Atlantic ocean currents that respond to climate change are hurtling toward a tipping point that could cause severe impacts before the end of this century, a new study finds.
The currents are those that form the Atlantic Meridional Overturning Circulation (AMOC), which loops around the Atlantic Ocean like a giant conveyor belt, bringing heat to the Northern Hemisphere before traveling south again along the seabed. Depending on how much carbon humans emit in the next few decades, the AMOC could reach a tipping point and start to collapse as early as 2055, with dramatic consequences for several regions, researchers found.
This scary prediction, based on a scenario where carbon emissions double between now and 2050, is considered unlikely — but the outcome of a much more likely scenario where emissions hover around current levels for the next 25 years isn't much better, according to the study. Even if we keep global warming this century to 4.8 degrees Fahrenheit (2.7 degrees Celsius) above preindustrial levels — a "middle of the road" scenario, according to the latest U.N. climate report — the AMOC will start to collapse in 2063, the results suggest.
"The chance of tipping is much larger than previously thought," Sybren Drijfhout, a professor of physical oceanography at the University of Southampton in the U.K. and Utrecht University in the Netherlands, told Live Science in an email. Overall, the chance of the AMOC collapsing this century is about 50-50, Drijfhout, who was not involved in the new research but recently led a similar study published in the journal Environment Research Letters, estimates.
In the study, Drijfhout and colleagues ran the latest climate models for a period extending beyond 2100 and found that high-emission scenarios, or those that cause around 8 F (4.4 C) of warming above preindustrial levels by the end of this century, always led to an AMOC collapse. Scenarios that aligned with the aim of the Paris Agreement to keep warming ideally below 2.7 F (1.5 C) also triggered a collapse in two of the models, suggesting a breakdown is more likely than scientists previously thought, he said.
The new modeling study, published Aug. 24 in the Journal of Geophysical Research: Oceans, tested 25 climate models and found an indicator that helped researchers determine when the AMOC might reach a tipping point. Unlike the parameters commonly used to monitor the AMOC indirectly, such as sea surface temperature, this new indicator is governed by the dynamics of Atlantic ocean circulation, study lead author René van Westen, a postdoctoral researcher in climate physics at Utrecht University, told Live Science in an email.
Van Westen and colleagues previously showed that the Atlantic's flow of fresh water at 34 degrees south, the latitude along the tip of South Africa, is a good marker of the AMOC's stability and can warn scientists of an impending collapse. This marker works for slowly changing environmental conditions, but it's less useful for identifying AMOC trends under a rapidly warming climate, van Westen said.
"Therefore, we were aiming to develop a new indicator that also works under climate change," he said.
A new marker for AMOC strength
To gauge when tipping points will be reached, the new study looked at the mass of water that sinks to the ocean floor in the North Atlantic.
Right now, surface water loses heat to the atmosphere when it reaches the cold North Atlantic. This surface water becomes so frigid, salty and dense that it sinks to the bottom of the ocean, forming currents that travel along the ocean floor to the Southern Hemisphere. The process of cold, dense water sinking is called deep water formation, and it is the engine that drives the AMOC. Deep water formation can be measured through changes in seawater density or by extrapolating ocean data in climate models.
"When this quantity reduces to zero, it means that the surface has become too light and no sinking takes place," which is essentially the moment when the AMOC starts to collapse, van Westen said.
Deep water formation is already declining due to both warming air temperatures in the North Atlantic and Arctic ice melt. Warm air means that surface water can't lose enough heat to sink, while ice melt is diluting the salt concentration of the water and thereby decreasing its density.
The researchers identified one component of deep water formation, the surface buoyancy flux, which was a "shortcut" for estimating deep water formation across the North Atlantic, van Westen said. The surface buoyancy flux is a parameter that combines changes in heat and salinity at the ocean surface to understand how these impact the water's density. Heat and salinity can be monitored directly using instruments or satellites, but the study examined existing heat and salinity data in simulations of sea surface dynamics, with the surface buoyancy flux standing out in different models and experiments as a clear marker of the AMOC's strength.
"The advantage of [the surface buoyancy flux] is that it can be calculated in many climate models," van Westen said.
The surface buoyancy flux was constant until 2020, van Westen said, meaning there were hardly any changes in the AMOC before then — a conclusion that is bolstered by research published in January.
Since 2020, however, the surface buoyancy flux has increased, suggesting the AMOC is weakening. The models showed that high-emission paths could trigger an earlier collapse of the AMOC than "middle of the road" emission paths could, so it is urgent to curb fossil fuel use, according to the study.
"An AMOC collapse scenario can possibly be prevented when following a low emission scenario," van Westen said, but this would require reaching net-zero carbon emissions around 2050.
A "serious climate wake-up call"
An AMOC collapse in the 2060s is plausible and "very worrying," Drijfhout said, but the uncertainties are too large to pinpoint precise years when the AMOC will collapse under different emission paths.
The consequences would be dramatic and global, but Europe would be hit particularly hard, Drijfhout said. An AMOC collapse would bring much colder temperatures to Northwestern Europe, as well as a decrease in precipitation that may lead to agricultural losses of about 30%, he said. The winters in Europe would be much harsher, with more storms and flooding along the Atlantic coast resulting from a redistribution of water around the ocean as the AMOC slows.
"Even larger sea level rise can be expected at the American east coasts" due to this redistribution, Drijfhout said. And places that don't border the Atlantic could also be impacted, such as monsoon regions in Asia and Africa, he said.
Wopke Hoekstra, the European commissioner for climate, net zero and clean growth, described the findings as a "serious climate wake-up call" in a social media post. "This new study says that the Gulf Stream could collapse in our lifetime," he warned.
However, the effects won't be felt immediately after the AMOC starts to collapse, according to the study. The authors estimate that it would take more than 100 years for the AMOC to weaken significantly and for new weather patterns to emerge.
But Drijfhout thinks the collapse could happen over just 50 years. The AMOC is like a campfire with a dwindling amount of fuel, he said. "If we stop throwing new wooden blocks on the fire, the fire does not immediately die, but it keeps smouldering for some time," Drijfhout said. "For the AMOC this 'smouldering time' is [about] 50 years."
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