The AMOC

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On the 25th of July in 2023, a scientific article by the Danish scientist couple Susanne and Peter Ditlevsen drew for some days a higher attention in the media, at least for media and private people concerned about the climate change development.

I asked ChatGPT to read the article and asked for help to understand this complex matter. Here is ChatGPT’s first overview of the paper:

The paper you’ve shared, authored by Peter Ditlevsen and Susanne Ditlevsen, provides a detailed analysis of the Atlantic Meridional Overturning Circulation (AMOC) and its potential collapse by mid-century under current emission scenarios. Here’s an overview of the key points:

AMOC’s Importance: The AMOC is a critical part of the Earth’s climate system, influencing climate patterns in the North Atlantic region. Its collapse could lead to severe climate impacts.
Current Status and Predictions: While the Intergovernmental Panel on Climate Change (IPCC) has suggested that a full collapse of the AMOC is unlikely within the 21st century, this paper presents a more alarming view based on the detection of early-warning signals (EWS) such as increased variance and autocorrelation, indicating a loss of resilience and a critical slowing down of the AMOC.
Methodology: The paper describes a novel statistical method to estimate the timing of the AMOC collapse by analyzing changes in mean, variance, and autocorrelation of the AMOC’s strength, without assuming a specific control parameter. This method involves stochastic modeling and statistical analysis to predict the collapse around mid-century, with a 95% confidence interval.
Findings: The analysis suggests a significant risk of AMOC collapse between 2025 and 2095, with a high probability of occurrence by 2057. This prediction is based on a data-driven approach and statistical significance, providing a more robust assessment than previous models.
Implications: The potential collapse of the AMOC underscores the urgency of addressing climate change and reducing greenhouse gas emissions. The paper emphasizes the importance of monitoring the AMOC and its early-warning signals to better predict and mitigate future climate risks.

This paper contributes significantly to our understanding of AMOC dynamics and the potential risks of its collapse, highlighting the need for immediate action to prevent severe climate impacts.

To coin it in scientific terms, I am quite confident that I won’t see the year 2057 personally, so why I care?

Two reasons: firstly, the couple Ditlevsen forecasts already 2025 onwards a chance that the AMOC breaks down, and secondly I hope my son will be alive in 2057.

But before we dive deeper in the understanding why a collapse of the AMOC could happen already in 21st century, let’s understand what the AMOC is:

ChatGPT:

The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that plays a crucial role in regulating the Earth’s climate, particularly in the North Atlantic region. Think of it as a giant conveyor belt in the ocean that moves warm water from the tropics towards the North Atlantic, where the water cools, sinks, and then flows back towards the equator and further on to the southern hemisphere. This process is driven by differences in water temperature and salinity, which affect the water’s density.

Why is the AMOC Important?

Climate Regulation: The AMOC helps distribute heat around the planet. By transporting warm water northward, it moderates the climate of Northern Europe and parts of North America, making these regions warmer than they would otherwise be at such latitudes.
Weather Patterns: It influences weather systems, particularly in the Atlantic, affecting rainfall patterns, storm paths, and the frequency and intensity of hurricanes.
Marine Ecosystems: The AMOC impacts marine ecosystems by moving nutrients around, supporting diverse marine life in both the deep and surface ocean layers. This has significant implications for fisheries and the broader marine food web.
Carbon Cycle: The circulation plays a key role in the global carbon cycle by transporting carbon dioxide from the atmosphere into the deep ocean, thus acting as a carbon sink and influencing the Earth’s carbon balance.
Global Climate System: The AMOC is interconnected with other parts of the global climate system, including the atmosphere and ice caps, meaning changes in the AMOC can have wide-reaching effects beyond just the Atlantic.

In summary, the AMOC is crucial for maintaining the climate we are accustomed to, supporting marine ecosystems, and regulating global climate dynamics. Disruptions or significant changes in the AMOC could lead to dramatic shifts in climate patterns, with wide-ranging impacts on weather, sea levels, and biodiversity, affecting societies and ecosystems around the globe.

