How scientists monitor the proximity of AMOC collapse

You may have heard that global warming risks total collapse of the AMOC - but how do we monitor this risk?

The Atlantic Meridional Overturning Circulation (AMOC) is a pivotal component of Earth's climate system, acting as a global conveyor belt that transports warm, salty water from the tropics to the North Atlantic.

In this region, the water cools, increases in salinity, and sinks to the deep sea, moving southward before eventually resurfacing in the Southern Hemisphere. This process plays a crucial role in regulating global climate patterns, particularly in Europe and North America.

However, the AMOC is facing significant threats due to climate change. A warming Arctic, with diminishing sea ice, leads to ocean temperatures that are more comparable to those of the North Atlantic, reducing the temperature gradient that drives part of the overturning process.

Additionally, the influx of freshwater from the melting Greenland ice sheet and increased precipitation decreases the salinity and density of the water, further hindering the sinking mechanism essential for the AMOC's function.

Why would an AMOC collapse be catastrophic?

The potential collapse of the AMOC would have profound implications. A phenomenon known as the 'cold blob' could develop over regions such as Ireland, the UK, Scandinavia, and Western Europe, leading to substantial drops in average temperatures and increased precipitation.

This climatic shift would severely impact agriculture, as colder temperatures and excessive rainfall could devastate crops. ^1 2

For instance, northern England and Scotland might experience temperature decreases of up to –10ºC, while southern England and Ireland could see reductions between –3ºC and –6ºC. Similar cooling effects are projected for other parts of Europe, with coastal Norway potentially becoming –5ºC to –10ºC cooler, and regions like Denmark, Northern France, the Netherlands, Belgium, and Northern Germany facing temperature drops ranging from –5ºC to –8ºC.

Conversely, the Southern Hemisphere could experience warming of approximately 1ºC–2ºC above current projections, potentially disrupting critical systems such as the South Asian monsoon, which is vital for freshwater supplies.³ Moreover, such a collapse could contribute to the melting of the West Antarctic ice sheet, leading to further global warming and sea-level rise.

Recent studies have raised concerns that an AMOC collapse may be imminent or closer than previously believed, with these drastic effects potentially unfolding in the coming decades.^4 5 6 

Given these potential consequences, monitoring the AMOC's strength and variability is essential for understanding its current state and predicting future changes, including the risk of collapse.

Monitoring techniques and instruments

Scientists employ a combination of direct and indirect methods to monitor the AMOC:

1. Transport Mooring Arrays (TMAs): Considered the "gold standard" for sustained AMOC observations, TMAs consist of moored instruments that measure dynamic height, current velocities, and other parameters across specific latitudes. The RAPID-MOCHA-WBTS array at 26°N has been operational since 2004, providing continuous data on the AMOC's strength and variability.
2. Expendable Bathythermograph (XBT) Transects: High-density XBT transects are used to resolve strong currents in the western boundary of the South Atlantic. These instruments measure temperature profiles of the upper ocean, contributing to estimates of the AMOC at 22.5°S.
3. Argo Profiling Floats: A global array of autonomous floats that measure temperature and salinity profiles down to 2,000 meters. In combination with satellite sea level data, Argo floats help map the AMOC's structure and variability across different latitudes.
4. Satellite Observations: Satellites provide sea surface height data, which, when combined with in-situ measurements, assist in estimating geostrophic currents and the AMOC's strength. Recent studies have developed new mapping methods that integrate satellite data with in-situ observations to estimate the AMOC at various latitudes.

How close are we to the collapse of the AMOC?

Determining how close the AMOC is to a potential collapse involves analyzing trends, identifying early warning signals, and utilizing predictive models:

1. Trend Analysis: Long-term observational data from TMAs and other instruments are analyzed to detect changes in the AMOC's strength. While some studies have reported a weakening trend over recent decades, others suggest variability within natural ranges. Continuous monitoring is crucial to distinguish between short-term fluctuations and long-term trends.
2. Early Warning Signals: Researchers look for indicators such as increased variance and autocorrelation in AMOC-related data, which may signal a loss of stability and an approaching tipping point. A 2023 study estimated that under current emission scenarios, a collapse could occur around mid-century, highlighting the importance of early detection.
3. Predictive Modeling: Climate models, ranging from intermediate-complexity models to advanced Earth system models, are used to simulate the AMOC's response to various forcing scenarios. Recent studies have employed machine learning techniques to predict tipping points, suggesting a potential collapse window between 2040 and 2065.

What don't we know?

Despite advancements in monitoring and modeling, significant uncertainties remain:

• Model Limitations: Differences in model structures and assumptions can lead to varying projections of the AMOC's future state. Some models may overestimate stability, while others might predict an imminent collapse. Continuous refinement and validation against observational data are necessary.
• Data Gaps: Comprehensive monitoring of the AMOC requires extensive spatial and temporal coverage. Regions like the South Atlantic have historically been under-observed, though recent efforts aim to address these gaps through new mapping methods and expanded observation networks.
• Natural Variability vs. Anthropogenic Influence: Distinguishing between natural variability and human-induced changes in the AMOC is complex. Long-term datasets are essential to identify anthropogenic impacts accurately.

Monitoring the AMOC is a multifaceted endeavor that integrates various observational techniques and modeling approaches. While current methods provide valuable insights, ongoing efforts to enhance observation networks, refine models, and understand underlying mechanisms are crucial as we face a collapse of the AMOC that will destroy food systems in the United Kingdom, Ireland, Scandinavia and Western Europe.

¹ Shifts in national land use and food production in Great Britain after a climate tipping point. Ritchie et al. Nature Food. 2020. 

² Running AMOC in the farming economy. Tim G. Benton. Nature Food. 2020.

³ Penultimate deglaciation Asian monsoon response to North Atlantic circulation collapse. Wassenburg et al. Nature Geoscience. 2021.

⁴ Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Peter D. Ditlevsen and Susanne Ditlevsen. Atmospheric and Oceanic Physics. 2023. 

⁵ Weakening of the Atlantic Meridional Overturning Circulation driven by subarctic freshening since the mid-twentieth century. Garbiel M. Pontes and Laurie Menviel. Nature Geoscience. 2024.

⁶ Open Letter by Climate Scientists to the Nordic Council of Ministers. 2024.