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This cataclysm occurred only in computer simulations, the product of complex models used by scientists to understand the effects of climate change. But van Westen, a climate scientist at Utrecht University in the Netherlands, said this is the first time this particular model has been used to show how the Atlantic’s sensitive circulation system breaks down. And it suggested that the oceans may be heading toward a “tipping point” that will end in sudden and irreversible change.
Van Westen’s study, published Friday in the journal Science Advances, is the latest attempt to understand what scientists call the Atlantic Meridional Overturning Circulation (AMOC). Evidence from Earth’s past shows that this important and complex ocean system has shut down before, and modeling studies like Van Westen’s suggest that as the planet warms due to human greenhouse gas emissions, It suggests that the same thing could happen again.
However, open questions remain about the likelihood of collapse and the timeline in which it will occur. Here’s what you need to know:
AMOC is a complex system
Scientists often liken the AMOC to a conveyor belt that transports water, heat, and nutrients across the Atlantic Ocean, driven by differences in water density.
It begins near the equator, where the ocean surface is warmed by the tropical sun. As that water moves north, some of it evaporates, increasing the salinity and density of the water it leaves behind. By the time the water approaches Greenland, it also becomes colder and more dense.
This cold, salty water sinks to the ocean floor, pushing water already on the ocean floor out of the channel. The expelled water begins to flow south along the ocean floor. Once back in the tropics, the water is drawn back to the surface through a process called upwelling, and the cycle begins again.
Although the Gulf Stream plays a role in AMOC, the two are not synonymous. While the AMOC is a complex system of many ocean currents, the Gulf Stream is a single fast current that transports warm water from the Gulf of Mexico along the east coast of the United States to northern Europe. This is primarily driven by winds and the Earth’s rotation, meaning that even if climate change disrupts the deep-sea portion of the AMOC, the Gulf Stream won’t necessarily stop.
AMOC was previously closed
The AMOC is considered the “overturning element” of the Earth system. This means that it can suddenly flip between two fundamentally different states, and once it flips, it’s difficult to get back on track.
Think of it like someone leaning back in a chair. As long as she doesn’t lean too far, the chair should be able to return to its original stable position with all four legs on the floor. However, if it leans beyond a certain threshold (tipping point), the chair will topple over and assume a new stable state of lying on the floor.
Currently, AMOC is self-reinforcing. Differences in water densities facilitate circulation, which creates density differences that continue the cycle.
But evidence from Earth’s past shows how an influx of freshwater can affect the Earth system. By studying seafloor sediments, scientists have discovered that a huge pulse of meltwater entered the North Atlantic Ocean at the end of the last Ice Age, when the huge ice sheets that covered Europe and North America began to break up. I know. Fossilized shells of small sea creatures show how the pulsations diluted the salt water that accumulated just beneath Greenland. This dilution made the water less dense and prevented it from sinking. That means there’s nothing left to push the deepwater part of the AMOC up.
This meant less warm salt water flowed north along the surface, further weakening the density gradient that drives circulation. Eventually, the feedback became so severe that AMOC could no longer sustain itself.
“There is evidence from various types of ocean circulation records that the AMOC has declined significantly; this is close to a state of collapse,” said Van Westen, a paleoceanographer at Woods Hole Oceanographic Institution in the study. said Sophie Hynes, who was not involved. .
Other geological records, including ice cores in Greenland and stalagmites found deep within caves, show that the AMOC’s closure led to major changes in weather around the world. Europe has become even colder, and parts of the tropics have stopped getting rain.
“Information about what actually happened in the past is critical to understanding exactly what conditions are needed for AMOC to switch from on to off,” Hines said. “That’s going to be really important as it applies to what might be happening today.”
There are growing signs that AMOC is in trouble.
For the past few decades, researchers have been using networks of floating robotic sensors to observe the ocean in real time. Drifting with the current and swimming through the water column, these sensors collect information about the ocean’s temperature and salinity and transmit it to satellites, which then transmit that information to scientists around the world.