The AMOC is very frequently described as a conveyor belt for transporting salt and heat through the oceans, here how it works in principle.

ChatGPT:

Heat Transportation:

Warm Start: The journey begins in the tropical regions of the Atlantic Ocean, where the sun’s heat is strongest. Here, the ocean water warms up significantly, making it lighter and causing it to stay on the surface.
Northward Journey: This warm, surface water then travels northward along the coast of North America towards Europe. As it moves, it carries the tropical warmth with it, like a massive, gently flowing river in the ocean.
Cooling Down: When this warm water reaches the North Atlantic, near Greenland and the Arctic, the colder surroundings cool it down. Cold water is denser than warm water, so as the surface water cools, it starts to sink.

Salt Transportation:

Evaporation: In warmer regions, some of the ocean water evaporates due to the heat, leaving behind salt. This makes the water saltier and denser.
Mixing and Sinking: As the water travels north and cools, it not only becomes denser from losing heat, but also from the saltiness left behind by evaporation. This extra density from the salt helps the water sink even deeper when it cools.

The Deep Return:

Deep Ocean Current: Once the water has sunk, it creates a deep ocean current that flows back towards the equator and further into the Southern Hemisphere. This deep, cold, and dense water makes its way around the globe in the deep ocean, eventually mixing with warmer waters and rising back to the surface in a process that can take hundreds of years.
Completing the Loop: As the deep water warms up and rises, it becomes less dense and starts its journey back to the surface, eventually warming up in the tropical regions and starting the cycle over again.

This continuous movement of warm and salty water towards the poles and cold water towards the equator creates a global circulation system, distributing heat and regulating climate around the world. It’s like a giant, natural thermostat that helps keep our planet’s climate in balance.

So even if you are, like me, no oceanographer nor climate scientist nor something related, you understand this conveyor belt is “somehow” important for us.

Again, before we try to understand why a forthcoming AMOC collapse in this century could appear, we should realize that the AMOC has already collapsed in ancient epochs several times:

ChatGPT:

The AMOC has experienced significant changes, including collapses, during Earth’s history, particularly during ancient epochs. These events are fascinating because they give us clues about how Earth’s climate system works and how it can change. Here’s a simplified explanation:

Ancient Epochs and AMOC Changes:

Ice Ages and Warm Periods: Throughout Earth’s history, there have been cycles of ice ages (glacial periods) and warmer intervals (interglacial periods). During these times, the AMOC has shown significant variations in strength and pattern.
Rapid Changes: Evidence from ancient sediments, ice cores, and other geological records shows that the AMOC can change rapidly, within a span of a few years to decades, leading to significant climate shifts. These rapid changes are often linked to sudden releases of freshwater into the North Atlantic, which can disrupt the delicate balance of the ocean’s salt and heat transport.

Effects of AMOC Collapses:

Cooling Events: When the AMOC weakens significantly or collapses, it can lead to rapid cooling in the North Atlantic region. This is because the warm, tropical waters are no longer being transported northward effectively, reducing the warming effect on these regions.
Climate Shifts: Such collapses have been associated with widespread climate changes, including cooler temperatures in Europe and North America, shifts in rainfall patterns, and even impacts on monsoons in Asia.
Past Examples: One of the most well-studied events is the Younger Dryas, a sudden cooling period about 12,000 years ago, which is thought to have been caused by a massive influx of freshwater from melting ice sheets into the North Atlantic, disrupting the AMOC.

Why It Matters:

Studying these ancient events helps scientists understand the potential impacts of current and future changes in the AMOC due to human-induced climate change. It provides a context for what might happen if the AMOC were to weaken or collapse in our time, emphasizing the importance of monitoring and understanding this critical component of Earth’s climate system.

In summary, the AMOC’s changes in the past have had profound effects on the global climate. By learning from these ancient episodes, scientists aim to better predict and mitigate potential future climate changes.

