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Researchers have found that North Atlantic water has "dramatically weakened".
The study shows that levels are at their lowest for 1,500 years – and that climate change is to blame.
Researchers warn that currents will have a "profound effect" on the North American and European climate.
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The weakening of the ocean: there is a pattern of traffic in the western North Atlantic during a strong transport episode (left) and weak (right) of the Labrador Current (LC). The oceanography of this region is characterized by the interaction of Labrador-based and westward moving bodies of water (LC and Labrador Sea Watershed (LSSW)) and water bodies eastward from the Gulf Stream (GS) and its temperate Atlantic slope. Water (ATSW). The exact location where these two systems of water masses meet (yellow dashed lines) is determined by the strength of the north recirculating gyre (white arrows), which then controls the temperature.
The researchers studied the Southern Atlantic Overflowing Circulation (AMOC), a branch of the North Atlantic circulation that brings warm surface waters to the Arctic and cold deep waters toward the Arctic Ocean. # 39; equator.
Research, co-led by Drs. Christelle Not and Benoit Thibodeau from the Department of Earth Sciences and the Swire Institute of Marine Science at the University of Hong Kong are interpreted as a direct consequence of global warming and the associated melting of the Greenland icecap .
A slowdown in traffic in the North Atlantic can cause profound changes in both the North American and European climate, but also on summer monsoon rains in Africa and Asia.
"AMOC plays a crucial role in regulating the global climate, but scientists struggle to find reliable indicators of its intensity in the past.
"The discovery of this new AMOC record will improve our understanding of its factors and ultimately help us better understand potential changes in the near future in the context of global warming," said Dr. Thibodeau.
The research team also found a weak signal during a period called the Little Ice Age (a cold spell observed between around 1600 and 1850 AD).
WHAT IS THE SMALL AGE OF ICE?
The reconstructed depth of the Little Ice Age varies according to the studies.
The Little Ice Age occurred between 1300 and 1850.
The Little Ice Age has gone through two phases, the first beginning around 1290 and continuing until the late 1400s.
There was a slightly warmer period in the 1500s, after which the climate deteriorated considerably, with the coldest period being between 1645 and 1715.
During this coldest phase of the Little Ice Age, average winter temperatures in Europe and North America were reported to be 2 ° C lower than current temperatures.
The Baltic Sea has frozen, as have many rivers and lakes in Europe.
The Frozen Thames, 1677, an original painting from the London Museum Collection
Winters were extremely cold and summers were often cold and wet.
These conditions led to a widespread poor harvest, famine and population decline.
Social unrest has increased as large sections of the population are reduced to famine and poverty.
If the signal is not as pronounced as the trend of the twentieth century, the signal could confirm that this period was also characterized by a lower circulation in the North Atlantic, which implies a decrease in heat transfer to Europe , thus contributing to the cold of this period.
However, further work is needed to validate this hypothesis.
"Although we can base our 20th century temperature reconstruction on an instrumental measure, it is not possible to do so for the Little Ice Age.
"Therefore, we need to do more analysis to consolidate this hypothesis," said Dr. Not.
The results were recently published in the prestigious journal Geophysical Research Letters.
The Southern Turnover Atlantic (AMOC) circulation is the branch of the North Atlantic circulation that brings warm surface waters to the Arctic and cold deep waters towards the Arctic Ocean. equator.
This transfer of heat and energy not only has a direct influence on the European and North American climate, but can also affect the monsoon systems of Africa and Asia because of its impact on sea surface temperature, hydrological cycle, atmospheric circulation and variations in the intertropical convergence zone.
Drs. Not and Thibodeau used microfossils, called foraminifer, found in a sediment core to estimate the past temperature of the ocean. In the photo, the species of foraminifer used in this study
Many climate models have predicted a weakening or even collapse of this branch of traffic under the effect of global warming, in part because of the release of fresh water from the Greenland ice sheet.
This fresh water has a lower density than salt water and thus prevents the formation of deep waters that slow down all the traffic.
However, this weakening is still the subject of lively discussions because of the scarcity of AMOC's long-standing archives.
Drs. Not and Thibodeau used microfossils, called foraminifer, found in a sediment core to estimate the past temperature of the ocean.
The sediment core used is in the Laurentian Channel, on the Canadian coast, where two important currents meet.
The strength of these currents will control the temperature of the water at the core site, implying that the temperature reconstructed from this core indicates the strength of the circulation in the North Atlantic.
With their collaborators from the United States of America, they validated their findings with the help of instrumental data and two numerical models able to simulate the climate and the ocean.
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