The Atlantic Meridional Upturn Cell — which takes warm waters from the Southern Hemisphere to the Northern Hemisphere, and brings cold waters from the Northern Hemisphere to the Southern Hemisphere — is a fundamental mechanism for regulating the planet’s climate. This cell has already collapsed in the past, due to natural factors, and its last collapse played a crucial role in the deglaciation process. Now, the cell is threatened again, because of global warming. And recent research has uncovered the sequence of events that triggers the collapse.
The study was carried out by German and Brazilian researchers, and one of the main authors was paleoclimatologist Cristiano Mazur Chiessi, professor at the School of Arts, Sciences and Humanities at the University of São Paulo (EACH-USP). An article about it was published in the journal Nature Communications with the title “Subsurface ocean warming preceded Heinrich Events”.
“By investigating marine sediments collected between Canada and Greenland, we found that, in the past, the warming of the oceanic subsurface of that region caused the great glaciers that once covered the territories corresponding to Canada and the northern United States to release colossal amounts of icebergs in the Atlantic”, says Chiessi to Agência Fapesp.
Once in the ocean, these icebergs melted and deposited continental sediments to the bottom. “It was precisely the identification of these sediments that, together with the reconstitution of the subsurface temperature of the region, made it possible, for the first time, to establish that subsurface warming preceded the massive release of icebergs.”
The enormous volume of fresh water produced by the melting of icebergs changed the composition of the ocean in the high latitudes of the Northern Hemisphere. And that has had a tremendous impact on the global climate, because the region between Canada and Greenland is a particularly sensitive site of the Atlantic Meridional Upturn Cell.
“This gigantic circulation transports, on the surface, warm, low-density waters from the South Atlantic to the North Atlantic. In the high latitudes of the North Atlantic, these surface waters release heat to the cold atmosphere. With that, they gain density and sink in the column of Once in the depths, the now cold, high-density waters return southward to the outskirts of Antarctica, where they return to the surface forced by an intense upwelling process. Upon reaching the surface, the waters are heated, losing density and close the Atlantic Meridional Upturning Cell”, describes Chiessi.
The highlight is that this circulation does not just transport a huge amount of water, approximately 18 million cubic meters per second. It also carries an exorbitant amount of energy: about 100,000 times the energy produced by the Itaipu hydroelectric plant. The spatial distribution of this energy decisively influences the climate in several regions of the planet, including Brazil. While a vigorous circulation maintains the climate we know, its collapse causes a marked redistribution of energy, altering the climate.
The research received support from Fapesp through the project “Past perspectives on critical thresholds of the climate system: the Amazon Forest and the Atlantic Meridional Upturning Cell”, coordinated by Chiessi.
The Atlantic Meridional Upturning Cell collapsed several times during the last glacial period, between approximately 71,000 and 12,000 years before the present. Other studies coordinated by Chiessi, based on marine sediments collected between the coast of Venezuela and Northeast Brazil, showed that these collapses caused a torrential increase in rainfall in the Northeast and a strong decrease in precipitation over Venezuela and the extreme north of the Amazon. Decreases in precipitation have also been described in the tropical regions of North Africa and Asia.
Finding that subsurface warming from the high latitudes of the North Atlantic preceded the massive release of icebergs from Canada and the United States into the Atlantic Ocean, the researchers were able to establish the sequence of events responsible for the collapse of the Atlantic Meridional Upturn Cell.
“The process starts with a seemingly minor weakening of the Cell, which causes subsurface warming in the high latitudes of the North Atlantic. This warming melts the base of ocean-ended glaciers and causes them to move rapidly toward the ocean, releasing colossal amounts of icebergs. The melting of icebergs reduces the salinity of the surface waters in the region, which no longer reach the density necessary to sink. And this causes the Cell to collapse”, informs Chiessi.
Monitoring the intensity of the Atlantic Meridional Upturning Cell over the past few decades shows that it is weakening. The three main causes for the weakening are intensified rainfall in the high latitudes of the North Atlantic, the melting of the existing ice cap over Greenland and the warming of the planet’s surface. The three causes are associated with the increase in the concentration of greenhouse gases (GHG) in the Earth’s atmosphere, due to human action.
The discovery now made indicates that ongoing weakening is likely to cause anomalous subsurface warming in the high latitudes of the North Atlantic, melting the base of ocean-terminated glaciers that exist today over Greenland. And that, ultimately, could collapse the Atlantic Meridional Upturn Cell, further aggravating, and with great repercussions, the climate crisis.
The article “Subsurface ocean warming preceded Heinrich Events” can be accessed at https://www.nature.com/articles/s41467-022-31754-x