So now we have I think a comprehensive overview of the AMOC. Why the researchers Ditlevsen see a high chance of a collapse for the AMOC within this century?

ChatGPT

“This paper presents a more alarming view based on the detection of early-warning signals (EWS) such as increased variance and autocorrelation, indicating a loss of resilience and a critical slowing down of the AMOC.“

What is the meaning of “early-warning signals (EWS) such as increased variance and autocorrelation” in the context of a global conveyor belt of salt and heat within Earth’s oceans?

ChatGPT

The early-warning signals are about the tipping point that the global conveyor belt interrupts and to make it clear: If it interrupts, then it will be interrupted for a very long time because there is no way at all from the current point of view that humans can turn it on (we can only turn it off unfortunately).

The term “increased variance and autocorrelation” contains scary statistic measures which need to be explained:

ChatGPT

Increased Variance:

Variance refers to how much the values of a particular variable differ from the average value over a period. In simpler terms, if the variance is high, it means that the measurements of that variable (like temperature or current strength in the AMOC) are more spread out — they fluctuate more widely around the average. An increase in variance in the context of the AMOC suggests that its strength and behavior are becoming more unpredictable and less stable, which could indicate a system approaching a critical threshold or change.

Autocorrelation:

Autocorrelation is a measure of how similar the values in a sequence are to each other at different times. For instance, if the AMOC’s strength today is similar to its strength yesterday and the day before, the autocorrelation would be high. This measure helps in understanding the persistence of a condition over time. An increase in autocorrelation in the AMOC would suggest that whatever happens to the system at one time is increasingly likely to influence its state at a subsequent time, indicating a slowing down of the system’s dynamics. In other words, it means that it gets “tired” and it doesn’t react anymore to the embedded system environment so quickly. Hence, it is a sign of approaching a tipping point where significant and potentially abrupt changes could occur. This is critical because it implies that once near a tipping point, the system might not recover or adapt easily to external stresses, leading to more profound and lasting changes.

In this case, I think two simple sketches will explain the terms variance and autocorrelation in context to a tipping point even better:

Top diagram (Stable Variance): This diagram shows a ball in a wide, shallow valley, illustrating a stable situation where the AMOC can easily return to its equilibrium state despite disturbances. The wide shape of the valley represents the system’s resilience, where external influences are unlikely to push it out of balance. This stability ensures that the AMOC can recover quickly from disruptions, maintaining its crucial role in regulating Earth’s climate.

Bottom diagram (Higher Autocorrelation): Here, the ball is in a deep, narrow valley. In compare to the upper scenario, the ball is at a higher energy level (hint: climate change is an increase in energy in oceans and atmosphere). This represents the system being more affected by change, taking longer to return to a stable state after being disturbed. The high autocorrelation means that the current state of the system is heavily influenced by its previous states, indicating a “memory” effect that can be a precursor to a tipping point, as it may not respond quickly to further disturbances.

To measure the variance and autocorrelation of the AMOC, the scientists Peter Ditlevsen and Susanne Ditlevsen used advanced statistical techniques on temperature and salinity data collected from the ocean. Here’s a simplified explanation:

Data Collection: They gathered historical data that tracks changes in ocean temperature and salinity over time, which indirectly reflect changes in the AMOC.
Calculating Variance: They calculated the variance by determining how much the data points (temperature and salinity) deviated from their average values over time. This tells them how much fluctuation or instability is present in the AMOC.
Calculating Autocorrelation: They then measured autocorrelation by examining how closely related data points are with their previous values over time. High autocorrelation indicates that present conditions are heavily influenced by past conditions, suggesting a slowing down of the system’s dynamics.

The Ditlevsens used multiple sources of data for their analysis, typically involving oceanographic datasets that include measurements of temperature, salinity, and flow rates, collected from satellite observations, ocean buoys, and ship-based sampling. These datasets are essential for tracking the behavior and changes in the AMOC over time.

But as it is part of such a scientific paper, also the criticism of their study appeared soon.

