Deep in the Atlantic Ocean, there's a massive current the size of 8,000 Mississippi Rivers. Its role in the Earth's climate is so powerful that it determines weather from the equator to Europe, crop production in Africa and sea level rise on the East Coast.
Scientists say there's a risk this vital current could shut down as the climate gets hotter, a collapse that could have dire consequences worldwide.
Researchers have been trying to determine when the Atlantic might cross that tipping point. But answering that is no easy task.
Now, a new study finds the collapse of the current, which is known as the Atlantic meridional overturning circulation, or AMOC, could happen far sooner than scientists have previously thought, possibly within a few decades, as a result of human-caused global warming.
"It's a worrisome result," says Peter Ditlevsen, professor of climate physics at the University of Copenhagen in Denmark and an author of the study. "It calls for quite immediate actions. We need to reduce emissions. We need more brakes on the train."
Other researchers caution that the timeline of such a collapse — or even whether the AMOC will collapse at all — remains unclear, given the sheer complexity of understanding an ocean system that stretches thousands of miles. Previous assessments have suggested a collapse is unlikely this century.
The new study adds to a growing body of research suggesting crucial tipping points in the climate system are incredibly hard to predict, and that humans are changing the fundamental processes of the Earth faster than we can understand them. Given the potential for catastrophic impacts, scientists say further research to understand the AMOC is more urgent than ever.
"The AMOC is a bedrock of our climate system," says Nicholas Foukal, an assistant scientist at the Woods Hole Oceanographic Institution who was not involved in the study. "It redistributes heat globally and it's something that we just take for granted."
A conveyor belt for heat
When it comes to weather, Europe has a lot to thank the AMOC for. Cities like London and Paris are warmer than their counterparts at similar latitudes in North America.
"In Scandinavia, we have a sort of pleasant, mild climate," Ditlevsen says. "And if you compare that with the U.S., we are at the latitude of Alaska, which is much colder than Scandinavia."
Milder winters in Europe are largely thanks to an influx of heat from the AMOC. The current carries vast amounts of warm water from the equator, which travels north up the East Coast of the U.S. before crossing to Europe. That's where the water cools off, releasing heat into the atmosphere.
The cold, salty water is denser and heavier, causing it to sink near Greenland. Like a massive ocean conveyor belt, the current then returns in the direction it came from, flowing south along the ocean floor.
Scientists know this conveyor belt has collapsed in the past. Around 12,000 years ago, temperatures around Greenland suddenly dropped by about 18 degrees Fahrenheit. That shift is attributed to a sudden shutdown of the AMOC — and demonstrates the potential impact of such a climate tipping point.
"A tipping point is a strong result to a small change," Ditlevsen says. "It's when you're pushed over the cliff. When you reach the cliff, you drop."
Looking for the tipping point
To determine how close that tipping point might be, Ditlevsen analyzed ocean temperature records near Greenland over the past 150 years and ran a statistical analysis to track the fluctuations in temperature. He and his co-author found increasing variability in temperatures, which they say is a sign the AMOC is weakening. Based on their analysis, they estimate the AMOC could collapse between 2025 and 2095. That's decades earlier than other studies have found.
While researchers disagree on the timing of such a collapse, there is broad consensus on the potential consequences. A collapse in the AMOC could have ripple effects around the planet. Temperatures in Europe could fall, while heat in the tropics would rise, exacerbating climate change that's already occurring.
Rainfall could decrease across the Sahel region of Africa, threatening crop production for millions of people. The summer monsoon could weaken across Asia and sea levels could rise even faster in the Eastern U.S. Scientists have already found that subtle shifts in Atlantic currents can have serious effects on marine life, like threatening endangered North Atlantic right whales.
"It's going to affect agriculture," Foukal says. "It's going to affect disease, especially in the equatorial region. It's going to affect mass migration."
When is still a big question
Still, a midcentury collapse is at odds with what other research studies have found. A report from the Intergovernmental Panel on Climate Change found the AMOC is unlikely to collapse this century.
"Whether it will collapse is still a question," Foukal says. "I think that there is still quite a bit of uncertainty."
Foukal says this most recent study relies on temperature records from a small part of the system and doesn't simulate what would happen to the entire current itself. He says it's also crucial to understand the cause of a collapse to estimate the timing — something Ditlevsen's study didn't address.
The last time the AMOC shut down, the Earth was coming out of an ice age. Scientists believe a vast amount of fresh water from melting glaciers poured into the Atlantic, interfering with the conveyor belt. Fresh water is lighter than salt water and can inhibit the sinking motion that powers the entire current.
A similar thing could happen again, as humans continue to heat the atmosphere by burning fossil fuels. Ice in the Arctic and Greenland's ice sheets are melting at an increasing pace, also adding fresh water to the Atlantic. But Foukal says researchers are still trying to determine whether that would be enough to cause a complete collapse.
What's more likely, he says, is that the AMOC could weaken this century. That could still cause some of the same serious impacts as a collapse, though to a lesser extent. Some studies have shown a weakening is already happening, but other researchers say that given the normal fluctuations in the current, it will take more time to make that call.
Direct measurements of the Atlantic circulation have only been made since 2004. Given the depths and distances the AMOC covers, it's challenging to keep tabs on it. But with the potential for such widespread impacts, scientific researchers say further research is more urgent than ever — as well as rapid action to limit how much the planet warms.
