When the fate of an ocean current system tops headlines during a historic heat wave, it’s a strange week for climate science.
A new study last week predicted the Atlantic Meridional Overturning Circulation, or AMOC, could pass a tipping point and collapse as soon as 2025. The bold prediction, which portends a future that could upend weather across the Atlantic Ocean, prompted news headlines fretting over “climate chaos,” “devastation” coming “ahead of schedule” and a “real-life Day After Tomorrow.”
Is AMOC on track to run amok? The risk of collapse remains a topic of hot debate and scientists are in disagreement over whether it is already slowing down or if it could be nearing a tipping point. Direct analysis of AMOC dates back to 2004 — too short a period to be certain of its trajectory.
It’s a debate with considerable stakes. The arguments illustrate scientists’ uncertainty about the next phases of climate change and just how to characterize the risk of extreme — hopefully unlikely — events as real-world climate disasters are taking place. The higher humans push temperatures, the less certain scientists are about where the climate is heading or if there are tripwires beneath humanity’s feet.
The term “tipping point” can be confusing — it’s often used interchangeably to describe sudden disruptions, irreversible shifts or feedbacks that rapidly amplify climate change.
Scientists are concerned the West Antarctic ice sheet could collapse and cause extreme, irreversible sea level rise or that Amazon forest dieback could metastasize Earth’s lungs, among other tipping points.
But some researchers worry that media fixation on tipping points in which there’s less confidence, like the recent AMOC study, could be a distraction when the perils of climate change are already in plain view.
“The focus on whether those changes will be abrupt or not or reversible or not … is missing the bigger picture,” said Kyle Armour, an associate professor in the departments of oceanography and atmospheric sciences at the University of Washington. “We have good reason to worry. We don’t have to think about the exotic tipping point.”
He said there already are robust predictions that AMOC will slow down because of human emissions. He thinks most climate change impacts are far more likely to play out in a steadier, harmful fashion — in line with human emissions.
AMOC describes a series of ocean currents that cycle water in the Atlantic Ocean. In general, currents push warm water northward, where a small portion becomes sea ice. The remaining water gets colder, saltier and dense. It sinks to depths in the North Atlantic and then slowly travels southward.
When it comes to weather, AMOC transports heat to Europe and also determines the location of a belt of precipitation in Central America, said Spencer Jones, a visiting assistant professor in physical oceanography at Texas A&M University.
A rapid halt to AMOC would cause rapid cooling in the North Atlantic, warming in the Southern Hemisphere and extreme changes in precipitation.
Climate models have long predicted AMOC would slow. A collapse would trigger extreme change — perhaps three to four times bigger, Jones said.
Scientists began directly measuring aspects of AMOC in 2004. The new study uses sea surface temperatures dating back to 1870 to estimate change over a longer period of time, and suggests with 95% confidence that the current could collapse between 2025 and 2095.
“We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions,” wrote co-authors Peter and Susanne Ditlevsen, countering a United Nations Intergovernmental Panel on Climate Change report that said collapse was unlikely this century.
Some experts said the recent study adds a new angle to other evidence suggesting collapse could happen this century.
Stefan Rahmstorf, a professor of physics of the oceans at Potsdam University in Germany, wrote that standard climate models are “underestimating risk” and that multiple methods of analyzing past climates suggest that a risk of collapse needs to be “taken very seriously.”
In an email to NBC News, Tim Lenton, a professor at the University of Exeter in the United Kingdom and founder of the Global Systems Institute, said the new study makes “important improvements to the method of providing early warning of a climate tipping point directly from data.”
But others are more skeptical of the research’s bold conclusions and reliance on a simple model.
“Proxies have to be taken with a grain of salt,” according to Eleanor Frajka-Williams, a physical oceanographer and climate scientist at the University of Hamburg in Germany, who said the science on how closely sea surface temperatures correspond to AMOC changes is unsettled and the new research risks simplifying a complex system for an overconfident prediction. She did note that the IPCC recently downgraded its confidence in the prediction that AMOC would not collapse by 2100.
Scientists have been directly tracking AMOC with sensors for about two decades, but it will likely take another 10 years before they can say for sure that the circulation system is slowing down, she said.
Some scientists aren’t sure that AMOC has a tipping point at all. There is evidence that AMOC has changed abruptly in the distant past, but researchers like Armour are unconvinced that it is likely in the near future.
“We’re just not in that climate anymore and we haven’t been for thousands of years. The drivers of AMOC right now are different,” he said. “What we do know is AMOC will slow with global warming in the future, but we haven’t yet seen the slowing and there’s no way to know if there’s an abrupt change or not.”
From Armour’s perspective, centering media conversation on uncertain predictions of systems where it’s not clear that tipping points exist could risk pushing people toward climate doomerism or could cost scientists credibility later on.
“We don’t have good evidence for anything being irreversible in the climate system except for ecosystems and ice sheets,” he said of large-scale physical climate systems. “We often find it’s much more complex than we think, and the complexity tends to reduce the likelihood of tipping points.”
Whether or not AMOC is heading for a slowdown or a frightening collapse, the solution remains the same.
From a policy perspective, “it doesn’t matter whether the AMOC is going to collapse. There are going to be hugely bad climate impacts if we don’t act today,” Jones said.