Beneath the subtropical paradise, Bermuda hosts a vast network of caves which contains records of Earth’s climate history. Inside are mineral deposits called speleothems – including stalagmites on the cave floors more than six-foot tall. These grow slowly as water drips down from the cave ceiling, gaining a millimetre every few years.

The stalagmites record the chemical signals of the dripwater that formed them. Cold weather tends to be windier, for example, leading to more sea spray and more seawater in the dripwater. Analysing the chemistry of one of these stalagmites has thus enabled us to indirectly reconstruct past sea surface temperatures.

Our latest research, published in the journal Communications Earth & Environment, harnesses this information to show a long-term picture of Atlantic Ocean temperatures, with a datapoint every ten days back to the year 1449. This record shows the Gulf Stream moved northward 300 years ago – a sign that a major system of ocean currents called the Atlantic Meridional Overturning Circulation (Amoc) started weakening then.

The Gulf Stream is a major ocean current that moves warm surface water from the Gulf of Mexico northward across the Atlantic, helping keep western Europe mild. As the water travels north it cools and sinks, flowing back south at depth. Together, these processes form part of the ocean conveyor belt known as Amoc.

If Amoc slows down too much, it could lead to dramatic regional climate change. Northern Europe would experience extreme cooling of up to 15°C, and rainfall and weather patterns across the tropics and subtropics would move and intensify.

Scientists agree that this system is crucial for regulating climate, but there is great uncertainty surrounding its stability. Although some studies suggest there has been no recent weakening, most agree the system has weakened in response to rising global temperatures. However, we don’t know for how long, and by how much, the Amoc has been slowing.

One fingerprint of Amoc change is the position of the Gulf Stream. When the Amoc weakens, the Gulf Stream moves northward, crossing the Atlantic at higher latitudes. This is what our Bermudan stalagmite has revealed: before the year 1720, ocean temperatures were unusually high. This period coincides with the little ice age, a cold interval in the northern hemisphere between approximately 1300 and 1850.

After 1720, Bermudan sea surface temperatures cooled substantially for more than a century. At the same time, records to the north (along the east coast of North America) show the opposite: warming where there had previously been cold temperatures.

This shift suggests the Amoc may have begun weakening a long time ago, starting around 1720 – before widescale industrialisation. This indicates that the system may be more sensitive than previously thought, because it responded to natural melting of ice sheets earlier than expected.

It could also mean the current Amoc is closer to a tipping point than expected. If a tipping point is crossed, the weakening would become self-perpetuating and lead to a near-complete shutdown of these vital ocean currents.

A warning signal

As global temperatures pass 1.5°C over the next few years, many climate models predict further weakening of the Amoc – and potentially even a collapse this century. The Intergovernmental Panel on Climate Change, the global committee assessing climate science, estimates there is up to a 10% chance of collapse before 2100 – but new research suggests this probability could be even higher.

Our study adds further historical context, showing that even small changes in ocean circulation can have large regional consequences. A sustained movement of the Gulf Stream would lead to changing regional temperatures, rainfall patterns and more extreme weather. This could have serious implications for wildlife and food security, as ecosystems struggle to adapt to the changing climate.

Even if the Amoc does not cross a tipping point soon, our research shows the weakening could still have a significant impact on regional climate patterns. The record does not just tell us about the past – it’s a warning that any amount of slowing down could have serious effects.

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James Baldini received funding from the European Research Council (grant number 240167).

Edward Forman does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.