Atlantic current shows steady decline, with scientists warning collapse could amplify global warming and disrupt climate systems

The complete collapse of the Atlantic Meridional Overturning Circulation (AMOC) could lead to the release of large amounts of carbon into the atmosphere, particularly from the Southern Ocean, turning it from a carbon sink into a source, according to a recent research paper published in the journal Nature Communications Earth and Environment.

This additional carbon dioxide (CO₂) could contribute up to 0.2 degrees Celsius (°C) of extra global warming. However, the regional impacts would vary sharply, with cooling of up to 7°C in the Arctic and warming of up to 6°C in parts of Antarctica.

A system nearing its tipping point

A separate study published in the journal Science Advances suggested that the AMOC may be approaching a tipping point, particularly in the western Atlantic Ocean.

Observational data from four mooring arrays across the western Atlantic, spanning latitudes from 16.5°N to 42.5°N, shows a consistent weakening of the western boundary overturning of the AMOC over the past two decades.

The AMOC is a complex conveyor belt of ocean currents that ferries warm surface water to the Arctic region, modulating the weather in northern North America and Europe, and brings back deep cold water to the tropics, constituting the feedback. 

AMOC is also one of the 16 Earth system climate tipping elements being studied by scientists for abrupt and irreversible changes that could lead to many cascading oceanic, atmospheric and ecological impacts across large regions of the planet. 

Why the AMOC is weakening

Most climate models suggest that the AMOC will weaken as greenhouse gas concentrations rise and global temperatures increase. One of the main drivers is believed to be the influx of freshwater from the melting Greenland Ice Sheet, which reduces ocean salinity and disrupts the circulation. 

Observations indicate that weakening is already underway, though the timeline for a full collapse remains uncertain. A 2023 research paper published in the journal Nature Communications estimates this could occur sometime between 2037 and 2109.

What happens if the AMOC collapses

The Nature Communications Earth and Environment study estimates that an AMOC collapse could release an additional 47-83 gigatonnes of CO₂ into the atmosphere.

Using the fast Earth system model CLIMBER-X, researchers simulated a transition to a “switched-off” AMOC state under different atmospheric CO₂ levels, ranging from 280 to 600 parts per million, and subsequent warming. The model is computationally efficient and has the ability to analyse the collapse of the AMOC on the climate and carbon cycle of the planet.

In the model simulations, scientists fed freshwater into the North Atlantic between specific latitudes over a period of 1,000 years to replicate the melting of ice sheets. To maintain the salinity balance of the global oceans, they also added saltwater to the tropical Pacific (30°N-30°S).

The CLIMBER-X model showed that the AMOC could collapse even without additional warming compared with the pre-industrial period. However, it also indicated that while the system may recover from such a collapse under stable conditions, this recovery may not occur in a warming world.

Cooling in the north, warming in the south

Under all CO₂ concentration scenarios, an AMOC collapse leads to cooling in the Arctic region by around 7°C, with the strongest cooling occurring in the North Atlantic across comparable latitudes. A shutdown of the AMOC results in a sharp reduction in heat transport to the northern North Atlantic Ocean, causing significant cooling that is further amplified by sea-ice-albedo feedback.

Sea-ice-albedo feedback occurs because sea ice reflects more sunlight than surrounding ocean water. “The cooling of the Arctic leads to a considerable increase in Arctic sea ice area, leading to an increase in albedo, which further causes cooling in the Northern Hemisphere,” the paper noted.

In contrast, the Antarctic region warms by around 6°C, with some areas becoming warmer by up to 10°C. Warming due to greenhouse gas concentrations in the atmosphere offsets some of the cooling in the northern hemisphere. Overall, the models show that the transition of the AMOC to an “off” state following collapse increases global average temperatures by 0.17-0.27°C.

The study also found that much of the excess CO₂ could be released due to circulation changes in the Southern Ocean. Carbon-rich waters from the deep ocean would rise to the surface, turning the Southern Ocean from a net carbon sink into a carbon source.

Evidence of decline continues to grow

Evidence pointing towards a weakening AMOC continues to grow. The Science Advances study found that changes in the western boundary of the circulation account for up to 90 per cent of the overall weakening observed.

“Maintaining a consistent methodology for continuously and accurately monitoring the AMOC across various latitudes of the Atlantic basin presents a considerable challenge for the physical oceanography community,” the study said. “Long-term observations of ocean bottom pressure (OBP) offer a promising approach for monitoring large-scale ocean circulation, as they provide high accuracy.”

When scientists analysed OBP data from different mooring arrays in the western Atlantic Ocean at depths between 1,000 metres and 4,000 m, they found the steepest decline in the strength of the western boundary transport of the AMOC at 16.5°N. 

At the other three latitudes — 26.5°N, 39.5°N and 42.5°N — the western boundary transport also showed a strong declining trend over the past two decades. The close correspondence between the western boundary transport and overall AMOC variability suggests that the circulation may be approaching a tipping point.