New study confirms AMOC is on tipping course

A new study published in Science Advances by René M. Van Westen, Michael Kliphuis, and Henk A. Dijkstra on February 9, 2024, unveils a physics-based early warning signal indicating the Atlantic Meridional Overturning Circulation (AMOC) is on a tipping course towards collapse. Utilizing the Community Earth System Model, this research not only simulates the first tipping event but also highlights the significant climate impacts that could ensue, offering a novel approach to predicting and potentially mitigating one of the most catastrophic climate tipping events.

The AMOC, a critical component of the Earth’s climate system known for its role in heat and salt transport across the global oceans, faces the threat of collapse due to increased freshwater input in the North Atlantic. This study, for the first time, uses the Community Earth System Model to simulate an AMOC tipping event, highlighting significant climate impacts and introducing a novel, observable early warning signal.

The AMOC’s significance in climate regulation cannot be overstated, with its variability closely monitored since 2004 through the RAPID-MOCHA array at 26°N. Observations suggest a fluctuation in AMOC strength, with a notable weakening since circa 1950, corroborated by proxy records indicating the weakest AMOC state in over a millennium.

The system’s sensitivity to freshwater influx, through precipitation or ice melt, has raised concerns about a potential rapid shift or collapse, a phenomenon supported by historical data during Dansgaard-Oeschger events.

Traditional early warning indicators based on sea surface temperature have hinted at an approaching tipping point, yet their reliability remains questionable due to underlying statistical assumptions.

The researchers made a simulation using the Community Earth System Model to explore the dynamics of an AMOC tipping event. By maintaining preindustrial levels of greenhouse gases and introducing a gradually increasing freshwater flux in the North Atlantic, the model exhibited a gradual decline in AMOC strength, culminating in a dramatic collapse after 1750 years. This collapse, characterized by a reduction in AMOC strength from approximately 10 Sverdrups (Sv) to slightly negative values, marked the first such event simulated in a complex global climate model.

The collapse’s climate repercussions were profound, with notable cooling in the Northern Hemisphere, significant changes in ocean salinity, and dynamic sea-level rise across the Atlantic.

The collapse led to extensive climate alterations, including atmospheric temperature shifts, Hadley Cell strengthening, and substantial sea-ice expansion in the Northern Hemisphere. These changes had far-reaching implications for regional climates, particularly in Europe, where dramatic cooling was observed. The study also highlighted the potential for significant alterations in precipitation patterns, notably in the Amazon rainforest, which could lead to ecosystem disruptions and cascading climate effects.

A key contribution of this study is the development of a physics-based, observable early warning signal for AMOC tipping, centered on the minimum of AMOC-induced freshwater transport at the southern boundary of the Atlantic. This signal presents a robust alternative to classical statistical indicators, which have shown sensitivity to the analyzed time interval before tipping. Reanalysis products suggest that the current AMOC trajectory is indeed headed towards a tipping point.

The study by Van Westen, Kliphuis, and Dijkstra represents a significant advancement in our understanding of AMOC dynamics and its susceptibility to collapse. By offering a tangible early warning signal, this research provides valuable tools for climate scientists and policymakers alike, emphasizing the need for immediate attention to the AMOC’s fragile state.

References:

¹ Physics-based early warning signal shows that AMOC is on tipping course – Rene M. Van Westen et al. – SCIENCE ADVANCES – February 9, 2024 – Vol 10, Issue 6 DOI: 10.1126/sciadv.adk1189

Featured image: Science Advances/The authors