The mechanism involves the warming of the ocean subsurface, leading to decreased surface salinity caused by the release of massive iceberg armadas from glaciers.
The Atlantic meridional overturning circulation (AMOC) is a crucial ocean current system responsible for regulating Earth’s climate by transporting warm and cold water across the globe. In the past, this system has experienced collapses due to natural factors, and one such collapse had a significant impact during the last deglaciation. However, scientists have now found evidence that AMOC is at risk from global warming, and a recent study has revealed the sequence of past breakdown events.
The findings discussed in the study were a result of collaborative research conducted by German scientists and Cristiano Mazur Chiessi, a paleoclimatologist and professor at the University of São Paulo’s School of Arts, Sciences and Humanities (EACH-USP) in Brazil. The research outcomes have been documented in an article published in the scientific journal Nature Communications.
Cristiano Mazur Chiessi, in an interview with Agência FAPESP, explained that the research involved studying marine sediments collected in the region between Canada and Greenland. The findings revealed a historical pattern in which the warming of the ocean surface led to the release of massive icebergs from the glaciers that once covered the areas now known as Canada and the northern United States.
The icebergs dissolved within the ocean, releasing continental sediments that settled on the seabed.
“By examining these sediments and reconstructing the past temperature conditions beneath the surface, scientists made a groundbreaking discovery. They found evidence indicating that subsurface warming occurred before the massive release of icebergs,” he explained.
The vast amount of freshwater introduced through the melting of the icebergs altered the composition of the ocean in the high latitudes of the northern hemisphere. This had an immense influence on the global climate, as the area between Canada and Greenland is an exceptionally delicate segment of the AMOC.
“This immense conveyor belt transports relatively lighter and warmer surface water from the South Atlantic to the North Atlantic. In the high latitudes of the North Atlantic, this surface water releases its heat into the frigid atmosphere, subsequently becoming denser and sinking deeper into the water column. The colder, denser water then flows southward until it reaches the vicinity of Antarctica, where it resurfaces due to strong upwelling forces. On the surface, it warms up, loses density, and thus completes the cycle,” explained Chiessi.
The AMOC serves as a conduit not only for an immense volume of water, estimated at around 18 million cubic meters per second, but also for an enormous amount of energy. This energy is equivalent to approximately 100,000 times the power generated by Itaipu, the world’s second-largest hydroelectric plant situated on the Brazil-Paraguay border. The spatial distribution of this energy plays a crucial role in influencing the climate across various regions of the world, including Brazil. The robust circulation of AMOC maintains the familiar climate patterns we experience, while its disruption leads to a significant redistribution of energy, ultimately causing alterations in the climate system.
The research was carried out within a project led by Chiessi and supported by FAPESP through its Research Program on Global Climate Change (RPGCC). During the last glacial period, approximately between 71,000 and 12,000 years before the present (BP), the AMOC experienced multiple collapses. Additional studies led by Chiessi, involving the analysis of marine sediments collected from the coast of Venezuela to Northeast Brazil, revealed that these collapses led to a significant increase in rainfall in Northeast Brazil while causing a sharp decrease in rainfall in Venezuela and the northern regions of the Amazon rainforest. Similar reductions in rainfall were observed in tropical areas of North Africa and Asia.
The researchers were able to establish the sequence of events that led to the collapse of the AMOC by uncovering the fact that subsurface warming in the North Atlantic at high latitudes occurred prior to the massive release of icebergs from Canada and the United States into the Atlantic.
“The initial stage involves a seemingly minor weakening of the Atlantic Meridional Overturning Circulation, leading to subsurface warming in the high latitudes of the North Atlantic. This warming triggers the rapid retreat of glaciers and the consequent release of massive fleets of icebergs. As these icebergs melt, the salinity of surface water in the region decreases. The reduced density of the surface water prevents it from sinking, ultimately resulting in the collapse of AMOC,” explained Chiessi.
The monitoring of the AMOC over the past few decades indicates a weakening trend. This can be attributed to three primary factors:
- increased precipitation in the high latitudes of the North Atlantic,
- the melting of the Greenland ice cap,
- and the overall warming of the Earth’s surface.
All three causes are closely linked to the elevated levels of greenhouse gases in the atmosphere resulting from human activities.
The recent findings indicate that the weakening of the AMOC could result in abnormal subsurface warming at the high latitudes of the North Atlantic. This warming effect has the potential to accelerate the melting of glacier sea snouts in Greenland. Ultimately, such a scenario could lead to the collapse of AMOC, amplifying the climate crisis and causing significant consequences.