28 July 2023

NEW STUDY: The AMOC Atlantic Meridional Overturning Circulation

The AMOC has only been monitored continuously since 2004 through combined measurements from moored instruments, induced electrical currents in submarine cables and satellite surface measurements...

  • We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.

Ocean current system could shut down as early as 2025, leading to climate disaster

By Keith Cooper
 published 

The Atlantic Meridional Overturning Circulation has been weakening – and new predictions suggest it could totally switch off between 2025 and 2095.

An illustration showing a visualization of the gulf stream and how heat flows.
Part of a NASA visualization of the Gulf Stream, showing surface temperatures and flow patterns. (Image credit: NASA/Goddard Space Flight Center Scientific Visualization Studio)

A major system of ocean currents that ferries heat from the tropics to the North Atlantic could shut down far sooner than expected, according to new predictions. Such a collapse would prove catastrophic for Earth's climate.

The system, known as the Atlantic Meridional Overturning Circulation (AMOC) had previously been measured to be dramatically weakening in conjunction with rising ocean temperatures. Despite this, however, the Intergovernmental Panel on Climate Change (IPCC) recently announced that climate scientists don't expect the AMOC to totally switch off within the century. 

But a new study is now challenging that conclusion, raising the specter of a halted AMOC to as early as 2025.

"Shutting down the AMOC can have very serious consequences for Earth’s climate, for example, by changing how heat and precipitation are distributed globally," study leader Peter Ditlevsen, from the Niels Bohr Institute at the University of Copenhagen, said in a statement.

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Finding that direct measurements of the AMOC's strength have only been made for the past 15 years, Ditlevsen's team applied sophisticated statistical tools to ocean temperature data going all the way back to the 1870s for an enhanced dataset. This detailed analysis ultimately suggested significant warning signs of the AMOC shutting down between 2025 and 2095, with a staggering certainty of 95%. More specifically, the team's results evidenced that the most likely time for this collapse would be around 2057.
Still, other climate scientists remain cautious, saying that there are still uncertainties in the data that could affect its accuracy. However, 
  • it's worth considering that even the mere possibility of the AMOC shutting down so soon is rather alarming.
The AMOC, which includes the Gulf Stream as part of its system, is our planet's main mode of transporting heat away from the tropics. Without it, the tropics would rapidly increase in temperature while vital tropical rains get disrupted. Such rains are essential for the environments of South America, western Africa as well as in India and other regions of south Asia.
> Meanwhile, northern and western Europe would lose their source of warm water from the tropics, leading to more storms and severely cold winters in these areas. 
  • The loss of the Gulf Stream in particular would also result in rising sea levels on the US’ eastern seaboard. 
"Our result underscores the importance of reducing global greenhouse gas emissions as soon as possible," Ditlevsen said.

In recent years, we’ve already seen the dangers of human-induced climate warming play out as heatwaves grip much of the northern hemisphere. And although the loss of the AMOC may see northern and western Europe cool, “this shutdown will contribute to an increased warming of the tropics," Ditlevsen said, "where rising temperatures have already given rise to challenging living conditions."
The findings were published on Tuesday (July 25) in the journal Nature Communications

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Warning of a forthcoming collapse of the Atlantic meridional overturning circulation

Nature Communications volume 14, Article number: 4254 (2023) Cite this article

Abstract

The Atlantic meridional overturning circulation (AMOC) is a major tipping element in the climate system and a future collapse would have severe impacts on the climate in the North Atlantic region. In recent years weakening in circulation has been reported, but assessments by the Intergovernmental Panel on Climate Change (IPCC), based on the Climate Model Intercomparison Project (CMIP) model simulations suggest that a full collapse is unlikely within the 21st century. Tipping to an undesired state in the climate is, however, a growing concern with increasing greenhouse gas concentrations. 
  • Predictions based on observations rely on detecting early-warning signals, primarily an increase in variance (loss of resilience) and increased autocorrelation (critical slowing down), which have recently been reported for the AMOC. 
  • Here we provide statistical significance and data-driven estimators for the time of tipping. 
  • We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.
The AMOC has only been monitored continuously since 2004 through combined measurements from moored instruments, induced electrical currents in submarine cables and satellite surface measurements22. Over the period 2004–2012, a decline in the AMOC has been observed, but longer records are necessary to assess the significance. For that, careful fingerprinting techniques have been applied to longer records of sea surface temperature (SST), which, backed by a survey of a large ensemble of climate model simulations, have found the SST in the Subpolar gyre (SG) region of the North Atlantic (area marked with a black contour in Fig. 1a) to contain an optimal fingerprint of the strength of the AMOC23,24,25. 

...

Fig. 1: The Atlantic meridional overturning circulation (AMOC) fingerprint, sea surface temperature (SST) and global mean (GM).
figure 1

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