Progressive climate change: Mediterranean forests threatened with desertification

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Pollen grains from Tenaghi Philippon, about 400,000 years old, under the microsc
Pollen grains from Tenaghi Philippon, about 400,000 years old, under the microscope. Due to their good preservability, they remain preserved in cores and thus make it possible to reconstruct vegetation and climate changes in the geological past. © Ulrich Kotthoff

Heidelberg geoscientists study natural climate fluctuations of the past 500,000 years

With the aim of predicting the consequences of man-made climate change for Mediterranean ecosystems, geoscientists at Heidelberg University have studied natural climate and vegetation fluctuations over the past 500,000 years. The focus was on how these fluctuations have affected forests in the Mediterranean region. To this end, the researchers led by Dr. Andreas Koutsodendris analyzed fossil pollen preserved in a sediment core from Greece. Their investigations indicate that if drought persists - as predicted by current climate modeling - desertification of Mediterranean forests can be expected in the near future.

Mediterranean forests are not only biodiversity hotspots, but also provide important ecosystem services. For example, they protect the soil from erosion, regulate regional climatic and hydrological conditions, and act as sources of food and timber. "Because they are very sensitive to climatic changes, there is growing concern about their continued existence in the face of human-induced CO2 emissions and the associated global warming," explains Koutsodendris. He is a member of Jörg Pross’ research group, which conducts research on the Earth’s environmental and ecosystem dynamics at the Institute of Geosciences at Heidelberg University.

In order to understand how Mediterranean forests reacted to climatic changes in the past, the Heidelberg scientists, in collaboration with colleagues from Germany, France, Greece and Great Britain, took drill cores in Tenaghi Philippon - a terrestrial climate archive in northeastern Greece - that cover the period of the past 500,000 years without gaps and in which fossil pollen grains have been preserved. Information on vegetation development during this period obtained from the pollen grains was correlated with geochemical data on contemporaneous precipitation variations. The results of Dr. Koutsodendris’ team show that in the past, Mediterranean forest landscapes turned into steppes within a few decades as soon as precipitation fell below certain thresholds.

With the help of ecological models, the scientists also investigated the question of what factors might have led to the change in precipitation patterns. Their analyses show that changes in atmospheric CO2 content affect the amount of precipitation in the Mediterranean region. "In the past, under natural conditions, a 40 to 45 percent decrease in rainfall was enough to herald the sudden transition from a forest to a steppe landscape," Koutsodendris explains. These results suggest that the Mediterranean forests could face such a transformation in the near future, according to the Heidelberg geoscientist, if no measures are taken to protect them.

The German Research Foundation (DFG), the State of Hesse as part of the State Offensive for the Development of Scientific and Economic Excellence, and the Wilhelm Schuler Foundation funded the research work. The research results have been published in the journal "Nature Communications".

A. Koutsodendris, V. Dakos, W. J. Fletcher, M. Knipping, U. Kotthoff, A. M. Milner, U. C. Müller, S. Kaboth-Bahr, O. A. Kern, L. Kolb, P. Vakhrameeva, S. Wulf, K. Christanis, G. Schmiedl, J. Pross: Atmospheric CO2 forcing on Mediterranean biomes during the past 500 kyrs. Nature Communications 14, 1664 (25 March 2023).