How We Know The AMOC Will Collapse Again

Key Concepts

• AMOC (Atlantic Meridional Overturning Circulation): A system of ocean currents that transports warm water from the tropics towards the North Atlantic and cold water southwards.
• Freshwater Forcing: The addition of freshwater to the North Atlantic, primarily from melting ice sheets and increased precipitation, which reduces the density of surface water and hinders the sinking process that drives the AMOC.
• Hysteresis: The dependence of a system's state not only on its current inputs but also on its past history. In the context of the AMOC, this means that the circulation may not recover immediately even if freshwater forcing is reduced.
• Early Warning Signals (EWS): Statistical indicators that can potentially predict an approaching critical transition or tipping point in a complex system. Examples include increased variance and autocorrelation.
• Critical Slowing Down: A phenomenon where a system becomes slower to recover from perturbations as it approaches a tipping point.
• Climate Models: Computer simulations used to project future climate scenarios and understand the behavior of complex climate systems like the AMOC.
• Proxy Data: Indirect measurements of past climate conditions, such as ice core data, sediment records, and tree rings, used to reconstruct historical AMOC behavior.
• Dansgaard-Oeschger Events: Rapid climate fluctuations observed during the last glacial period, characterized by abrupt warming followed by gradual cooling, potentially linked to AMOC instability.
• Heinrich Events: Massive iceberg discharge events into the North Atlantic during glacial periods, associated with significant freshwater forcing and AMOC weakening or shutdown.
• Bifurcation Point: A critical threshold where a small change in a parameter can lead to a qualitatively different state of the system. In the AMOC context, this is the point where the circulation collapses.

Evidence for AMOC Collapse

The video presents multiple lines of evidence suggesting that the AMOC is at risk of collapse, drawing from climate models, proxy data, and theoretical understanding of the system's dynamics.

1. Climate Model Projections

• CMIP6 Models: The latest generation of climate models (Coupled Model Intercomparison Project Phase 6) show a wide range of AMOC responses to future warming scenarios. While most models project a weakening of the AMOC, some models even simulate a complete shutdown under high-emission scenarios.
• Freshwater Input Sensitivity: Models demonstrate that increased freshwater input into the North Atlantic, primarily from melting Greenland ice and increased precipitation, weakens the AMOC. The video highlights that the rate of Greenland ice melt is accelerating, increasing the risk of freshwater forcing.
• Uncertainty and Model Limitations: The video acknowledges that climate models have limitations in accurately representing all the complex processes that govern the AMOC. However, the consistent signal of weakening or collapse across many models raises serious concerns.

2. Proxy Data and Past AMOC Behavior

• Dansgaard-Oeschger Events: The video discusses the rapid climate fluctuations observed during the last glacial period, known as Dansgaard-Oeschger events. These events are characterized by abrupt warming followed by gradual cooling, and are thought to be linked to changes in the AMOC. The rapid warming phases may have been triggered by sudden AMOC strengthening, while the cooling phases may have been caused by AMOC weakening or shutdown.
• Heinrich Events: The video explains that Heinrich events were massive iceberg discharge events into the North Atlantic during glacial periods. These events introduced large amounts of freshwater into the ocean, which significantly weakened or shut down the AMOC. The video argues that these past events demonstrate the AMOC's sensitivity to freshwater forcing and its potential for abrupt changes.
• Sediment Core Analysis: Analysis of sediment cores from the North Atlantic provides evidence of past AMOC variability. Changes in sediment composition and grain size can indicate changes in ocean currents and water mass properties, which can be used to reconstruct historical AMOC behavior.

3. Early Warning Signals and Critical Slowing Down

• Increased Variance and Autocorrelation: The video explains that as a system approaches a tipping point, it often exhibits increased variance and autocorrelation. This means that the system becomes more sensitive to small perturbations and takes longer to recover from them.
• Observational Evidence: The video cites studies that have found evidence of increased variance and autocorrelation in observational data of the AMOC, suggesting that the system may be approaching a critical threshold.
• Critical Slowing Down as a Predictor: The concept of critical slowing down is presented as a potential early warning signal for AMOC collapse. By monitoring the system's response to perturbations, scientists may be able to detect when it is approaching a tipping point.

4. Hysteresis and Irreversibility

• Multiple Stable States: The video explains that the AMOC may have multiple stable states, including a strong circulation state and a weak or collapsed state.
• Hysteresis Loop: The concept of hysteresis is introduced, meaning that the AMOC's response to changes in forcing may depend on its past history. Even if freshwater forcing is reduced, the AMOC may not immediately recover to its previous strong state.
• Potential for Irreversible Change: The video warns that if the AMOC collapses, it may be difficult or impossible to restore it to its previous state, even if climate change is mitigated. This is because the system may have crossed a bifurcation point, beyond which the dynamics are fundamentally different.

Implications of AMOC Collapse

The video outlines the potential consequences of an AMOC collapse:

• Significant Cooling in Europe: The AMOC transports warm water to the North Atlantic, which helps to moderate the climate of Europe. An AMOC collapse would lead to significant cooling in Europe, particularly during the winter months.
• Sea Level Rise Along the US East Coast: Changes in ocean currents associated with an AMOC collapse would lead to sea level rise along the US East Coast.
• Shifts in Precipitation Patterns: An AMOC collapse would disrupt global precipitation patterns, potentially leading to droughts in some regions and floods in others.
• Disruption of Marine Ecosystems: Changes in ocean currents and water mass properties would disrupt marine ecosystems, potentially leading to declines in fish populations and other marine life.

Conclusion

The video concludes that the AMOC is at risk of collapse due to increasing freshwater forcing from melting ice sheets and increased precipitation. Multiple lines of evidence, including climate model projections, proxy data, and early warning signals, suggest that the system may be approaching a critical threshold. An AMOC collapse would have significant and potentially irreversible consequences for the climate and ecosystems of Europe, North America, and the world. The video emphasizes the need for further research to better understand the AMOC's dynamics and to develop more accurate projections of its future behavior. It also underscores the urgency of reducing greenhouse gas emissions to mitigate climate change and reduce the risk of an AMOC collapse.