The Atlantic meridional overturning circulation (AMOC) is a vast system of ocean currents which carry warm water from the tropics into the North Atlantic ocean and transport cold water from the northern to the southern hemisphere, thus playing a major role in the regulation of the Earth’s climate. This system has collapsed in the past due to various natural factors, with its most recent collapse playing a key role in the last deglaciation.
Since AMOC is currently threatened by anthropogenic global warming, understanding the reasons for its past collapses is essential. According to a new study led by the University of São Paulo in Brazil, AMOC past collapse was caused by the warming of the ocean subsurface resulting in a reduction of surface salinity due to the release of a large number of icebergs from glaciers into the sea.
“An investigation of marine sediments collected between Canada and Greenland led to the discovery that, in the past, glaciers covering the territories that now correspond to Canada and the northern United States released colossal numbers of icebergs into the Atlantic owing to ocean surface warming in the region,” explained study co-author Cristiano Chiessi, a paleoclimatologist at the University of São Paulo.
In the ocean, these icebergs melted and deposited continental sediments on the seabed. “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 said.
The massive volume of fresh water added by the melting of the icebergs changed the composition of the ocean at the high latitudes of the northern hemisphere, which had a tremendous impact on the global climate.
According to the scientists, AMOC collapsed several times during the last glacial period (between 71,000 and 12,000 years ago), causing torrential increases in rainfall in northeast Brazil and a sharp drop in rainfall in Venezuela, the north of Amazonia, and tropical areas of North Africa and Asia. By discovering that North Atlantic subsurface warming preceded the massive release of icebergs from the U.S. and Canada into the Atlantic ocean, the scientists were able to establish the sequence of events responsible for AMOC’s past collapse.
“The process begins with an apparently insignificant weakening of AMOC, which causes subsurface warming at high latitudes of the North Atlantic. This warming melts the glaciers’ sea snouts, moving the glaciers rapidly seaward and releasing colossal armadas of icebergs. As the icebergs melt, surface water salinity decreases in the region. The surface water isn’t dense enough to sink, and AMOC collapses,” Chiessi explained.
In recent decades, monitoring of AMOC has shown evidence that it is once more weakening due to three main reasons: the intensification of rainfall at high latitudes; the melting of the ice cap over Greenland; and the warming of the Earth’s surface. According to the experts, all three causes are associated with greenhouse gas emissions due to human activities. The new findings regarding AMOC’s collapse in the past suggest that the current weaker AMOC will cause anomalous subsurface warming at high latitudes, which could melt glaciers in Greenland, ultimately leading to another AMOC collapse and thus exacerbating the current climate crisis.
The study is published in the journal Nature Communications.
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