How will our future be shaped by global climate change?

With the aim of gaining a better understanding of our future, an international collaboration involving researchers from the Centre de Recherche et d'Enseignement en Géosciences de l'Environnement (AMU/CNRS/IRD/INRAE/Collège de France) is exploring ancient warm periods in the Earth's history. In their study, published on 29 January in the journal Nature Geoscience, they show that an increase in greenhouse gases in the atmosphere 56 million years ago led to a sudden change in rainfall and vegetation in Central Asia.

Today, vast treeless steppes characterise the landscape of Central Asia. The rare rainfall that reaches the interior of Asia is mainly due to the monsoon in summer or to mid-latitude westerlies in winter and spring. These steppes are particularly sensitive to changes in temperature and precipitation, with all the consequences that this implies for their inhabitants, such as the Saiga antelope or Przewalski's horse, an endangered species”, explains Dr Niels Meijer, before adding: “One of the major uncertainties of global climate change is how the Asian monsoon and the Central Asian regions that depend on it will react to future climate change.”

As part of an international research team that has been working in the region for more than twenty years, Niels Meijer recently focused on the warm period of the early Palaeogene, where he identified a hyperthermal event of extreme warming precisely dated to 56 million years ago.

In order to reconstruct the precipitation patterns of this period, the researchers combined their expertise to develop an innovative multi-proxy approach in which they combined fossil pollen and spores with geochemical data from fossil soils. “During the hyperthermal event we studied, precipitation temporarily doubled due to higher temperatures and the regional steppe was replaced by a forested landscape,” explains Meijer, describing the results and adding, “But most importantly, we were able to use geochemical data to show that the soils dried out in winter, meaning that, contrary to expectations, most of the precipitation fell during the summer period - which is comparable to the modern monsoon.”

Scientists associate this hyperthermal event with the “Paleocene/Eocene temperature maximum”, a phase of global warming associated with a considerable increase in greenhouse gases in the Earth's atmosphere and oceans. During this period, the global temperature rose by an average of six degrees Celsius in the space of a few thousand years. Their research shows that in Asia this event was accompanied by exceptionally wet conditions and an inland expansion of precipitation, which they have called a proto-monsoon.

“The abrupt greening of the Central Asian steppe desert as a result of the monsoon climate probably also allowed the spread of new mammal species and may also have played a role in feedbacks in the global carbon cycle”, Meijer sums up before concluding: “Our work shows an abrupt and non-linear response of the Asian monsoons to extreme greenhouse conditions. Although the seas and mountains of Asia 56 million years ago were very different from those of today, these data highlight the possibility of abrupt changes in precipitation and ecosystems in Central Asia under future global warming. Current warming is imposing extreme temperatures and increased drought on the Central Asian steppe and its fragile flora and fauna, which are already under threat from anthropogenic land use. Even more dramatic abrupt changes are expected if temperatures continue to rise”.

29.01.2024