A new study predicts that the Atlantic meridional overturning circulation (AMOC) could potentially collapse within this century.
What is the Atlantic meridional overturning circulation?
The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents in the Atlantic Ocean that plays a critical role in Earth’s climate system. The AMOC is critical in the current climate around large parts of the world, particularly North America and Europe.
The Atlantic Meridional Overturning Circulation is a component of the global ocean conveyor belt, playing a key role in regulating the climate in the North Atlantic region.
The main characteristic of the AMOC is warm northward flow of salty water in the upper layers of the Atlantic and southward flow of cold water at deeper levels. Water from the tropics moves northward, where then it cools and some of the water evaporates that leads to increased saltiness. Colder and denser water moving to the south warms up and goes up to the surface.
This circulation is part of the larger Thermohaline circulation that has wide impact on overall global climate. The AMOC is responsible for much of the temperate and relatively stable temperature conditions in large parts of Europe. The AMOC also affects sea level change.
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When are climate models predicting the collapse of the AMOC?
There is some disagreement by climate scientists about how quickly or if the AMOC will collapse.
Based on climate model projections and global temperatures increasing over the 21st century, the Intergovernmental Panel on Climate Change (IPCC) believes any collapse of the AMOC will not occur until after the 21st century. The potential collapse, however, could occur and has occurred in the distant past, which may have led to long episodes of climate change.
Scientists have known that accumulation of freshwater and heat in areas of downwelling could potentially lead to a collapse in circulation — warm freshwater is lighter and does not sink as readily as cold salty water. Melting glaciers introduce a lot of freshwater and could reduce salt content in colder northern hemisphere waters — warmer waters would alter circulation closer to the Equator.
However, not all scientists have agreed on the degree and effects of melting glaciers and warmer waters are having on the AMOC. Since 2004, the AMOC has been continuously monitored and there have been periods of declining circulation, which was thought to have contributed to major storms and sea level rise, such as in New York in 2009-2010.
Some scientists believe the AMOC will collapse within the next 25 years
In a recent paper, using statistical significance and data-driven estimators in developed statistical models, researchers estimate that a collapse of the AMOC could happen much earlier than previously thought. The overall trend in the AMOC is not certain given that continuous monitoring has been relatively recent. Temperatures prior to when waters and the AMOC were not monitored could be estimated so that a forward prediction of when the AMOC could collapse could now be estimated.
Recent weakening episodes could effectively foreshadow a tipping point where change in the AMOC occurs much faster. Under current emissions and global temperature change, scientists conclude that the AMOC could collapse by the middle of this century. That is, within the next twenty-five years.[1]
Contradiction in predictions
These results contradict United Nations commissioned scientific assessments and, therefore, not everyone is convinced. The IPCC report from 2019 states clearly that the collapse of the AMOC is not estimated to occur until sometime after this century.[2]
The new research uses an updated statistical model to critique this report. Scientists express the need for further study, particularly direct evidence of a potential collapse of the AMOC before such a model and conclusion for a near-term collapse of the AMOC could be widely accepted.[3]
Lack of historical monitoring of the Atlantic meridional overturning circulation complicates predictions
The main problem is continual monitoring of the AMOC is relatively recent and estimates of a general trend towards collapse depend on weakening conditions over decades that is uncertain. The modeling effectively has to back forecast previous AMOC conditions which are uncertain. While not everyone is convinced by the arguments of near-term collapse of the AMOC, scientists do indicate the general trend or weakening of the AMOC is still concerning and a potential future collapse is a strong possibility.
What will happen if the AMOC collapses?
The result of the AMOC collapse would be would much higher temperatures near the equator, more extreme winters to the United States, including major fluctuations between warmer and colder weather in Europe and northern climates. The relatively moderate climates seen in northern hemispheres would be more susceptible to more extreme long-term climate change.
The AMOC is perhaps the most well-known and studied global Thermohaline circulation. The recent research suggesting a collapse of the AMOC before the century is concluded also raises questions about the overall Thermohaline circulation that governs wider global temperatures. If the AMOC could collapse this century, the overall global Thermohaline circulation may significantly alter as well.
More research might be needed across oceanic circulation and changing current and temperature conditions to determine if there are long-term changes that can be detected. The timing of any collapse within the Thermohaline circulation will have major climate implications for the world and currently changing climate will accelerate even more in many parts of the world with dramatic consequences such as long-term drought, major heat and cold fluctuations that lead to increased storms and sea level change, and other changes that might be difficult to foresee.
Studies of the Thermohaline circulation have been increasing recently but a lot more needs to be done if we are to better understand if there are near-term threats to our oceanic circulation. Even if threats are not immediate, we will need to develop long-term plans that adapt to changes in the AMOC or the wider global Thermohaline circulation.
References
[1] For more on a recent article arguing for a potential collapse of the AMOC by mid-century, see: Ditlevsen P, Ditlevsen S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nat Commun. 2023;14: 4254. doi:10.1038/s41467-023-39810-w.
[2] The IPCC report can be found here: https://www.ipcc.ch/srocc/chapter/summary-for-policymakers/.
[3] Views by scientists sceptical of a mid-century collapse of the AMOC can be seen here: https://www.smithsonianmag.com/smart-news/a-vital-ocean-current-system-could-collapse-as-soon-as-2025-study-predicts-180982605/.
