Is A Mega-ocean Current About to Shut Down?

CLIMATEWIRE | An enormous ocean current that warms some continents and cools others as it snakes around the world could collapse decades earlier than scientists predicted.

It would be a dire outcome that disrupts weather patterns in nearly every place on Earth. That makes the findings of a new study this week alarming because of its timing. The current could shut down in as little as two years — triggering chaotic weather changes worldwide in real time.

But the finding is also controversial.

Previous studies have found that the current, known as the Atlantic Meridional Overturning Circulation, or AMOC, is weakening over time, but that it’s unlikely to collapse before the end of the century. The new study marks the first time that researchers have tried to pin down when the AMOC could stop working — the authors said it could be anytime between 2025 and 2095.

That raises new questions about the current’s vulnerability. But, first, let’s look at the basics: What does the AMOC do, why is it important in the context of climate change, and what would happen if its flow is interrupted?

What is the AMOC?

The AMOC (pronounced A-mock) is a gigantic ocean current system. It works like an underwater conveyor belt that extends for thousands of miles to ferry warm surface water from the equator toward the Arctic, where the water cools and sinks to the bottom of the sea, before flowing back in the opposite direction and, eventually, welling up to the surface again.

The fast-flowing Gulf Stream, which surges past the U.S. East Coast, is one part of the system. Additional branches of the AMOC stretch into the Southern Hemisphere, where the same overturning and upwelling process occurs.

It’s a critical component of the Earth’s climate system. The AMOC helps distribute heat throughout the Atlantic Ocean basin, which in turn helps regulate climate and weather patterns around the globe. The warm water it carries through the North Atlantic, for instance, is the reason much of Europe is known for its mild winters.

Why is it weakening?

Multiple studies have found that the AMOC is slowing down as time goes on. One paper, published in 2021, estimated that the current is likely at its weakest point in the past 1,000 years.

Some of the weakening may be driven by natural variations in the Earth’s climate system — but human-caused climate change is also to blame, scientists say. And climate models, which simulate the Earth’s future, suggest that continued warming could cause the system to get weaker.

Melting ice is a major reason. The vast Greenland ice sheet, situated in the middle of the North Atlantic, is pouring about 250 billion metric tons of ice into the ocean each year on average — and it’s accelerating as the planet warms. This influx of cold, fresh water into the sea can destabilize the AMOC’s flow over time.

If the current weakens enough, it can eventually cross a threshold of no return, causing the system to collapse. In fact, scientists believe it’s happened before. Studies of the Earth’s ancient climate suggest that the AMOC probably shut down around 13,000 years ago, during a natural warming period when large volumes of melting ice were pouring into the ocean.

If it happened in the past, it could happen again, experts warn. But where the tipping point is on the time scale and temperature arc is a major scientific debate.

Climate models have generally indicated that the AMOC will continue to weaken in the coming decades but that it’s unlikely to fully collapse within the next 100 years. The U.N. Intergovernmental Panel on Climate Change stated in its most recent assessment report that there was “medium confidence” the AMOC would not collapse before the end of this century.

But some scientists say there’s reason to believe standard climate models may be underestimating the AMOC’s weakening.

These experts say the AMOC’s representation in the models is too stable, said David Thornalley, an ocean scientist and AMOC specialist at University College London. As a result, the models may “underestimate the chance of an abrupt change in AMOC,” he said in an email to E&E News.

Stefan Rahmstorf, an ocean expert at the Potsdam Institute for Climate Impact Research, said climate models also don’t adequately simulate the influx of fresh water from Greenland, a major contribution to the slowing current.

That means the AMOC could weaken faster than the models suggest.

What happens if it collapses?

If the AMOC shuts down, it would have widespread global consequences, scientists say.

Many studies predict a significant cooling over parts of Europe, Thornalley said — potentially by as much as 5 or 10 degrees Celsius. Tropical rain belts might shift their positions, causing some regions to experience more droughts and others to suffer more floods.

Rahmstorf added that the North Atlantic may see a major increase in rising seas. If the AMOC can’t ferry large volumes of water around the world, the ocean may absorb less carbon dioxide from the atmosphere. Parts of the deep ocean may receive less oxygen. Marine ecosystems could change in ways scientists are still trying to understand.

In short, there could be dramatic consequences. But it’s still a matter of debate whether those looming alterations could happen within the next few decades.

Is the new study right?

The study published this week suggests that the AMOC is likely to collapse within this century — and potentially within the next few years.

That finding conflicts with most previous studies. As Rahmstorf and Thornalley pointed out, there’s some evidence that the models may be underestimating the AMOC’s weakening. But that doesn’t mean the new study overturns the narrative.

“No, I don’t think one study does overturn the IPCC assessment, and we should view the results of this new study with some skepticism,” Thornalley said.

The study takes a different approach from previous modeling attempts. It relies on observations of sea surface temperatures from one region in the North Atlantic — then it uses a statistical method to extrapolate the future of the entire ocean system using those narrow observations.

There are pros and cons to this approach, experts say.

The statistical method is sound, said Levke Caesar, an AMOC expert at the University of Bremen in Germany, who commented on the new study for E&E News. But the study assumes that the entire AMOC can be adequately represented by using observations from just one region of the ocean.

In some ways, it’s hard to get around that difficulty. Scientists have only been monitoring the entire AMOC system with ocean sensors for a decade or so. Using measurements from individual regions with longer data sets is still necessary for these kinds of studies.

But the assumption that these observations can represent the whole system “needs to be further tested,” Caesar said.

There are other uncertainties about the data used in the new study, Thornalley added. It relies on sea surface temperature measurements from one region in the subpolar North Atlantic and suggests that changes in these temperatures are a kind of “fingerprint,” or signal, of the shifting AMOC.

But Thornalley cautioned that the slowing AMOC may not be the only factor that’s changing this region of the ocean — and if that’s the case, the study’s findings may be less robust.

On the other hand, Rahmstorf pointed out that the new study isn’t the only research to suggest that the AMOC may be weakening faster than scientists previously expected.

A study published in 2021, and another one published in 2022, also suggested that the AMOC may be approaching a tipping point that could accelerate its eventual collapse. Those studies didn’t go as far as to suggest that a full collapse could be imminent within a few years — but they did indicate that the AMOC may be destabilizing faster than anticipated.

“Individual studies always have weaknesses and limitations, but when several studies with different data and methods point to a tipping point that is already quite close, I think this risk should be taken very seriously,” Rahmstorf wrote in a recent blog post.

Caesar added that there are still questions about how the AMOC will behave as it weakens. It’s possible that the current may have several tipping points that lead to progressively weaker states but don’t cause the whole system to shut down.

“It could be that crossing the first tipping point does not lead to a complete collapse of the AMOC, but that the system stabilizes at a weaker level,” she said.

On its own, the latest study adds to a growing body of evidence that the AMOC is in trouble. But there are still a lot of uncertainties about its exact findings, especially the timeline it presents for collapse.

If all the questions and concerns about the study’s methods and assumptions could be addressed, “then this is a very concerning result,” Thornalley said.

But he added that “there are some really big unknowns and assumptions that need investigating before we have confidence in this result.”

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2023. E&E News provides essential news for energy and environment professionals.