For generations, the mild and temperate climate of north-western Europe has been credited to one legendary force: the Gulf Stream. This idea is so deeply entrenched in our cultural identity that in James Joyce’s Ulysses, the protagonist Stephen Dedalus refuses to take a bath, arguing that “all Ireland is washed by the Gulf Stream”.
However, the Gulf Stream is just one part of a much more complex system called the Atlantic Meridional Overturning Circulation or AMOC.
To explain this better, scientists often use the image of a giant ocean conveyor belt, where warm waters move northwards across the surface of the Atlantic from the tropics. As these waters reach the North Atlantic, they release their heat into the atmosphere, much like a radiator. The AMOC also carries the moisture that gives us our temperate landscape. After the waters have released their heat, they become colder and denser, which makes them sink into the deep ocean. These waters then return southward, at great depths.
When scientists talk about the AMOC “slowing down” or “changing,” they are essentially describing a reduction in the strength of our natural radiator. Specifically, they measure how much water is moving north and south at different depths across the Atlantic. This allows them to estimate how much heat is being carried from the tropics toward the North Atlantic and back again at depth.
More than a conveyor belt
Although this “conveyor belt” analogy is a helpful starting point, modern research suggests it is incomplete and potentially misleading. For example, the system is incredibly sensitive to how seawater changes its weight and density as it interacts with the atmosphere, freshwater, ice and incoming solar radiation. Because of these additional processes, the AMOC behaves less like a single, steady loop and more like a network of interconnected regional components.
Different parts of the system can change independently, sometimes with only regional effects and sometimes with consequences for the entire system.
The Subpolar Gyre (SPG), a system of wind-driven ocean currents occupying the region from the Labrador Sea to the west of Ireland, is a powerful example of why the network perspective matters. This regional AMOC component can show a significant degree of independence from the global AMOC. It is controlled by local winds and pulses of freshwater, linked to changes in sea-ice.
Crucially for those of us in Ireland and the UK, a sudden weakening of the SPG could trigger abnormally cold winter weather, similar to conditions seen during the “little ice age”. This period of intense regional cooling, which lasted roughly from the early 14th century to the mid-19th century, was characterised by winters so severe that the River Thames froze over.
Scientific research suggests that this cold period was likely sustained and amplified by a regional change in the SPG while the AMOC remained relatively stable. This means we could face local climate shifts, including increased storminess and colder winters, because of a “flicker” in our regional component of the AMOC network, long before the entire global circulation reaches a tipping point.
This is why scientists are now focused on identifying early warning signs of instability within the AMOC.
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Are there signs that the AMOC has already begun to change? While climate models agree that it is likely that the AMOC will destabilise this century due to global warming, direct scientific observations of the AMOC are still too short to give us a definitive answer.
Networks of monitoring tools like Rapid or OSNAP that measure the transport of water both at depth and at the surface have only been in place for about 20 years. In the life of a massive ocean system, this is just a heartbeat. Scientists estimate we may need 30 to 40+ years of continuous observations to clearly detect a long-term AMOC decline against the ocean’s natural variability.
Why does it matter?
For generations, societies, economies and infrastructures in north-western Europe have been built around a stable, mild and wet climate. If this natural radiator fails or even significantly weakens the consequences will ripple across Ireland, the UK and the European continent.
We should care about this because the AMOC currently moves a massive amount of heat
from the tropics to the North Atlantic, where it is released into the atmosphere. A weakening of this system means that a portion of this tropical warmth is no longer delivered to our region as effectively, leading to cooling across northwestern Europe.
While Hollywood depicted a sudden ice age in the film The Day After Tomorrow (2004), the scientific reality of a slowdown is no less concerning. We could face significantly colder winters resulting in more frequent harsh freezes, snow and severe frosts. During the little ice age a weaker SPG led to agricultural failures and famines. We could also experience an increase in storminess shifting rainfall patterns, and drier summers, all of which could damage critical infrastructures like roads and crop harvests.
The AMOC is also essential for keeping carbon and heat stored in the deep ocean, effectively locking it away from the atmosphere. At the moment the world’s oceans absorb approximately 25-30% of all human-made carbon dioxide emissions each year.
However, should the AMOC slow down it is expected that the rate at which carbon is stored in the deep ocean also slows down. The AMOC also redistributes the nutrients that sustain marine ecosystems. A disruption here wouldn’t just change our weather; it would weaken the ocean’s ability to act as a carbon sink, potentially accelerating global warming in a dangerous feedback loop.
Keeping an eye on the AMOC is a matter of national and regional security.
Whether the decline is gradual or approaches a tipping point, the impact on our way of life will be profound. By listening to the signals coming from the deep ocean today, we can better prepare for the climate of tomorrow.
Audrey Morley receives funding from Research Ireland, The Marine Institute, The Department of Foreign Affairs and Trade (Ireland) and the Geological Survey of Ireland