ChatGPT

The scientific community has expressed varied reactions to the Ditlevsens’ article predicting a potential collapse of the AMOC. Some key points from experts include concerns about the data used, the assumptions made, and the models employed in their study.

Concerns About Proxy Data: Some critics point out that the Ditlevsens’ predictions rely heavily on proxy data from sea surface temperatures, which might not accurately reflect the true state of the AMOC. Continuous direct measurements of the AMOC have only been available since 2004, making long-term conclusions less certain (Science Media Centre).
Model Assumptions: The model assumptions have also been questioned. The Ditlevsens assume that the AMOC is bi-stable (having an “on” and “off” state) and can switch rapidly between these states. This bi-stable assumption is debated, with some suggesting that the AMOC might respond more smoothly to changes in the climate system. If the assumptions in their model are correct, the implications would indeed be profound; however, other plausible models might yield different conclusions.
Skepticism from Broader Climate Models: Some experts argue that the broader, more comprehensive climate models do not show the bifurcation (a sudden, drastic shift) suggested by the Ditlevsens. This has led to skepticism about the urgency and likelihood of an AMOC collapse as portrayed in the study.
Importance of Continued Monitoring: Despite these criticisms, there is a consensus on the importance of continuing to monitor the AMOC. Variability in its strength is crucial for understanding regional and global climates, and thus, even those critical of the study acknowledge the necessity of ongoing research and observation.

Overall, while the study by the Ditlevsens has sparked significant discussion and brought attention to the potential risks associated with changes in the AMOC, it also highlights the complexities and uncertainties involved in predicting such critical transitions in Earth’s climate system.

But the story does not end here. In February 2024, a Dutch research team published as well a scientific work about the AMOC: “Physics-based early warning signal shows that AMOC is on tipping course”. The scary thing is that they did it much more advanced and reliable in compare to the statistical approach of the Ditlevsens, but the outcome is pointing into the same direction at least.

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Here, the details presented from ChatGPT 4.0 analyses:

The Dutch research team led by René M. Van Westen, Michael Kliphuis, and Henk A. Dijkstra conducted a comprehensive study focusing on the Atlantic Meridional Overturning Circulation (AMOC) and the potential for its collapse. Here’s an overview of their research as detailed in the document you provided:

Key Findings

Physics-Based Early Warning Signal: The team developed a new, physics-based early warning signal to detect the tipping point of the AMOC collapse. This signal is based on monitoring the minimum AMOC-induced freshwater transport at the southern boundary of the Atlantic.
AMOC Simulation: They conducted simulations using the Community Earth System Model (CESM), showing a gradual weakening of the AMOC under increasing freshwater forcing, which led to a simulated collapse after approximately 1750 model years.
Impact of AMOC Collapse: The study outlined significant climate impacts due to the AMOC collapse, such as dramatic cooling in the Northern Hemisphere and alterations in sea ice distribution and regional climates globally. This includes profound changes in temperature and precipitation patterns, affecting ecosystems like the Amazon rainforest.

Methodology

Modeling Approach: They used a quasi-equilibrium approach with added freshwater flux anomalies in the North Atlantic, maintaining other climatic forcings at pre-industrial levels during the simulations.
Innovative Measurement: The researchers focused on the freshwater transport dynamics at 34°S as a critical indicator, emphasizing its role in the salt-advection feedback mechanism which is crucial in AMOC stability.

Significance and Novelty

This study is one of the first to identify an AMOC tipping event in a complex General Circulation Model (GCM) used for climate projections. Their findings suggest that traditional statistical early warning signals might not be sensitive enough and highlight the necessity for a more physics-integrated approach.

Critiques and Further Research

The simulation showed that while the traditional early warning indicators like increased variance and autocorrelation were not consistently reliable, the newly proposed physics-based indicators showed more promise in predicting AMOC’s behavior.