These observations reveal the existence of a strange cooling spot on the southern tip of Greenland. He is one of the only places on earth where the oceans are not hot. This suggests that the AMOC is not supplying as much warm water to the North Atlantic, suggesting that the system is slowing down. Other studies, combining direct observations and computer simulations, have concluded that the AMOC has already weakened by about 15 percent since the 1950s.
In a study in Science Advances, Van Westen and his colleagues found that the amount of freshwater moving around the southernmost tip of the Atlantic Ocean is a good indicator of the strength of the feedback system driving the AMOC. did. If that indicator is a positive number, it means that the system is self-reinforcing. But when Van Westen’s team looked at real-world data, they found that the measure of freshwater transport was negative.
“AMOC is becoming unstable,” he said.
However, there is a big difference between a linear process of decreasing stability and an abrupt transition beyond a tipping point.
AMOC’s collapse would have catastrophic consequences
Van Westen’s research modeled how quickly the effects of an AMOC collapse could unfold. He said if the system passes a tipping point, Western Europe will begin to cool by up to 3 degrees Celsius (5.4 degrees Fahrenheit) every decade. The accumulation of water in the North Atlantic would cause sea levels to rise by up to 1 meter, starving the ocean floor of oxygen and killing deep-sea life. Meanwhile, the rest of the world will continue to warm, thanks to heat-trapping greenhouse gas pollution caused by humans.
“Once AMOC tilts, it goes very quickly,” Van Westen said. “And the response has been very harsh.”
The world is currently warming by 0.2 degrees Celsius (0.36 degrees Fahrenheit) every 10 years, and societies are already struggling to cope, he added. If the temperature fluctuations caused by an AMOC outage are an order of magnitude larger, “it’s going to be very difficult to adapt,” Van Westen said.
Climate scientist Tim Lenton, director of the Global Systems Institute at the University of Exeter, also used computer models to assess how the closure of AMOC would affect global food supplies. Dramatic cooling in the Northern Hemisphere will cause changes in the cloud belt and rainfall that surrounds the Earth in the tropics. The monsoon, which normally brings rain to West Africa and South Asia, will become unreliable, and large areas of Europe and Russia will be plunged into drought. Half of the world’s corn and wheat acreage could become dry.
“in simple words [it] “There will be a combination of food and water security crises on a global scale,” Renton said.
Scientists aren’t sure if closure is near
Despite signs that AMOC is weakening, scientists still don’t know how quickly a shutdown will occur. AMOC is huge and slowly moving. It takes hundreds of years for a single drop of water to circulate throughout the ocean. This means that just 30 years of observations is not enough to assess how much the system is changing.
So researchers turned to computer models to simulate how the Earth would respond to specific perturbations (such as releasing Arctic meltwater into the North Atlantic), pushing the system beyond its limits. I am checking what is needed.
Many of these simulations do not predict that AMOC will cross the tipping point in the near future. In its latest assessment report on the state of the Earth’s climate, the United Nations Intergovernmental Panel on Climate Change derived multiple climate models and concluded with “moderate confidence” that AMOC will not completely collapse this century. I concluded.
But Renton, who was not involved in the new Science Advances study, believes the model is too conservative and doesn’t fully capture the complex interactions that shape our planet. He said it was “important” that Van Westen and his colleagues were able to reproduce the AMOC collapse using one of the world’s most sophisticated climate models.
A definitive answer to AMOC’s vulnerability will require more direct observations of the ocean and improved computer simulations by scientists. Further research into past collapses could also help identify triggers and early warning signs that scientists should be looking for today, Hines said.
And even if the AMOC tipping point is unlikely to be exceeded this century, the potentially devastating consequences will require us to invest in better research and start thinking about how the world will adapt. That’s a good reason, the researchers agreed.
“We’re not taking this seriously enough,” Renton said.
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