The Gulf Stream is weakening year after year. This ocean current which crosses the Atlantic and allows Europe to benefit from a mild climate in winter, is currently at its lowest level for a millennium, according to a study published at the end of February in the journal Nature Geoscience.
To arrive at this conclusion, Irish, English and German researchers studied one of the components of the Gulf Stream: the meridian circulation of Atlantic reversal (AMOC). “This system works like a giant treadmill, explains Stefan Rahmstorf, co-author of the study. Hot, salty water moves from south to north where it cools and thus becomes more dense. When it is heavy enough, water flows to deeper ocean layers and returns to the south. “ In total, it displaces nearly 20 million cubic meters of water per second thanks to AMOC, or nearly a hundred times the flow of the Amazon River.
AMOC only started to be measured directly in 2004. Before that, there was very little data. To precisely analyze this phenomenon, the researchers therefore had to observe indirect indicators. “For example, we looked at the grain size in ocean sediment cores, explains Levke Caesar, the study’s first author. This is because a faster current can carry larger grains. We also looked at the species composition of corals, as they are distributed according to different water temperatures, and finally we compiled historical data, for example from logbooks. “ Together, these indicators paint a picture of how circulation has changed over the years.
The mechanism of currents disturbed by warming
And the observation is clear: “We have seen that ocean circulation remained stable until the end of the 19th century.e century, explique Stefan Rahmstorf. With the end of the Little Ice Age around 1850, ocean currents began to decrease, before a second more drastic decline from the mid-20th century.e century. “ According to all the data studied, the ocean current has decreased by around 15% over the past seventy years.
This phenomenon has worried researchers for several years. A slowdown in AMOC has long been predicted by climate models in response to global warming. According to a number of studies, this is probably the major reason for the observed weakening. Global warming indeed disrupts the mechanism of the currents. Increased precipitation and increased melting of the Greenland ice sheet add fresh water to the ocean surface; for its part, global warming increases the surface temperature of the oceans. This reduces the salinity and therefore the density of the water, which has less tendency to dive deep. “We do not yet know all the causes with certainty, explains Didier Swingedouw, researcher at the CNRS, who did not participate in this work. There is this hypothesis of climate change, but natural cycles could also come into play. ”
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