by A new study predicts that the Atlantic meridional overturning cycle (AMOC) may collapse within this century.
What is the Atlantic meridian solstice circulation?
The Atlantic overturning cycle (AMOC) is a system of ocean currents in the Atlantic Ocean that plays a critical role in Earth’s climate system. The AMOC cycle is very important in light of the current climate that surrounds large parts of the world, especially North America and Europe.
The Atlantic overturning circulation is a component of the global ocean conveyor belt, and plays a major role in regulating climate in the North Atlantic region.
The main feature of the AMOC cycle is the northward flow of warm salt water in the upper layers of the Atlantic Ocean and the southward flow of cold water at deeper levels. Water from the tropics moves north, where it cools and evaporates some of the water, leading to an increase in salinity. The colder, denser water moving southward heats up and rises to the surface.
This circulation is part of the larger thermohaline cycle that has a broad impact on the overall global climate. The AMOC is responsible for much of the relatively mild and stable temperature conditions in large parts of Europe. AMOC also affects sea level change.
When do climate models predict AMOC collapse?
There is some disagreement among climate scientists about how quickly or whether the AMOC will collapse.
Based on climate model projections and increasing global temperatures through the 21st century, the Intergovernmental Panel on Climate Change (IPCC) believes that any collapse of the AMOC will not occur until after the 21st century. However, the potential collapse could have occurred in the distant past, which may have led to prolonged periods of climate change.
Scientists have known that a build-up of fresh water and heat in bottom regions can lead to circulatory collapse—warm fresh water is lighter and does not sink as easily as cold, salt water. Melting glaciers provide plenty of fresh water and could reduce the salt content of colder waters in the northern hemisphere – warmer waters would also change circulation near the equator.
However, not all scientists agree on the degree and effects of glacier melt and warm water on the AMOC. Since 2004, the AMOC has been continuously monitored and it has happened Periods of decreased blood circulationwhich were thought to have contributed to major storms and sea level rise, as occurred in New York in 2009-2010.
And some scientists believe that the AMOC will collapse within the next 25 years
In a recent paper, using statistical significance and data-based estimators in developed statistical models, researchers estimate that AMOC breakdown could occur much earlier than previously thought. The overall trend in the AMOC is not certain since continuous observation was relatively recent. Temperatures can be estimated prior to the time when the water and AMOC were not observed, so that a future prediction of when the AMOC could collapse can now be estimated.
Recent bouts of weakness could effectively herald a tipping point as the change in the AMOC occurs much faster. With current emissions and changing global temperatures, the scientists conclude that the AMOC could collapse by the middle of this century. That is, during the next twenty-five years.[1]
Contradictory expectations
These findings contradict scientific assessments ordered by the United Nations, and therefore, not everyone is convinced of them. The 2019 IPCC report clearly indicates that the AMOC collapse is not expected 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, especially direct evidence of possible AMOC collapse, before such a model and conclusion of near-term AMOC collapse can be widely accepted.[3]
The lack of historical observation of the Atlantic meridian overturning cycle complicates projections
The main problem is that continuous monitoring of the AMOC is relatively recent, and estimates of the general trend towards a breakdown are based on conditions of weakness over decades, which is uncertain. Modeling should effectively support predictions of uncertain prior AMOC conditions. While not everyone is convinced by the arguments for an AMOC collapse in the near term, scientists point out that the overall trend or weakening of the AMOC remains worrisome and that a possible future collapse is a strong possibility.
What would happen if AMOC collapsed?
The result of the collapse of the AMOC cycle will be higher temperatures near the equator, and more severe winters in the United States, including large swings between warmer and cooler weather in Europe and northern climates. The relatively mild climates we see in the Northern Hemisphere will be more susceptible to more extreme climate change in the long term.
The AMOC is perhaps the most well-known and studied global thermohaline cycle. Recent research that the AMOC collapsed before the end of the century also raises questions about the overall thermal cycle that governs global temperatures more broadly. If the AMOC could collapse in this century, the overall global thermal cycle could change dramatically as well.
Further research across the oceanic circulation, current conditions and changing temperatures may be needed to determine if there are detectable long-term changes. The timing of any collapse of the thermohaline cycle will have major climatic implications for the world, and climate change will now accelerate further in many parts of the world with disastrous consequences such as prolonged drought, large heat and cold fluctuations leading to increased storms, sea level change, and other changes that It can be hard to predict.
Studies of the thermohaline cycle have increased recently, but there is much more we need to do if we are to better understand whether there are near-term threats to our ocean circulation. Even if the threats are not immediate, we will need to develop long-term plans that adapt to changes in the AMOC or the broader global thermohaline cycle.
References
[1] For more information on a recent article advocating a possible collapse of the AMOC by mid-century, see: Ditlevsen P, Ditlevsen S. Warning of imminent collapse of the Atlantic meridional overturning cycle. Nat Common. 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 of scholars skeptical about the 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/.
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