Saclay (France) (AFP)

"This piece of ice is 500,000 years old": in the Paris region, researchers are deciphering past climates in ice, trees or sediments to better understand the current climate and predict the changes linked to global warming.

In Saclay, the ICE building brings together 300 people from the Laboratory for Climate and Environmental Sciences (LSCE: CEA / CNRS / UVSQ). State-of-the-art, internationally recognized teams are working on past climates, their impact on ecosystems and climate modeling.

"What interests us is to understand how the climate works," says Didier Roche, deputy director of the LSCE and CNRS researcher. Since there have only been direct measurements for about 70 years, "we have to go in the past," he continues.

They are based on cores of ice, marine or lacustrine sediments, trees ... Within the framework of a European project, EPICA (European Project for Ice Coring in Antarctica), an ice core of 3,270 meters has been extracted from the cap covering the Antarctic continent, to establish the content of the atmosphere in carbon dioxide and methane over 800,000 years.

The ice offers "several tracers in the same archive," says the paleoclimatologist Anaïs Orsi, presenting small ice cubes hundreds or even thousands of years old, filled with small bubbles of air. "We can reconstruct the composition of the atmosphere", but also thanks to the dust "if there were a lot of wildfires in Patagonia" or "aridity in Australia," says the researcher at the CEA (Office of the atomic energy).

Trees are also good recorders of climatic variations. "They can tell us about temperature, sunshine, humidity," says Valérie Daux, professor of Versailles-Saint Quentin University in Yvelines. She is involved in a project in Patagonia that allows to go back in time over 200 years thanks to the rings (growth rings) of trees, Fitzroy cypress, which plans to go up to 1,000 years.

Another line of research is to see "how quickly an ecosystem can adapt to climate change", with the help of carbon 14, says Christine Hatté, researcher at CEA.

- The time of the dinosaurs -

In parallel, scientists model past climates, that is to say, they try to reproduce them from a very complex software, from certain elements (atmosphere, oceans, hydrology, vegetation ... ). They then compare their models to the data collected by their colleagues via the samples.

"We can look at whether each model is good relative to the data," explains Masa Kageyama, research director at the CNRS, who is responsible for LSCE modeling. This allows "to select the models for the future".

Having the most reliable models possible is essential in the face of climate change. French scientists from the National Center for Scientific Research (CNRS), the Atomic Energy Commission (CEA) and Météo-France participate in the work that is the basis of the IPCC, the UN climate experts.

If emissions continue at the current rate, the planet could heat up from 3.4 to 3.9 ° C by the end of the century, according to the United Nations Environment Program (UNEP). And even if the signatory states of the Paris agreement respect their commitments, the mercury will rise by 3.2 ° C.

An increase of 6 to 8 ° C compared to the pre-industrial period would correspond to "climates comparable to the time of the dinosaurs (disappeared 66 million years ago) with very hot temperatures" but the comparison is difficult and these climates are less well known because "the continents were different and not in the same place", says Didier Roche.

Paleoclimatologists, on the other hand, are better acquainted with the climates which prevailed 21,000 years ago, when the average global temperature was 3 to 4 degrees colder. At that time, "there was a kilometer thick ice in New York, three kilometers in Norway, the sea level was 120 meters lower," describes the researcher.

"We have disrupted the climate very strongly and in a very short time, the more we continue, the more it changes," he warns.

© 2019 AFP