronger contrasts between seasons.

Food security and fisheries

The slowdown also threatens marine food systems. Deep currents help recycle nutrients that sink from the surface to the seafloor. In upwelling regions, these nutrients return to sunlit surface waters, where they support plankton. Plankton feed small fish, which in turn feed larger fish, seabirds and marine mammals.

If deep circulation weakens, more nutrients can remain trapped in the deep ocean. That means less biological productivity at the surface and growing pressure on fisheries. This is especially important for regions that rely on marine protein and for Europe's seafood supply chains, which are connected to fisheries far beyond European waters.

Why it matters

Ocean currents move slowly, but they shape the living conditions of the planet. They influence weather, sea level, oxygen supply, carbon storage and food webs. Their weakening is not a Hollywood-style instant catastrophe. It is a slow-moving risk multiplier.

The solution is also clear. The less the planet warms, the lower the risk of destabilising the ocean systems that help keep Earth's climate liveable. That means cutting greenhouse gas emissions, protecting carbon sinks, improving climate adaptation in coastal regions and monitoring the ocean more closely – from the North Atlantic to the Southern Ocean.

illuminem Voices is a democratic space presenting the opinions of leading Sustainability Thought Leaders, their views do not necessarily represent those of illuminem.

The world needs sustainability knowledge. At illuminem, no interest group or shareholder can influence our work. Thank you for supporting our mission to make high-quality and independent sustainability information free for all. Every contribution helps. Thank you for donating today.