A 2024 study reveals AMOC is nearing a tipping point, with new early warning signals predicting potential collapse and climate impacts.
Atlantic Meridional Overturning Circulation (AMOC)
The Atlantic Meridional Overturning Circulation (AMOC) is one of Earth's most critical climate systems, transporting heat and salt across the Atlantic Ocean and regulating regional and global climate patterns.
A 2024 study by van Westen and colleagues has provided alarming new evidence that AMOC may be approaching a tipping point, a state from which it could collapse due to continued input of freshwater from ice melt and precipitation. This study introduces a physics-based early warning signal that can help predict the timing of such a collapse, offering a crucial tool for understanding the risks of a rapidly shifting climate.
AMOC’s Role in Global Climate
The AMOC plays a fundamental role in modulating the Earth’s climate by transporting warm water northward in the upper layers of the Atlantic and returning cold water southward at depth. This system affects weather patterns, sea levels, and heat distribution, especially in Europe and the Americas.
Since 2004, continuous observations from the RAPID array at 26°N have shown fluctuations in AMOC’s strength. While these fluctuations are a normal part of its behavior, the long-term trend points toward a weakening, likely linked to increased freshwater input from melting ice sheets and changes in precipitation.
The study cites that the AMOC is currently at its weakest point in over a millennium, with proxy records suggesting that past rapid AMOC changes contributed to abrupt climate shifts during the last glaciation.
A Physics-Based Early Warning Signal
Traditional early warning signals based on statistical analysis of sea surface temperatures (SST) have suggested that AMOC could collapse by the end of the century. However, these signals have limitations due to their assumptions and sensitivity to time intervals. The authors of this study introduce a more reliable early warning system based on a physics-based model.
Their model focuses on a key indicator: the minimum of AMOC-induced freshwater transport at the southern boundary of the Atlantic (34°S). This indicator, known as FovS, reflects the salt-advection feedback—a process critical to AMOC stability. As freshwater from the Greenland Ice Sheet and other sources enters the North Atlantic, it weakens the AMOC by reducing salinity, which is essential for maintaining the system's strength.
AMOC Collapse Simulation
Using the Community Earth System Model (CESM), the study simulates an AMOC tipping event. The model shows that gradual increases in freshwater forcing in the North Atlantic lead to an eventual collapse of AMOC. Initially, natural variability dominates AMOC behavior, but after about 1,750 years of simulation, the model shows a sudden collapse from a strength of 10 Sverdrups (Sv) to just 2 Sv in the following 100 years, with the circulation eventually becoming negative. This collapse drastically reduces heat transport across the Atlantic and cools the Northern Hemisphere while warming the Southern Hemisphere—a pattern known as the Atlantic seesaw effect.
Climate Impacts of AMOC Collapse
An AMOC collapse would have profound implications for global and regional climates. The study finds that the Northern Hemisphere would cool significantly, with parts of northwestern Europe experiencing temperature drops of 5°C to 15°C in just a few decades. This rapid cooling would have severe consequences for agriculture, infrastructure, and energy systems. Meanwhile, the Southern Hemisphere would experience warming, disrupting global weather patterns and rainfall distribution, particularly in regions like the Amazon, where shifts in precipitation could exacerbate ecosystem stress and lead to tipping cascades, such as Amazon rainforest dieback.
Additionally, the AMOC collapse would cause dynamic sea-level rise, with coastal regions in the Atlantic experiencing rises of over 70 cm due to changes in ocean circulation.
Early Warning and Action
One of the most critical findings of the study is that the FovS minimum occurs about 25 years before the AMOC tipping point, making it a valuable early warning signal. The study emphasizes the importance of monitoring this signal to estimate how close we are to an AMOC collapse. Current reanalysis data already shows a negative trend in FovS, suggesting that the AMOC is on its way to tipping.
The study underscores the need for sustained ocean observations and further model simulations to refine our understanding of how and when AMOC could collapse. Given the devastating potential impacts, understanding and preparing for this event is crucial for climate resilience.
Conclusion: A System on the Edge
The 2024 study provides critical evidence that the AMOC, one of Earth's most important climate regulators, is on the verge of a tipping point. With the introduction of a physics-based early warning signal, we now have a more accurate tool to anticipate this potential collapse. However, with current trends pointing towards further weakening of the system, immediate global action to curb climate change and mitigate its impacts is essential to avoid the catastrophic consequences of an AMOC collapse.
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