About 34 million years ago, the Earth shifted from the warm climate of the Eocene to a much colder climate, allowing the establishment of a perennial polar ice cap in Antarctica during the Oligocene. Recent studies based on sedimentary records of very high temporal resolution, show that this global climate transition was however punctuated by local and rapid climate oscillations, of the order of a few tens of thousands of years, linked to periodic changes in the Earth's orbit. These orbital variations, due to the interaction of the Earth with the other planets of the solar system, modify the distribution of the insolation received at the surface of the globe. This has repercussions on atmospheric and oceanic circulations, and therefore on the climate. These orbital variations are for example involved in the alternation between glacial and interglacial cycles that have marked the last million of years climate.

Sampling the cyclic alternations of gypsum with red muds in the 50 to 30 million year old sedimentary archives near the city of Xining in China. Photo: Niels Meije

The precise reconstruction of the climates of this Eocene-Oligocene transition period is complex. Indeed, for such ancient times, the various paleo-environmental indicators available are often dated with too little precision to account for these "rapid" terrestrial orbital changes on the geological time scale. Climatic reconstructions are currently based on compilations stacking many fossils, particularly of plants, which are dated at about the same period, which can span a few millions of year. These compilations therefore represent a climatic "snapshot" but can sometimes offer contradictory view of the climates considered.

In a study published on October 22, 2021 in the journal Science Advances, and led by scientists from the Institut de Physique du Globe de Paris, University of Paris, CEREGE (CNRS, Aix Marseille University), LSCE (CNRS, University of Paris-Saclay), the Universities of Stockholm, Amsterdam, Rennes and Potsdam, they sought to assess the extent to which the lack of consideration of orbital variations by models and botanical compilations biases the representation of the paleoclimates of that time.

Shrubby environment. Photo: Natasha Barbolini

Based on French models recently adapted to the simulation of ancient climates (IPSL-CM5A2 earth system model and ORCHIDEE continental surface model), this study has performed a large panel of simulations testing different orbital configurations (changes in precession, obliquity and eccentricity parameters). These simulations allow to improve the correspondence to the botanical data available for the period and to map the regions of the globe presenting an important sensitivity of the vegetation to the orbital modifications for the two periods considered.

These results show that the vegetation of the tropics, for a constant CO2 level, could have oscillated between tropical rainforest and open shrub forest conditions, or even desert. These local but major environmental changes are linked to the impact of precession, and to a lesser extent obliquity, on intertropical temperature gradients, allowing the intermittent establishment of a monsoon-like climate.

The international team also demonstrates that the combined impact of CO2 decrease and obliquity variations induces a fragmentation of bioclimatic corridors in Anatolia and Siberia. The biogeographic implications of these results are important, as these migratory corridors linking Europe to Asia were crucial in the migration of Asian fauna to Western Europe during the faunal dispersal event called the Grande Coupure.

This article was originally published on the Department of Ecology, Environment and Plant Sciences