A sudden halt in the Atlantic's currents, which could put large parts of Europe into a deep freeze, appears more likely and closer than ever, as a complex new computer simulation finds a “cliff-like” tipping point looming in the future.
A new study in the journal Science Advances found on Friday that the long-awaited nightmare scenario, triggered by the melting of the Greenland ice sheet due to global warming, is still at least decades away if not longer, but perhaps not the centuries it once seemed. . The study, the first of its kind to use complex, multi-parameter simulations, uses a key measurement to track the strength of global ocean circulation, which is slowing down.
A collapse of the current — called the Atlantic Overturning Circulation, or AMOC — would change weather around the world because it would mean shutting down one of the planet's major climatic and oceanic forces.
This would cause temperatures in northwestern Europe to drop by 9°F to 27°F (5°C to 15°C) over decades, expand Arctic ice farther south, and warm more in the Southern Hemisphere. And changing global rainfall patterns, the study said, would disrupt the Amazon region. Other scientists said that would be a disaster that could cause food and water shortages around the world.
“We are getting close (to collapse), but we are not sure how close we are,” said the study's lead author Rene van Westen, a climate scientist and oceanographer at Utrecht University in the Netherlands. “We are heading towards a turning point.”
The likelihood of such a global climate catastrophe — fictionalized in the movie The Day After Tomorrow — occurring is “the million-dollar question, which unfortunately we cannot answer at the moment,” Van Westen said. He said it would likely happen a century later but it could still happen in his lifetime. He has just turned thirty years old.
“It also depends on the rate of climate change that we as humans are causing,” Van Westen said.
Studies have shown that the AMOC slows down, but the problem is a complete collapse or shutdown. The United Nations Intergovernmental Panel on Climate Change, a group of hundreds of scientists that provides regular official updates on global warming, said it had moderate confidence that there would be no collapse before 2100, and generally played down disaster scenarios. But Van Westen, several outside scientists and a study conducted last year say that may not be true.
Stefan Rahmstorf, head of Earth systems analysis at the Potsdam Institute for Climate Research in Germany, was not part of the research, but called it “a major advance in the science of AMOC stability.”
“The new study adds significantly to the growing concern about AMOC collapse in the not-too-distant future,” Rahmstorf said in an email. “We will ignore this at our peril.”
Tim Linton, a climate scientist at the University of Exeter, who is also not part of the research, said the new study makes him more concerned about collapse.
A collapse of the AMOC would cause numerous ripples across the world's climate that are “so sudden and severe that it would be almost impossible to adapt to in some locations,” Linton said.
There are indications that the AMOC has collapsed in the past, but when and how it will change in the future remains uncertain, said Wei Cheng, a oceanographer at the US National Oceanic and Atmospheric Administration, who was not part of the research.
AMOC is part of a complex global conveyor belt of ocean currents that move different levels of salt and warm water around the world at different depths in patterns that help regulate Earth's temperature, absorb carbon dioxide and fuel the water cycle, according to NASA.
When the AMOC closes, there is less heat exchange around the world, “and that really hits Europe hard,” Van Westen said.
For thousands of years, the Earth's oceans have relied on a circulation system that acts like a conveyor belt. It's still going, but it's slowing down.
The drive of this conveyor belt is located off the coast of Greenland, where more fresh water is flowing into the North Atlantic Ocean, with more ice melting due to climate change, slowing everything down, Van Westen said. In the current system, colder, deeper, fresher waters move south through the Americas and then east through Africa.
Meanwhile, warmer, saltier ocean waters, coming from the Pacific and Indian Oceans, flow across the southern tip of Africa, deflect into and around Florida, and continue up the east coast of the United States to Greenland.
The Dutch team simulated 2,200 years of its flow, adding what human-caused climate change does to it. They found after 1,750 years a “sudden collapse of the AMOC”, but so far they have been unable to translate this simulated timeline into Earth's real future. The key to monitoring what's happening is the complex measurement of flow around the tip of Africa. The more negative this measurement is, the slower the AMOC will run.
“This value becomes more negative under climate change,” Van Westen said. He added that when it reaches a certain point, it does not mean a gradual stop, but rather something “like an abyss.”
Joel Hirschi, head of the department at the UK's National Oceanographic Centre, said the world should be alert to the possibility of the AMOC collapsing. But he said there was a larger global priority.
“To me, the rapidly increasing temperatures we have seen in recent years and associated temperature extremes are a more pressing concern than the AMOC,” Hershey said. “Rising temperatures are not hypothetical but are already happening and affecting society now.”
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