The phenomenon at the heart of this concern is the potential weakening or collapse of the Atlantic Meridional Overturning Circulation (AMOC)—a crucial system of ocean currents that transports warm water from the tropics to the North Atlantic. Some studies suggest AMOC is already slowing, and if it were to reach a tipping point, the UK's climate could be drastically altered, with bitterly cold winters replacing the more temperate conditions the country currently experiences.
The Ocean Current Keeping Britain Mild
The AMOC plays a crucial role in Europe's climate by distributing heat across the Atlantic. It helps explain why London is considerably milder than other cities at a similar latitude, such as Moscow. The circulation works by carrying warm, salty water from the equator northwards, where it cools, sinks, and flows back towards the tropics in a deep ocean current. This continuous process maintains the heat balance and climate stability of the North Atlantic region.
However, climate change is disrupting this system. Rising global temperatures are causing glaciers and ice sheets, particularly in Greenland, to melt at an accelerated rate. This influx of freshwater into the North Atlantic makes the ocean less salty and reduces the density of surface waters, preventing them from sinking and potentially slowing down or even halting the circulation.
Is AMOC Already Slowing?
Although direct measurements of AMOC strength have only been available since 2004—too short a period to confirm a long-term trend—indirect evidence suggests the system has already weakened by around 15% over the last two centuries. Scientists cite multiple indicators of this decline, including sediment analysis from the ocean floor and the presence of the so-called ‘cold blob'—a mysterious patch of cooling water south of Greenland that runs counter to the rest of the world's warming trend.
"A slowdown in AMOC is a very clear signature of climate disruption," says Matthew England, a professor of oceanography at the University of New South Wales. "If it were to collapse, the consequences would be far-reaching."
The Intergovernmental Panel on Climate Change (IPCC) expects AMOC to continue weakening over the coming century. But the main concern is whether this decline will be gradual or whether the system could suddenly ‘switch off'—something believed to have happened multiple times in Earth's past.
Could AMOC Collapse Completely?
One of the greatest uncertainties in climate science is how close we are to a tipping point for AMOC. While the IPCC previously stated with "medium confidence" that a full collapse is unlikely this century, more recent studies suggest the risks may be greater than previously thought.
"We don't know exactly where the tipping point is, but some models suggest it could happen within decades rather than centuries," warns David Thornalley, a professor of ocean and climate science at University College London.
More than 40 leading scientists signed an open letter last year calling for AMOC risks to be taken more seriously, arguing that even if the likelihood of a collapse remains uncertain, the potential consequences are so severe that governments must start planning for them.
What Would an AMOC Collapse Mean?
If AMOC were to slow further, or even collapse, it could drastically alter weather patterns across the world. In the UK, winters could become far colder, akin to those experienced in northern Norway. With colder ocean temperatures, the mild conditions Britain has long enjoyed would be replaced by harsh and prolonged winter freezes.
"It could become horribly, horribly cold," says Prof Thornalley. "Our infrastructure is not set up for that kind of climate."
Other effects would include an increase in winter storms due to shifting air currents, as well as dramatic disruptions to tropical rainfall patterns, potentially devastating food production in regions reliant on monsoons, such as West Africa and South Asia.
How Can the UK Prepare?
Despite mounting concerns, a recent report found that the UK government has not yet formally assessed the economic or infrastructural impact of a potential AMOC collapse. Scientists argue that this is a "glaring national security blind spot" and that the UK must start preparing for both gradual AMOC weakening and the possibility of a sudden collapse.
Prof Tim Lenton from the University of Exeter compares the situation to the COVID-19 pandemic—another crisis scientists warned about but governments were ill-prepared for. "We need to take the risks seriously now," he says, "rather than scramble to respond when it's too late."
Reducing the Risks
The most fundamental way to reduce the threat of an AMOC collapse is to limit global warming. "Every fraction of a degree matters," says Prof England. "Reducing greenhouse gas emissions can slow ice melt in Greenland, stabilising the ocean currents and buying us more time."
While the UK is already feeling the effects of climate change through rising summer temperatures and extreme weather events, the prospect of a dramatically colder future may seem counterintuitive. But for scientists studying ocean currents, it is a possibility that cannot be ignored. As Prof Thornalley warns, "We are playing Russian roulette with our climate system. The more we delay action, the greater the risks become."