A study of clam shells suggests Atlantic Ocean currents may be approaching a "tipping point".
Scientists studied records of quahog clams (which can live for over 500 years) and dog cockles - because shell layers provide an annual record of ocean conditions.
They studied these natural archives to understand long-term patterns in Atlantic Ocean currents such as the Atlantic Meridional Overturning Circulation (AMOC) and the subpolar gyre (SPG).
Recent studies have debated possible AMOC and SPG tipping points - transitions that would transform climate patterns. For example, AMOC collapse would have far-reaching global effects, from harsher winters in north-west Europe to shifts in global rainfall patterns, while a weakening of the SPG would be less catastrophic but still bring substantial impacts, including more frequent extreme weather in the North Atlantic region.
The new study, led by the University of Exeter, finds evidence of "stability loss" which suggests the region is "moving towards a tipping point".
"Anticipating a tipping point requires good data, covering a long period with no gaps," said Dr Beatriz Arellano Nava.
"We don't have ocean observations going back into the distant past, but the bands in clam shells give us an unbroken annual record covering hundreds of years."
The annual difference in shell growth does not reveal specific details of the past climate, as growth depends on multiple factors, but it gives an overall picture of ocean conditions from year to year.
"When a system is stable, there will still be variations - but we would typically see a rapid return to the normal state after a change," said Professor Paul Halloran, from Exeter's Global Systems Institute.
"When a system destabilises, it doesn't recover as quickly - and this could be a sign of an approaching tipping point."
The study reveals two "destabilisation episodes" over the last 150 years.
The first - likely involving the SPG - happened in the early 20th Century, and may have played a role in a documented warming of the Arctic and North Atlantic in the 1920s.
The second, stronger destabilisation began around 1950 and continues to the present.
"Although we cannot yet say which part of the system is losing stability, or what may be causing it, our results provide independent evidence that the North Atlantic has lost stability - suggesting that a tipping point could be approaching," Dr Arellano Nava said.
"Because these circulation systems are interconnected, it is not clear whether the AMOC, the SPG, or both are driving the signals we see - but any tipping point would have major implications for the climate.
"Melting of polar ice due to climate change is certainly contributing to the weakening of ocean currents and pushing them closer to a tipping point, so rapidly reducing greenhouse gas emissions is the best way to prevent tipping points in the Atlantic Ocean."
The study was funded by European Union's Horizon 2020 COMFORT and the ARIA AdvanTip projects.
