Deep in the Atlantic Ocean, a fundamental engine of the globe’s climate is losing momentum. The Gulf Stream and other torrential currents in the Atlantic, which help transport heat and energy around the world, are at their weakest point in more than 1,000 years, according to a study published on February 25th in Nature Geoscience.
That change will likely be exacerbated in the decades to come and could mean an increase in sea-level rise and extreme weather in places along the Atlantic coast, including New York and New Jersey. If the world continues to warm at the current rate and the Gulf Stream system shuts down completely, scientists say that the whole globe would feel the catastrophe.
“It’s something that has been predicted as a consequence of global warming,” said Stefan Rahmstorf, head of Earth System Analysis at the Potsdam Institute for Climate Impact Research and one of the authors of the study. “It’s another prediction coming true, and it potentially has massive impacts.”
The Gulf Stream is part of a system of ocean currents known as the Atlantic Meridional Overturning Circulation (AMOC). It acts like a conveyor belt and transports warm water from the tropics to the North Atlantic and Europe, impacting local climates along the way. The water then cools and sinks in the North Atlantic Ocean near Greenland and flows back south along the ocean floor, and the process starts again. The new study adds to a growing body of research that has found that this web of water is slowing down, its pace decreasing 15 percent since circa 1950.
This decline is as predicted by climate models in response to #globalwarming & responsible for a particular “finger print” pattern of sea surface temperature change including the northern Atlantic ‘𝗰𝗼𝗹𝗱 𝗯𝗹𝗼𝗯’ or ‘𝘄𝗮𝗿𝗺𝗶𝗻𝗴 𝗵𝗼𝗹𝗲’. See https://t.co/Mf1eqkJbKu 4/11 pic.twitter.com/9xvMJkmEEq
— Stefan Rahmstorf 😷 (@rahmstorf) February 25, 2021
Although direct measurements of the AMOC are only available starting in 2004, Rahmstorf and his colleagues used proxy records like ocean temperatures, deep-sea currents, and sediment data to model changes dating back to A.D. 400. They found an unprecedented decline in the Gulf Stream system in recent decades, said Rahmstorf.
The oceanic network matters because it has an outsized influence on making the world habitable, said Peter de Menocal, the president and director at Woods Hole Oceanographic Institution.
The problem now is that the Arctic is warming at least twice as fast as the rest of the planet. That means a lot of extra freshwater flowing off of the Greenland ice sheet in the polar oceans called the “cold blob.” This freshwater is more buoyant than saltwater and sits on the surface, inhibiting the normal sinking of cold salty water. Scientists believe this cold blob barrier could be a critical factor in slowing down the whole operation of ocean current circulation.
When this Atlantic Ocean circulation slows, more hot water gets trapped along the East Coast, including the tri-state area.
“You can think of it almost like a sewer backing up,” said Jennifer Francis, senior scientist at the Woodwell Climate Research Center. “It’s just not letting that warm water flow as easily as it normally would.”
You can think of it almost like a sewer backing up.
Given heat equals energy, that backup creates more fuel for storms and hurricanes in the coastal areas around New York and New Jersey, as well as more evaporation which contributes to heavier downpours that have been documented in the area, said Francis. The New York area lies alongside the Gulf Stream.
Sea level rise would also be magnified in the region, and here’s why. Under normal circumstances, the Coriolis Effect from the Earth’s rotation pushes the water away from the East Coast. But when the ocean currents slow down, the Coriolis force weakens, too, which results in water moving closer to the shore and more sea-level rise, said Rahmstorf.
“Recent observations show sea level rise indeed accelerated in these coastal regions during the past decade. The AMOC weakening is an important factor to explain it,” Jianjun Yin, an associate professor at the Department of Geosciences at the University of Arizona, said in an email.
The East Coast wouldn’t be alone in feeling the dramatic fallout. The real concern for scientists is that if the Gulf Stream system slows down too much, all of the Atlantic Ocean currents will lose momentum. ”If it were to stop, everyone on the planet would know it.” said De Menocal.
The last time deep water circulation shutdown in the Atlantic around 12,000 years ago, Western Europe turned into the equivalent of the Siberian tundra because the warm waters didn’t flow as easily across the Atlantic. Some scientists think this might happen again if the Gulf Stream system shuts down and that North Africa, which is already water-stressed, would become even drier.
Another concern is that the ocean absorbs almost a third of human-made emissions, and a weakening of Atlantic Ocean circulation likely interferes with this process. Circulation is what pumps carbon dioxide deep into the ocean and away from our atmosphere.
All of these omens come with uncertainty, even though a growing body of work supports the Gulf Stream system’s slowdown. Arnold L. Gordon, a professor in the Department of Earth Environmental Sciences at Columbia University, said it’s hard to predict the future because there are so many factors changing in the oceans under global warming, including an increase in salty water entering the AMOC from the Indian Ocean by way of the South Atlantic. That might offset the effect of freshwater melt in Greenland, which is contributing to the slowdown.
Rahmstorf said he isn’t convinced this additional salty water would make it all the way to Greenland to balance out the freshening effect of the cold blob. “I would say that that is speculation.”
While many climate impacts are gradual—like a light dimmer—Atlantic Ocean circulation is like an on-off switch. Once the switch is flipped, the global chain reaction would be drastic, De Menocal said, adding that this tipping point could happen in less than a decade. But predicting the exact moment is difficult. “It’s like steering a ship into uncharted waters,” Rahmstorf said. “There are rocks under the surface, but you can’t see them.”
“Would you board a plane that has a 10 percent chance of crashing?
According to some climate models, the Gulf Stream system could decline around 40% by 2100, but some scientists like Yin believe that if emissions are unabated, the AMOC could weaken by 50% or even more during this century. The Intergovernmental Panel on Climate Change reported in 2019 that a continued decline is very likely this century, but a shutdown is very unlikely (which means a probability of 10 percent or less).
Rahmstorf isn’t comforted by those odds: “Would you board a plane that has a 10 percent chance of crashing? Probably not. Running a 10 percent risk of this happening is totally unacceptable. It’s something we must avoid.” If we implement the Paris agreement and limit warming to well below 2 degrees, shut down is very unlikely, Rahmstorf said, “but anything above that starts running a real risk.”
Because the shutdown of the Atlantic Ocean’s currents is dependent on how much the planet warms, it’s not too late to reverse course and avert the resulting catastrophe to the global climate system.
“Every year that we commit to increasing emissions, we commit to a sooner arrival of this very avoidable thing,” said De Menocal. “It’s very human to just put your hands up in the air and say, ‘we’re screwed.’ But that is exactly the opposite reaction that we should be taking because we do have control over this.”