A Closer Look at the Weakening AMOC and Its Ramifications

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A Closer Look at the Weakening AMOC and Its Ramifications

When it comes to climate change, tipping points refer to irreversible change that happens when a certain threshold is reached, for example, a specific temperature.

Scientists and policymakers agree that global warming beyond 2°C above the pre-industrial average would pose significant and escalating risks to human life.

In a climate science update, scientists revealed that the world is warming at an unprecedented rate of over 0.2° C per decade between 2013 and 2022, caused by high GHG emissions. 

Regarding tipping points, scientists will refer to the Atlantic Meridional Overturning Circulation (AMOC) as a major tipping element in the climate system.

Climate models project that a weakening AMOC is one of the consequences of climate change, along with other problems like sea level rise, increasing frequency of extreme precipitation, droughts, and heat waves.

Studies claimed that they have found evidence to suggest that the AMOC is weakening with severe impacts on the climate in the North Atlantic region.

What is the AMOC? It is a system of ocean currents that circulates water within the Atlantic Ocean, bringing warm water northward and cold water towards the south. 

Circulation begins as warm water near the surface moves towards the poles, where it cools and forms sea ice. The salty water left when ice forms sinks and is carried southwards in the depths, a process called thermohaline circulation. Eventually, the water comes to the surface and warms up, a process known as “upwelling”, and completes the cycle.

These currents are carried in a large “global conveyor belt, including the AMOC that transports heat worldwide.

The warm water the AMOC carries towards the north pole releases heat into the atmosphere and keeps Western Europe warm. For instance, without it, winters would be about 5°C colder in the UK. So, any changes in the AMOC will seriously impact Europe’s weather and the global climate.

The AMOC’s entire circulation cycle is slow – it is estimated to take 1,000 for a parcel of water to complete its journey along the belt. However, recent studies have shown evidence that the AMOC is slowing down further.

A study in Nature published in April 2018 identifies a “fingerprint” of a changing AMOC. Researchers provide evidence for a weakening of the AMOC by about 15% since 1950. “This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project.”

According to Carbon Brief, this finding is consistent with direct measurements of the AMOC by the RAPID project  – a collection of instruments tethered to the seafloor across the Atlantic Ocean that monitors temperature, salinity and current speed at different depths.

Read the study: Observed fingerprint of a weakening Atlantic Ocean overturning circulation

Another study published in the same issue of Nature also concludes that the AMOC has weakened. Researchers provide several lines of paleoceanographic evidence that the Labrador Sea deep convection and AMOC have been anomalously weak in the last 150 years compared to the preceding 1500 years.

According to an article in Real Climate, scientists based their findings on a more extended climate history context. The authors used two types of data: sediment cores from shells of the marine organisms; this allows researchers to know temperature conditions reaching back to 1600 years.

The second data is based on the grain size of the sediments based on currents from the Gulf Stream – coarse grain sizes indicate a strong flow, while finer grain size indicates a weaker flow.

Based on these two data sets, researchers conclude that the AMOC has never been as weak in all those previous centuries as in the last hundred years.

Read the study: Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years

A recent study published on 25 July 2023 in Nature Communications finds that the continued warming will push the AMOC over its tipping points by 2050. It means that the AMOC will grind to a halt, leading to devasting changes in the weather on both sides of the Atlantic.

The study based its findings on the modelling and reconstruction of the AMOC, which shows that it is bistable. Instead of getting weaker or stronger, it turns on and off like a switch if pushed too far. And when it turns off or stops moving, it is very hard to turn it on again. The authors project that this change in the AMOC could occur by the middle of this century.

The study’s finding is based on the new calculations by researchers from the University of Copenhagen contradicting the latest IPCC report. They predict that the ocean current system that distributes cold and heat between the North Atlantic regions will stop if the GHG emissions trend continues. 

The study used advanced statistical tools and ocean temperature data from the last 150 years. Researchers calculated that the ocean current, known as the Thermohaline Circulation or the Atlantic Meridional Overturning Circulation (AMOC), will collapse with 95% certainty between 2025 and 2095 and likely happen in 34 years in 2057.

Read the study: Warning of a forthcoming collapse of the Atlantic meridional overturning circulation

Impacts if AMOC continues to slow down or collapse

According to the study, the collapse of the AMOC will bring warming in the tropics and increased storminess in the North Atlantic region. Heatwaves will also occur more frequently around the globe.

AMOC’s weakening due to global warming will cause the freshwater from melting ice at the poles to shift the rain belt in South Africa, causing droughts for millions of people. It would also cause sea level rise across the US East Coast (What is the, 2023).

Sources:

Hood, M. (2023, June 8). World warming at record 0.2 C per decade, scientists warn. Phys.Org. Retrieved from https://phys.org/news/2023-06-world-decade-scientists.html

What is the Atlantic Meridional Overturning Circulation (AMOC)? (2023, January). National Ocean Services. Retrieved from https://oceanservice.noaa.gov/facts/amoc.html

McSweeney, R. (2018, April 11). Atlantic ‘conveyor belt’ has slowed by 15% since mid-20th century. Carbon Brief. Retrieved from https://www.carbonbrief.org/atlantic-conveyor-belt-has-slowed-15-per-cent-since-mid-twentieth-century/

Caesar, L., Rahmstorf, S., Robinson, A. et al. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556, 191–196 (2018). https://doi.org/10.1038/s41586-018-0006-5

Thornalley, D.J.R., Oppo, D.W., Ortega, P. et al. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature 556, 227–230 (2018). https://doi.org/10.1038/s41586-018-0007-4

Stronger evidence for a weaker Atlantic overturning circulation. (2018, April 11). Real Climate. Retrieved from https://www.realclimate.org/index.php/archives/2018/04/stronger-evidence-for-a-weaker-atlantic-overturning-circulation/

Ditlevsen, P., Ditlevsen, S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nat Commun 14, 4254 (2023). https://doi.org/10.1038/s41467-023-39810-w

University of Copenhagen – Faculty of Science. (2023, July 25). Gloomy climate calculation: Scientists predict a collapse of the Atlantic ocean current to happen mid-century. Science Daily. Retrieved from https://www.sciencedaily.com/releases/2023/07/230725123122.htm

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