According to researchers, the carbon hidden in the depths of the ocean has thrown a former climate switch

A million years ago, a prolonged alternation of glaciations and warm periods radically changed, when the ice age suddenly became longer and more intense. Scientists have long suspected that this was linked to the slowing down of a key current system in the Atlantic Ocean, which is slowing once again today. A new study of the sediments of the Atlantic bottom directly associates this slowdown with a massive accumulation of carbon driven air into the abyss. With the system running at full speed, this carbon would have settled in the air rather quickly, but it stagnated in the depths during this period. This suggests that carbon sequestration has cooled the planet – the reverse of the greenhouse effect we are currently seeing, as humans pump carbon into the atmosphere. But if the current continues to slow down, we should not expect it to help us by storing our emissions; perhaps on the contrary. The study, led by researchers from the Lamont-Doherty Earth Observatory of Columbia University, appears this week in the newspaper Nature Geoscience.

Scientists have targeted a system of currents called southern inversion circulation of the Atlantic, or AMOC. Sending northward near the surface, it carries warm, salty water from near the equator to the nearby latitudes of Greenland and northern Europe. . Here it hits the colder Arctic water, densifies and sinks into the abyss, bringing with it large amounts of carbon absorbed by the atmosphere. The deep water then turns south, where much of it re-merges into the Southern Ocean to release carbon into the air. The trip takes place over decades, even centuries.

A 2014 study by Lamont-Doherty geochemist Steven Goldstein and his student Leopoldo Pena, both co-authors of the new study, showed that the current had slowed sharply about 950 000 years. The new study shows that this slowdown is directly linked to a huge accumulation of carbon in the depths of the Atlantic and a corresponding decrease in carbon in the air. This event was the apparent trigger of a series of glacial ages that occurred every 100,000 years, compared to previous ones that occurred approximately every 40,000 years and that accumulated less ice than those that occurred. are produced later. Scientists call this turning point the transition between the mid-Pleistocene and the new pattern persisted until the end of the last ice age, which ended about 15,000 years ago. Nobody knows exactly why this trend has continued, but the study clearly shows that the carbon missing in the air was found in the ocean and had a powerful effect on the climate.

"It's a one-to-one relationship, it was like switching," said lead author Jesse Farmer, who did the work while a PhD student. student at Lamont-Doherty. "This shows us that there is an intimate relationship between the amount of carbon stored in the ocean and what the climate does."

The researchers drew their conclusions by analyzing the deep-water sediment cores taken from the south and north of the Atlantic, where the old deep waters passed and left chemical clues to their contents in the shells of microscopic creatures. Their analysis confirmed the 2014 study showing that AMOC was weakening to a point never seen before, around 950,000 years ago, and for an unusually long time. As a result, deepwaters have collected about 50 billion tonnes more carbon than in previous glaciations, equivalent to about one-third of human emissions consumed by all oceans around the globe. (By context, the oceans today absorb about a quarter of our emissions, the land and vegetation absorb a third, and the rest stays in the air.)

During the warm period that preceded this event, the atmosphere had retained about 280 parts per million of carbon; with slowing down, airborne carbon dioxide fell to 180 ppm as measured by ice cores. Atmospheric carbon had also flowed during previous glaciations, but from 280 ppm to about 210 ppm. (Due to human emissions over the last two centuries, this normal figure of 280 ppm in hot weather has become obsolete, with atmospheric carbon now reaching about 410 ppm.)

At one point, the current again woke up and things warmed for a while before falling back into a new extreme ice age, after 100,000 years. "There are a lot of ideas about the cause of these changes, but it's hard to say what was the trigger," said Bärbel Hönisch, Farmer's advisor and co-author of the program. ;study. "There are several screws that you could imagine turning and a lot of loosened screws."

An idea, retained in particular by the Goldstein group: in the north, repeated accumulations of glaciers eventually erase everything on the mainland up to the rock in place. Subsequent glaciers are then able to quickly stick to the rock in place and accumulate even more, before releasing icebergs into the ocean. This introduces more fresh water to mix with AMOC, making it less dense and ultimately unable to flow. At the other end, the ice would also develop in Antarctica and release more icebergs, which would make the ocean waters cooler and less salty, thus promoting the growth of more sea ice. This, theoretically, would cover the surface and prevent deep water from rising and releasing their carbon. But if this is indeed the case, we do not know exactly what starts or ends the processes; it is a question of hen and egg.

The strength of AMOC is thought to fluctuate naturally, but it appears to have weakened by 15% since the mid-20th century. Nobody knows what is behind it or what effects it could have if the slowdown continued. Another study by Lamont-Doherty last month showed that an intense slowdown about 13,000 years ago, at the end of the last ice age, was followed, 400 years later, by an intense cold wave that lasted centuries.

"We have to be careful not to draw a parallel with that," said Farmer, now a postdoctoral fellow at Princeton University. "We are seeing a similar weakening today, and we could say," Great! Ocean circulation will save us from global warming! "But it's not correct, because of the way different parts of the climate system talk to each other." Farmer said that if AMOC continued to weaken, it is likely that less water loaded with carbon will flow in the north, while in the Southern Ocean, all the carbon that already arrives in the deep waters will probably continue to bubble without this happening. problem. The result: carbon will continue to accumulate in the air, not in the ocean.

The researchers point out that AMOC is just one part of a much larger system of global circulation linking all oceans – the so-called Great Ocean Conveyor, a term coined by the scientist Lamont-Doherty, Wallace Broecker, who laid the groundwork for much of the current research. Much less is known about the carbon dynamics of the Indian and the Pacific, which together negate the Atlantic. So there are still many missing pieces in the puzzle. Current research at Lamont-Doherty aims to establish a carbon chronology of these other waters over the next few years.