2023-12-08 01:22:00
Current levels of carbon dioxide in the atmosphere are the highest reached on Earth in 14 million years, revealed a study released this Thursday, which evokes the inhospitable climates towards which humanity is heading.
“This shows us to what extent what is being done now is truly unprecedented in the history of the Earth,” the study’s lead author, Baerbel Hoenisch, a researcher at Columbia University in New York, told AFP.
The last time the planet’s atmosphere contained the same concentration of the main greenhouse gas (CO₂) as today, around 420 ppm (parts per million), was between 14 and 16 million years ago.
This dates back much longer than scientists had previously estimated (between three and five million years).
Between 14 and 16 million years ago, for example, there were no ice sheets in Greenland.
Climates unknown to humanity
However, “our civilization is accustomed to the current sea level, to the warm tropics, to the cold poles and to temperate regions that benefit from a lot of precipitation,” warns Baerbel Hoenisch.
“Our species (…) only began to evolve three million years ago,” recalls the scientist. “We have never experienced anything similar to these warm climates.”
Before the industrial era, the concentration of CO₂ in the atmosphere was around 280 ppm. With human activities it increased by 50%, which has caused an increase in temperatures of around 1.2 °C.
And if emissions continue, the concentration could rise to 600 or 800 ppm, rates reached during the Eocene (-30 to -40 million years ago), before Antarctica was covered in ice and when wildlife and flora planetaria were very different, with, for example, huge insects.
The study published Thursday in Sciences is the result of seven years of work by a group of 80 researchers from 16 countries. Its conclusions are considered a scientific consensus.
The contribution of this research lies not so much in the collection of new data, but in the meticulous work of re-evaluation and synthesis of existing works to update them and classify them according to their reliability, which made it possible to use the best data to design a general image.
To reconstruct past climates, a well-known technique consists of recovering from the depths of the polar caps the air bubbles that trapped the composition of the atmosphere at the time.
But this technique only allows us to go back a few hundred thousand years. To go further, you must resort to indirect markers. The chemical study of ancient leaves, minerals or plankton has thus made it possible to deduce the concentration of CO₂ in older times.
In the last 66 million years, the warmest period the Earth has known dates back to about 50 million years ago, with a CO₂ concentration of 1,600 ppm and temperatures 12 °C warmer than today.
Those levels slowly decreased until 2.5 million years ago and the time of the ice ages, when the CO₂ concentration fell again to 270-280 ppm.
Then they remained stable, until humanity began burning fossil fuels on a large scale.
According to the study, a doubling of the CO₂ concentration rate would gradually warm the planet over hundreds of thousands of years, reaching between +5 and 8°C, due to the cascading effects of rising temperatures. .
Thus, the melting of polar ice reduces its ability to reflect the sun’s rays, further accelerating the process.
The study shows that 56 million years ago, the Earth’s atmosphere experienced a rapid increase in CO₂ concentration similar to what we know today and that caused massive changes in ecosystems that took about 150,000 years to dissipate.
“We will be there for a long time, unless we capture carbon dioxide from the atmosphere and stop our emissions as soon as possible,” summarized Baerbel Hoenisch.
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