Through Centuries

The term conveyor belt refers to the set of deep currents of thermohalin origin that transport heat from the equator to the poles. This happens thanks to the warm current (that of the Gulf) that is formed in the equatorial regions and rises up to Iceland, where it meets the icy winds coming from Canada. Here the surface water of the sea cools, yielding heat to the air and mitigating the climate in Northern Europe. Evaporating, the water becomes denser and sinks, generating a deep current that takes the name of conveyor belt that brings very cold water to the oceans in an extraordinary time: 1000 years!

Credit: Inmeteo

This transport of water circulation in the Atlantic has undergone and continues to undergo changes over the centuries. Our purpose is to be able to understand if in some way human action affects these inversions and, if it is possible to understand the future of the starting point of this very important circulation that generates a delicate balance on a world level. The section of the current we are examining is the so-called Atlantic Meridional Overtourning Circulation (AMOC) which is the one in which the Atlantic water sinks to the North Atlantic. It has been observed from experiments that this water is slowed down about 400 years before the cold periods as for the Yunger Drias (13000 years ago) and is strengthened about 400 years before global warming and, therefore, the hottest periods (as for the periods of deglaciation).

It can be thought that these results can be obtained with the study of the radioactive isotope 14C that afterwards allows to date the organisms and the surrounding sediment and, obviously, in this way also the water. In reality the carbon 14 that is created in the atmosphere has a penetration time in water that is quite long and, therefore, when it reaches the sediment it may already have hundreds or thousands of years.

This is why scientists have had to invent different methods for dating water and sediments. The carbon 14 content was measured in a nearby lake bed. This is because the layers at the bottom of the lake contain decomposing plants whose age can be dated. In this way they were able to combine the sedimentary states of the lake with those of the sea and, hence, also to trace the age of the marine layers. In addition, ash layers from Icelandic volcanic eruptions (which we know are frequent due to their position along the Mid-Atlantic ocean ridge) were also used for dating. Subsequently, the real age of marine sediments was compared with the age obtained from carbon 14 measurements of the deep ocean. The difference between these two data made it possible to obtain an estimate of the time taken by atmospheric carbon 14 to reach the seabed. In other words, the speed with which the water sank in this area in the deep water formation process that is essential for the circulation of the AMOC was detected. Greenland ice cores were also analyzed to study temperature and climate change over the same time period. At this point it was possible to compare the order of events between changes in ocean circulation and climate change.

The interesting thing now, is that the Atlantic Meridional Overtourning Circulation is undergoing a process of weakening as it happened before the glaciations for already about 150 years. However, this process will not take place in 400 years but will take much longer due to the global warming caused by man. Obviously, these are just assumptions based on studies of the past. What is clear is how Svensson says: “As long as we do not understand the climate of the past, it is very difficult to limit the climate models necessary to achieve realistic future scenarios”.

Nothing is certain in the future of the earth, but surely man has a certain incidence on it. Triggered processes to date are not understood and may never be understood. We will never be enough to understand the action of animate and inanimate things and this will lead us to always be a step backwards and to dare against a world that would only want to breath and take its course.

Maria Bruno

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