A crucial system of ocean currents may already be collapsing, according to a new report, with worrying implications for sea level rise and global weather – causing temperatures to drop dramatically in some areas and rise in others.
Using exceptionally complex and expensive computer systems, scientists have found a new way to detect an early warning signal of the collapse of these currents, according to the study published Friday in the journal Science Advances. As the planet warms, there are already signs that it is headed in this direction.
The Atlantic Overturning Circulation (AMOC) – of which the Gulf Stream is a part – acts like a giant global conveyor belt, taking warm water from the tropics towards the extremes. The North Atlantic, as the water cools, becomes saltier and sinks deeper into the ocean, before spreading south.
Currents carry heat and nutrients to different regions of the globe and play a vital role in keeping the climate of large parts of the Northern Hemisphere relatively mild.
For decades, scientists have been sounding the alarm about circulatory stability, as climate change warms oceans and melts ice, upsetting the heat-salt balance that determines the strength of currents.
While many scientists believe that the AMOC will slow under climate change, and could even stop altogether, there is still a great deal of uncertainty about when and how quickly this will happen. AMOC has only been monitored continuously since 2004.
Scientists know – by building a picture of the past using things like ice cores and ocean sediments – that the AMOC stopped more than 12,000 years ago after the rapid melting of glaciers.
Now they are scrambling to find out if it could happen again.
This new study provides an “important breakthrough,” said Rene van Westen, a marine and weather researcher at Utrecht University in the Netherlands and co-author of the study.
Scientists used a supercomputer to run complex climate models over three months, simulating a gradual increase in fresh water to the AMOC, which accounts for melting ice as well as precipitation and river runoff, which could dilute ocean salinity and weaken currents.
As they slowly increased fresh water in the model, they saw the AMOC gradually weaken until it suddenly collapsed. This is the first time the collapse has been detected using such complex models, which represents “bad news for the climate system and humanity,” the report says.
But what the study does not do is give time frames for a possible collapse. Van Westen told CNN that more research is needed, including models that also simulate the effects of climate change, such as increased levels of pollution from heating the planet, which this study did not do.
“But we can at least say we are moving toward a tipping point with climate change,” Van Westen said.
The effects of AMOC’s collapse could be disastrous. The study found that some parts of Europe could see temperatures drop by as much as 30°C over the course of a century, leading to a very different climate over just a decade or two.
“There are no realistic adaptation measures that can cope with such rapid temperature changes,” the study authors wrote.
On the other hand, countries in the Southern Hemisphere may see temperatures rise, while wet and dry seasons in the Amazon could flip, leading to serious disruption to the ecosystem.
A collapse of the AMOC could also cause sea levels to rise by about 1 meter (3.3 feet), Van Westen said.
It was a “major advance in the science of AMOC stability,” said Stefan Rahmstorf, a physical oceanographer at the University of Potsdam in Germany, who was not involved in the study.
“It confirms that the AMOC has a tipping point beyond which it collapses if the North Atlantic is diluted with fresh water,” he told CNN.
Previous studies that found the tipping point for the AMOC used much simpler models, giving hope to some scientists that it may not be found in more complex models, he said.
Rahmstorf said this study crushes those hopes.
Joel Hirschi, associate head of marine systems modeling at the UK’s National Oceanographic Centre, said the study was the first to use complex climate models to show that the AMOC can switch from “on” to “off” in response to relatively small amounts of fresh water that… You enter the ocean.
But he added that there are reasons to be cautious. He said that although the study used a complex model, it was still low-resolution, meaning there may be limitations in representing some parts of the currents.
This study adds to the growing body of evidence suggesting that the AMOC may be approaching a tipping point, and may even be close.
A 2021 study found that the AMOC was weaker than at any other time in the past 1,000 years. A particularly alarming – and somewhat controversial – report published in July last year concluded that the AMOC could be on track to potentially collapse as early as 2025.
However, huge doubts remain. Jeffrey Cargill, chief scientist at the Planetary Science Institute in Arizona, said he doubts the theory of a possible imminent shutdown of the AMOC “will still be somewhat controversial until we know a year later that it’s happening.”
He likened its potential collapse to “the wild fluctuations of the stock market that precede a major crash” — it is almost impossible to determine which changes are reversible, and which are a precursor to disaster.
Recent data shows that the strength of the AMOC fluctuates, but there is no noticeable evidence yet of a decline, Hershey said. “Whether abrupt changes in the AMOC similar to those seen in the past will occur as our climate continues to warm is an important open question.”
This study is part of that puzzle, Rahmstorf said. “(This) significantly adds to the growing concerns about the collapse of the AMOC in the not-too-distant future,” he said. “We will ignore this risk at our peril.”