Thermohaline circulation

The Gulf Stream is breaking apart at breakneck speed. It's very bad news.

We had suspected this for several years, but today the scientists are formal and this is a first. Two studies have just been published simultaneously in the journal Nature. They note an alarming disruption in the circulation of ocean currents in the Atlantic, which contribute to the regulation of the global climate. If we have a temperate climate in Western Europe, it is thanks to the Gulf Stream. If its circulation slows down or stops, the scientists do not dare to imagine the consequences. That is what is happening.
Vou probably remember that hit film by Roland Emmerich released in 2004. The Day After imagined an interruption in the circulation of the ocean current that would cause the northern hemisphere to suddenly enter into glaciation. This catastrophic film could become a reality. Researchers have been studying the thermohaline circulation, i.e. the circulation of ocean currents, particularly in the Atlantic, for some fifteen years. All the studies show evidence of disruption. The two studies published on April 11 in NatureThe ocean current has never been so weak in 1600 years. In addition, the melting pack ice due to global warming is pouring huge quantities of freshwater into the Atlantic. This less salty water compromises the ocean circulation process between the South and the North of the planet.

Planetary conveyor belt

The ocean circulation works like a giant conveyor belt, driven by the difference in the density of seawater, which is caused by differences in the temperature and salinity of the water masses. Warm water in the tropical South Atlantic cools as it approaches the latitudes of the North Atlantic. It becomes denser and sinks deeper. The current then continues southward, warms up and rises to the surface. Scientists refer to this phenomenon as AMOC (Atlantic Meridional Overturning Circulation). This circuit crosses the world and regulates the climate. The Gulf Stream, for example, provides Western Europe with a temperate climate. If, as it approaches from the north, the ocean water does not cool down enough and the salinity level of the water decreases, the conveyor belt no longer works properly. The water is no longer sufficiently dense.
The two studies published in Nature are a continuation of decades of research. All have found anomalies, but today the scientists are definitive. They've conclusively validated the hypothesis that AMOC currents are weakening. " The evidence we are now able to provide is the strongest to date. " says Stefan Rahmstorf of the Potsdam Institute, who designed the study. He continues: " We analyzed all available sea surface temperature datasets including data from the late 19th century to the present day. The specific trend pattern we found in the measurements resembles exactly what is predicted by computer simulations, namely a slowing of the Gulf Stream ".

Global warming

This weakening is caused by a number of factors related to global warming, including increased precipitation, melting sea ice, glaciers and ice shelves, which release freshwater, which is less dense than salt water, into the North Atlantic". Freshwater weakens AMOC because it prevents the water from becoming dense enough to sink. "...says to AFP David Thornalley of University College London, co-author of one of the studies." If the system continues to weaken, it could disrupt weather conditions from the United States and Europe to the Sahel and cause sea levels to rise faster on the east coast of the United States. "warns the Woods Hole Oceanographic Institution, which participated in the research.
Several studies have already warned that the slowing down of the ocean current would exacerbate the rise in sea level on the American coast, directly threatening cities such as New York and Boston. The weakening of the Gulf Stream would also have consequences for Europe by affecting the trajectory of storms coming from the Atlantic. More generally, European weather conditions would be disrupted with intense heat or cold waves. Meteorologists thus explain that the heat wave of summer 2015 is linked to a record cold wave recorded in the North Atlantic. This seems paradoxical but can be explained by the variations in atmospheric pressure which carries warm air from the south to Europe.

READ IN UP' : Melting ice around Greenland is seriously disrupting the Gulf Stream. Glaciation and rising waters in Europe?

Sand grains

One of the two studies published by Nature, that of David Thornalley's team from University College London, is a first. It looks at the analysis of sand grains deposited by currents on the seabed over time. The larger the sand grains found in the sediments, the stronger the currents that carried them. The results show that AMOC was relatively stable between 400 and 1850 and began to weaken in the early industrial era. These paleoclimatic data provide independent confirmation of earlier findings that the recent weak circulation is unprecedented for at least a millennium.
The second study looked at ocean surface temperatures and concluded that AMOC has declined over the past 50 years, probably due to human-induced climate change. While it is difficult to know for certain the role played by global warming, "... it is difficult to know for certain the role played by global warming. the fact that AMOC remained weak and weakened during the 20th century, with a notable decline from about 1950 onwards, is most certainly related to human factors "Scientists estimate that the South Atlantic circulation has decreased by 15 % since the middle of the 20th century to a "new record". This represents a decrease of 3 million cubic metres of water per second, the equivalent of nearly 15 Amazonian rivers.
I think it's happening. "alert Stefan Rahmstorf." And I think it's bad news. "he adds.

Cascading Consequences

Meteorology is not the only area affected by this major disruption, according to the European Research Fund. ATLASwho also participated in the studies, commercial fishing could be affected by changes in the position and depth of ocean currents and some regions would lack oxygen-rich waters." A weakening of AMOC can also lead to temperature increases or decreases of several degrees, affecting some important (to humans) fish species, as well as the amount of plankton, fish, birds and whales. "he said in a statement.
To top it all off, if the ocean currents lose strength again, it would lead to " leave more carbon dioxide in the atmosphere, where it contributes to global warming It's a vicious circle," adds David Thornalley, "a real vicious circle.
What will the future bring? Stefan Rahmstorf predicts that the circulation will only get weaker as climate change progresses. It may not be slow and steady: there is a great fear that there will be a "tipping point" where the circulation stops abruptly. We would then find ourselves in the situation of a breaking point in any complex system. A point that cannot be predicted but which can lead to chaos.

READ IN UP' : Global warming could lead us into a new ice age...

This cataclysmic scenario would correspond to what scientists call "abrupt climate change". Studies of the planet's history suggest that such a sudden change in the North Atlantic has occurred many times in the Earth's past, most recently probably 13,000 years ago. But no one knows when this tipping point will occur." I think Greenland is going to melt even faster, so I think the prospect for that ocean circulation system is that it's going to weaken even more."...predicted Professor Rahmstorf." And I think it's going to affect all of us, fundamentally, in a negative way. "
Sources: Nature, Washington Post, AFP

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