Global warming – worrying lessons from the past

Fifty-six million years ago, the Earth experienced an exceptional episode of global warming. In a very short time, on a geological scale, in 10 to 20,000 years, the average temperature has increased from five to eight degrees, returning to its initial level a few hundred thousand years later. On the basis of sediment analysis of the southern slopes of the Pyrenees, researchers from the University of Geneva (UNIGE) have measured the impact of this warming on the surrounding floods and landscapes. The magnitude of the floods has increased eight-fold, and sometimes even a factor of 14, and the vegetated landscapes may have been replaced by arid and stony plains. Their disturbing conclusions, to be published in Scientific reports, show that the consequences of global warming may have been much larger than predicted by current climate models.

"The method we used to analyze this global warming is directly inspired by cell signaling in systems biology, where researchers analyze the response of cells to external stimuli and signal transmission," explains Sébastien Castelltort, professor at the Department of Earth Sciences. the Faculty of Sciences of UNIGE and responsible for the study. "We are interested in how a system, in this case the hydrological cycle through the behavior of rivers, reacts to an external signal, here, global warming." This project focused on an extreme climatic case well known to scientists: a 5- to 8 – degree warming occurred 56 million years ago between the Paleocene and the Eocene, also known by the acronym PETM (Paleocene-Eocene Thermal Maximum). Named Land Surface Signaling System (ESSS), this project is supported by the Swiss National Science Foundation (SNSF).

Palm trees in polar latitudes

As early as the 1970s, scientists observed a strong anomaly in the ratio of stable carbon isotopes (δ13C), due to the relative increase in the proportion of the light isotope (12C) relative to the heavy isotope ( 13C). the carbon cycle, both in oceans and on continents, associated with global warming and its dramatic consequences. Palm trees have flourished in polar latitudes and some marine plankton, such as the dinoflagellate Apectodinium, normally restricted to tropical waters, have suddenly spread around the world. Geologists use this type of observation as true "paleothermometers" which, in this case, show an increase in the temperature of the surface water which has reached almost 36 degrees in places, a deadly temperature for many organisms. Several phenomena are cited as possible causes of this global warming, from intense volcanic activity in several regions of the globe to this period destabilizing methane hydrates, these "ice cubes" methane which remain stable only under certain conditions of pressure and temperature. and who by degassing would have released their greenhouse gas.

But if the event is known and its causes have been widely explored, what about the consequences? "The question is important because there is an obvious analogy with the current global warming.There are lessons to be learned from this event, especially as the rise in temperatures we are witnessing appears to be much faster." , points out Sébastien Castelltort.

Pebbles that reveal the history of rivers

The Spanish Pyrenees offer sediments that allow you to observe the ancient channels of the river and determine their size. Chen Chen, a Ph.D. student in the Department of Earth Sciences at UNIGE's Faculty of Science, says thousands of pebbles have been measured in the field. Little by little, thanks to the direct relationship between the size of the pebbles and the slope of the rivers, the researchers were able to calculate their flow velocity and their flow. They unveiled the history of these rivers and the dramatic changes that affected them.

Fifty-six million years ago, the Pyrenees were formed and their buttresses were crossed by small canals isolated in a floodplain where they deposited alluvium very fertile, favoring the development of a vegetation whose roots anchor the ground. Leaving the Pyrenean foothills, these small rivers then headed west towards the Atlantic, then about 30 kilometers away.

"With global warming, the landscape has changed dramatically, with floods averaging every two or three years and measuring the flow of water 14 times higher than when the climate was cooler. "explains Sébastien Castelltort. During the PETM, streams were constantly changing course, they no longer adapted to increased landfills by incising their bed, but they widened, sometimes dramatically, from 15 to 160 meters wide in cases the most extreme. Instead of being trapped in the flood plains, the alluvium was transferred directly to the ocean and the vegetation seemed to disappear. The landscape is transformed into vast plains of arid gravel, crossed by rivers ephemeral and torrential.

Much more risk than expected

Scientists still do not know how precipitation patterns have changed, but they know that this warming has resulted in more intense floods and increased seasonality, with the summers being much warmer. Higher evaporation resulted in an unexpected increase in the magnitude of the flood. A degree of temperature increase implies a 7% increase in the atmosphere's ability to retain moisture, and this ratio is typically used to assess increased precipitation. "But our study shows that there are thresholds, nonlinear evolutions that go beyond this ratio.With a ratio of 14 for the magnitude of the floods, we are facing effects that we do not We do not understand global factors that are not yet integrated into current climate models: our study shows that the risks associated with global warming can be much greater than we generally think, "Sébastien Castelltort concludes.

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More information:
Chen Chen et al, Estimation of regional flows during Paleocene-Eocene warming, Scientific reports (2018). DOI: 10.1038 / s41598-018-31076-3