A million years ago, the Earth's climate changed abruptly for reasons that are still unknown.
The great masses of continental ice accumulated in the polar regions, the glacial cycles became longer and colder - the most intense in the history of the Quaternary -, and as a consequence, the global climate system was altered on a planetary scale.
The true nature of the mechanisms that radically transformed the planet climate In the aforementioned period, it has been the subject of debate in the international scientific community for decades.
According to a study published in the journal Nature Geoscience, an abrupt reduction in the intensity of the deep oceanic circulation or thermohaline 950,000 years ago - a phenomenon already documented by oceanographers Leopoldo Pena and Steven Goldstein (Science, 2014) - enhanced the capture and storage in the deep ocean of atmospheric carbon dioxide (CO2) on a planetary scale.
As an effect of this slowdown in global ocean circulation, "a part of that CO2 was trapped in the deep ocean and this could contribute to a drastic climate change in the planetary system", detail the researchers Leopoldo Pena and Maria Jaume-Seguí, members from the Department of Earth and Ocean Dynamics of the University of Barcelona and co-authors of the work.
The authors of the new work have estimated that, during the most extreme phases of this climate transition, the deep Atlantic stored about 50 billion tons of carbon additional, compared to less intense glacial cycles that existed before one million years.
With these large amounts of carbon confined to the depths of the ocean, the level of carbon dioxide decreased in the atmosphere, global temperatures became colder, and ice sheets spread across the planet during this particular Quaternary period.
“The deep ocean has acted and acts as a reservoir or storehouse for CO2. When this gas accumulates for hundreds or thousands of years at the bottom of the ocean, there is a decrease in CO2 in the atmosphere that has global climatic consequences. Now, it is important to emphasize that the opposite mechanism is also possible ", alerts Leopoldo Pena.
In the ocean depths, marine sediments preserve the climate record of this exceptional period that meant a turning point in Earth's climate.
In the framework of the investigation, the experts have analyzed the isotopic composition of the fossilized remains of planktonic and benthic foraminifera, unicellular organisms capable of generating a calcium carbonate mineral shell. The study of these protozoa, which abound in the fossil record of oceanic sediments, is decisive for understanding the characteristics of the climate and marine ecosystems of the past.
Climate change: from the past to the future of the planet
Understand the nature of past climate changes it is key to improving forecasts on the evolution of the climate in the future.
Today, some of the great challenges in paleoclimatology and paleoceanography They lie in knowing exactly the mechanisms for capturing and emitting CO2 in oceanic ecosystems, identifying and determining the size of these reservoirs, and discovering their response capacity to changes in ocean circulation.
The new study describes some of the climatic mechanisms that contributed to the Middle Pleistocene transition and provides new perspectives for making future climate predictions with greater accuracy and reliability. According to experts, it will still be necessary to unravel many unknowns about the changes in deep ocean circulation that marked this period.
"The Earth's climate system, as we know it, is not stagnant. Our study shows that there are mechanisms that control the climate of our planet that we do not fully understand, ”explains Leopoldo Pena.
“One million years ago,” he continues, “multiple components of the Earth's climate system worked together to push the global climate toward a state of extreme and long-lasting ice age. Today, human-induced increases in atmospheric carbon dioxide concentrations could also drive the climate system into a totally different state. '
Nowadays, there is evidence that ocean circulation in the Atlantic has slowed by 15% since the mid-20th century. Regardless of the causes that cause this phenomenon, "it is important not to draw simplistic parallels: one could be tempted to say that, if circulation slows down, atmospheric CO2 will decrease, but that would be a very serious mistake", warns Leopoldo Pena.
“In this case - he continues - surface waters rich in CO2 would not be transported to the deep ocean, while, in regions such as Antarctica, deep waters rich in CO2 would continue to reach the surface, and consequently atmospheric CO2 would continue to increase ».