Concerns have been raised about the effects of global warming on the Atlantic Meridional Overturning Circulation (AMOC) as regards a possible collapse. Researchers have now discovered the mechanism that could cause the circulation system to collapse, with dire effects on Earth’s climate.
Conditions in the North Atlantic
The vast Atlantic Ocean circulation system had collapsed multiple times in the past. Researchers say the most recent of these played a key part in the last deglaciation.
This new study done by Brazilian paleoclimatologist Cristiano Mazur Chiessi and researchers from Germany shed light on how past collapse events occurred.
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These researchers examined marine sediments obtained from the region between Canada and Greenland. They found that glaciers covering present-day Canada and the northern U.S. in the past released high amounts of icebergs into the Atlantic due to rising ocean surface temperatures to trigger AMOC collapse.
The icebergs left continental sediments on the ocean floor.
“Identification of these sediments and reconstitution of the subsurface temperature in the region enabled scientists to establish for the first time that subsurface warming preceded the mass iceberg release,” Chiessi, a professor in the School of Arts, Sciences and Humanities at the University of Sao Paolo, told Agência FAPESP.
The research team published its findings in Nature Communications.
Atlantic Ocean’s conveyor belt
AMOC is a system for circulating the currents of the Atlantic. It helps to carry cold water from the Northern Hemisphere to the South and warm water in the opposite direction. The system is key to the proper control of Earth’s climate and constantly regulates temperatures in North America and Europe.
The circulation system not only transports a high amount of water but also an enormous quantity of energy. Researchers compared the energy that it carries to 100,000 times the amount of power produced by Itaipu, which is the second-largest hydroelectric plant in the world.
The distribution of this colossal energy impacts the climate in some parts of the world. Strong circulation helps to maintain the climate in a good state while a slowdown could promote extreme weather.
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As per researchers, the melting of the icebergs released by glaciers in the past released a massive amount of freshwater, which altered ocean makeup in the Northern Hemisphere. This impacted the global climate radically.
“This gigantic conveyor belt carries lighter, warmer surface water from the South Atlantic to the North Atlantic. At high latitudes of the North Atlantic, this surface water releases heat into the cold atmosphere, becoming heavier and sinking down the column,” Chiessi explained. “The deeper, colder, and denser water then flows southward again until it reaches the vicinity of Antarctica, where it returns to the surface, forced by an intense upwelling. On the surface, it warms up, loses density, and completes the circulation.”
Collapse of AMOC
Researchers say this vast circulation system of the Atlantic collapsed multiple times during the last glacial period, which is estimated to be in the period from around 71,000 to 12,000 years before present (BP). These were caused by natural factors.
By analyzing marine sediments obtained off the seacoast of Venezuela and Northeast Brazil, Chiessi and his team observed in other research extreme weather changes from past collapses. There was a massive rise in heavy rainfall in Northeast Brazil and an acute decline in rainfall in Venezuela and north of Amazonia.
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Researchers in this study were able to work out the sequence of events that can cause AMOC to collapse from their discovery of how subsurface warming brought about immense iceberg release at high latitudes.
Chiessi explained that the process begins with AMOC’s weakening, which may seem trivial. However, this results in high-latitude subsurface warming that melts glacier sea snouts and moves glaciers swiftly seaward while releasing huge amounts of icebergs.
The salinity of surface water around where icebergs melt drops. This reduces the density of the surface water and causes AMOC to collapse.
Sadly, different reports suggest that this great circulation system has been growing weaker in recent decades.
Growing greenhouse gas levels from human activity are linked to the weakening of AMOC. They contribute to this mainly by intensifying rainfall at high latitudes, melting Greenland’s ice covering, and increasing the planet’s surface temperatures.
The discovery in this research hints that adverse subsurface warming could result from a weaker AMOC and, ultimately, bring about its collapse. Certainly, this will worsen the climate crisis if it were to happen.
