Collapse of AMOC ocean current may already be locked in

AMOC collapse – New computer simulations on the Atlantic Meridional Overturning Circulation (AMOC) estimate that there is a 10% to 23% chance the collapse may already be “locked in,” depending on emissions timing and how fast Greenland ice keeps melting. Researchers say the w

By the time the ocean current is visibly changing, it can already be too late for most fixes.

That’s the unsettling message coming out of new model simulations of the Atlantic Meridional Overturning Circulation—AMOC—the massive conveyor belt that carries warm. salty water from the tropics into the North Atlantic. There, it cools, sinks, and returns south. The circulation regulates climate across Europe, Africa and the Americas. And lately. there have been signs that it is weakening: in some areas it has slowed. in part because melting Greenland ice sheets—driven by climate change—freshen the water. making it less dense so it sinks more slowly.

Researchers now estimate that a potentially catastrophic AMOC collapse—something some scientists say could push Europe toward near-Arctic conditions and weaken monsoon systems around the world—may already be “committed” in a meaningful fraction of futures. In their simulations, there is a 10 to 23 per cent chance that the collapse is already inevitable.

Phil Holden at the Open University, UK, puts it bluntly: “There is a significant probability that we’re already committed to collapse, and we can’t change that even now.”

The AMOC collapse has been hard to pin down in human terms. Holden describes why: “The AMOC collapse has just been so intangible. So far there’s been no firm quantification of whether it is going to happen or when it is going to happen.”

Tim Lenton at the University of Exeter, UK, says even among specialists the picture is not settled. “There’s a lot of uncertainty, and a lot of different opinions among the expert scientific community.”

To reduce that uncertainty to something that can be compared, Holden, Lenton and colleagues ran 21 computer simulations. The setup varied two key levers: rates of Greenland ice melt and the date when greenhouse gas emissions peak.

The simulations started in intervals of 2005 through to 2135, at 10-year steps. After the peak, the team assumed greenhouse gas emissions would fall to net zero over 35 years, while the Greenland ice melt rate would remain constant. Each simulation then ran for a total of 300 years.

The researchers also had a clear definition for what “commitment” means in the model. Under very conservative assumptions—where emissions peak in 2025 and the Greenland ice sheet contributes just 54 millimetres to sea-level rise by 2100—the team estimates a 10 per cent chance that the AMOC collapse is already inevitable. They define inevitability as the point when the circulation only occurs at lower latitudes and the overturning current that brings heat to high latitudes has stopped.

If net zero doesn’t arrive until 2100, the model predicts that the probability of collapse rises dramatically to 80 per cent.

Under less conservative assumptions, the numbers shift again. When melting ice from Greenland adds 274 mm to sea-level rise by 2100—as projected by 2100—the probability that the world is already committed to collapse is 23 per cent.

Even when commitment happens, the ocean doesn’t switch off instantly. In the simulations, the average delay between the world becoming committed to collapse and the collapse taking place was 84 years. The earliest collapse occurred around 2060.

Till Wagner at the University of Wisconsin-Madison. who is not trying to oversell the implications. frames the research as a tool for decision-making rather than a stopwatch. “This idea of talking about committed collapse. rather than when the collapse actually happens. frames it in a way that’s quite interesting for risk management.” But he warns against straight-line extrapolation from model to reality: “I think there’s fairly good evidence that there’s going to be a weakening. but the actual larger-scale dynamical outcome is still very much up in the air.”.

The work also comes with a practical limitation that matters for the numbers themselves. Jonathan Baker at the Met Office. the UK’s national weather service. says the simulations are valuable for exploring how the AMOC responds to different scenarios. But the model used in this study has low resolution. and that could affect how sensitive it is compared with other climate models. Many state-of-the-art climate simulations compute the globe in grids of 1° of latitude and longitude. a choice that demands huge computational resources—especially when running long-term simulations. This study uses 5° grids.

Lenton says that lower resolution was deliberate, because higher-resolution experiments at this scale aren’t currently feasible. “There isn’t the compute power. or no one’s had the ability to do this exercise with a higher-resolution model.” He adds that while higher resolution could shift probability estimates. recent research on the AMOC using a higher-resolution model suggests the risk estimates might rise rather than fall.

Baker calls for caution before locking conclusions into policy: “Further work using multiple climate models and comparison with the wider body of evidence will be important before drawing reliable conclusions about the magnitude of future AMOC collapse risk.”

Still, the central message of the simulations is hard to ignore. If the world is indeed already leaning toward collapse in some futures, that changes how people might think about urgency. Lenton argues the logic is straightforward: “This is because the model indicates that the probability of AMOC collapse rises sharply if net zero is delayed.”.

His numbers underline the trade-off. If emissions continue unabated for 10 extra years beyond the point of commitment, the actual collapse would happen faster—after 57 years on average rather than 84.

“What the model is saying to me is ‘let’s do everything in our power to get to net zero as quickly as possible to try to keep this probability down at the 10 per cent level’,” Lenton says.

The grim prospect has also met a thread of hope. The study’s findings chime with research published last month that hinted the slowdown of the AMOC may be reversible—if carbon dioxide emissions come down enough.

Between “reversible slowdown” and “committed collapse” lies a narrow corridor where emissions cuts could still change the trajectory. What these simulations are asking now is whether that corridor might already be smaller than anyone wanted to believe—and whether the next decade’s choices matter not just for the future. but for whether certain outcomes have already been set in motion.

AMOC Atlantic Meridional Overturning Circulation ocean currents climate change Greenland ice melt sea-level rise tipping point net zero Met Office computer simulations