A vital system of ocean currents that governs weather patterns across the northern hemisphere could collapse by the middle of the century and potentially as early as two years’ time, according to a study warning that the ocean climate system is close to an irreversible tipping point. But other researchers have doubts about the accuracy of the projections.

The Atlantic meridional overturning circulation (AMOC) is a large system of ocean currents that acts like a conveyor belt, carrying warm water from the tropics into the North Atlantic.

Scientists have already warned that climate change is weakening the AMOC and say the system could potentially collapse in the future – an outcome heavily dramatised in the film The Day After Tomorrow.

But the strength of the AMOC has only been consistently monitored since 2004, leaving researchers without enough long-term data to make clear estimates of when such a collapse might occur.

The Intergovernmental Panel on Climate Change (IPCC) says a full AMOC collapse is unlikely in the current century, based on climate modelling.

In their new study, siblings Peter and Susanne Ditlevsen, both at the University of Copenhagen, Denmark, used sea surface temperature data from the sub-polar North Atlantic – which dates back to 1870 – as a proxy for the stability of the AMOC.

Their analysis suggests that the AMOC is becoming increasingly unstable and will soon hit a critical tipping point. Assuming greenhouse gas emissions continue on their current trajectory, they conclude that AMOC collapse is most likely to occur around the middle of this century – but they warn it could happen at any time between 2025 and 2095.

“The IPCC has said in their latest report that it is ‘very unlikely’ that AMOC collapses this century. Our results are actually much more negative,” says Susanne Ditlevsen. “What we would say is that if we continue emissions as now… it will probably happen between 2050 and 2080.”

The collapse of the AMOC could lead to rapid sea level rise in North America, a sudden and severe drop in temperatures across northern Europe and serious disruption to monsoons across Asia.

But researchers not involved in the study urge caution. Penny Holliday at the UK’s National Oceanography Centre warns that it isn’t clear whether sea surface temperatures can serve as a direct proxy for the resilience of the AMOC. “Sea surface temperature is affected by many other things,” she says. “It isn’t a one-to-one relationship.”