This research adds significantly to our understanding of AMOC dynamics and tipping points by integrating physics-based indicators, which could potentially offer more precise predictions and early warnings about AMOC’s future states. This advancement in modeling and warning systems is crucial for preparing for and potentially mitigating the severe climatic impacts of an AMOC collapse.

Remarks:

In climate modeling, “model years” typically refer to the passage of time within the simulation rather than actual calendar years. The starting point of a model can be any year, depending on the preindustrial or historical setup used, and the progression follows the conditions set by the experiment (like greenhouse gas concentrations, solar forcing, etc.). Therefore, stating that a collapse happens after 1750 model years doesn’t mean it predicts a collapse in the year 3750 AD; it indicates how long it takes after the initial conditions are set for the system to collapse under those specific simulated conditions.

Could a Collapse Happen This Century?

According to the findings from the Dutch research, they noted potential early warning signs and shifts within their simulations that could imply increased risks of an AMOC collapse under sustained or specific stresses like significant freshwater influxes. Real-world observations and other models have also suggested that while an AMOC collapse this century is possible, it is still considered unlikely. However, they do show that the AMOC is weakening, which aligns with concerns about its stability due to ongoing climate change.

The real-time risk of an AMOC collapse this century hinges on various factors, including the rate of global warming, ice melt rates, and changes in salinity and temperature gradients across the Atlantic. It’s a subject of active research, with different models providing varying timelines based on their specific parameters and scenarios.

The Dutch research team conducted their AMOC simulation using the Dutch National Supercomputer Snellius. This detail indicates the computational power required for such complex climate modeling. The procession of the simulation took about half a year.

Finally, we should take a look at this diagram:

The graph shows the daily sea surface temperature from 1981 to 2024 for a broad region of the world, tracking annual fluctuations and highlighting the mean temperature and standard deviations over this period. The apparent sharp increase in average temperature in 2023 and 2024 is particularly noticeable.

Interpretation of the Graph:

Annual Lines: Each gray line represents the sea surface temperature for a specific year, plotted daily.
Dashed Black Lines: These indicate the mean (average) sea surface temperature calculated from data spanning 1982 to 2011. The lines marked as “plus 2σ” and “minus 2σ” represent two standard deviations above and below the mean, respectively. This range indicates the typical variability in temperature expected due to natural fluctuations.
Orange Lines: The temperatures for 2023 and 2024 show a significant deviation above the historical average, especially in 2024, where the temperature remains consistently higher than the mean.

Potential Reasons for the Anomaly:

Climate Change: The most likely explanation for sustained increases in sea surface temperature is global warming driven by increased concentrations of greenhouse gases in the atmosphere. This trend aligns with global observations of rising ocean and atmospheric temperatures.
El Niño Events: These periodic climatic events can cause significant, temporary increases in sea surface temperatures across large areas of the Pacific Ocean, which can affect global weather and climate patterns. An El Niño event could explain a sudden spike in temperatures.
Changes in Ocean Currents: Shifts in major ocean currents could redistribute warm water differently, potentially explaining regional increases in sea surface temperatures.

And to make it clear, a slowdown of the AMOC, which we already see in the measurement data, can cause the following:

Heat Redistribution: The AMOC transports warm water from the tropics northward across the Atlantic, contributing to milder climates in Western Europe and affecting weather patterns across the Northern Hemisphere. If the AMOC slows down or alters its path, this could redistribute heat across the globe, potentially leading to increases in sea surface temperatures in other regions.
Climate Feedbacks: A weaker AMOC could lead to less heat being removed from the tropics and more remaining in the equatorial regions, which could increase global sea surface temperatures. Additionally, changes in the AMOC can impact sea ice, cloud cover, and atmospheric circulation patterns, all of which play roles in the global climate system and can further influence temperatures.
Interactions with Other Systems: The AMOC is also linked with other oceanic and atmospheric patterns, such as El Niño, the Southern Oscillation, and the Pacific Decadal Oscillation. Changes in the AMOC could exacerbate or modify the effects of these systems, leading to broader impacts on global climate.

Bottom line: Houston, we have a problem!