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Most models of climate simulations and vegetation indicate that ongoing climate change will have a global and far-reaching impact on terrestrial species and ecosystems for decades to come. Field observations would probably give researchers more confidence in future projections and thus allow more appropriate decisions to be made. An original approach has been implemented based on the observations and expertise of over 40 paleoecology researchers around the globe.Each researcher assessed changes in ecosystem structure and composition during the transition from the last ice age, centered around 20,000 years ago to about 14,000 years ago, in fossil sequences in his study area. These fossil recordings represent a unique and invaluable archive and provide concrete observations of the relationship between global climate change and the composition and structure of ecosystems. The chosen period of time is very interesting in terms of increasing temperatures and changing ecosystems on a global scale even if it can not be considered as analogous to the expected climate changes, in particular because of the concentrations of CO2 that were weaker than currently.Mountain forests in Cameroon (A © A.-M. Lézine) and Morocco (B © R. Cheddadi) where the impact of current climate change is visible on populations already located at altitudinal limit.This expert-based approach examined several hundred published paleoecological records to assess changes in the composition and structure of global terrestrial vegetation. The synthesis of the results published in the journal Science and their comparison with simulations shows that in the case of the RCP2.6 scenario (temperature increase of about 1.5 ° C, in line with that of the Paris Agreement of 2015) the impact would be limited but in the case of the RCP8.5 scenario, terrestrial ecosystems would be strongly threatened by rising temperatures and most ecosystems would risk profound and irreversible changes such that ecosystem services and biodiversity would be irretrievably affected.Researchers from the Institut des Sciences de l’Évolution de Montpellier, the Institut méditerranéen de biodiversité et d’écologie marine et continentale, the  Centre européen de recherche et d'enseignement de géosciences de l'environnement and the Laboratoire d'océanographie et du climat were involved in this study.Sources :CNRS – INEEPast and future global transformation of terrestrial ecosystems under climate change. Connor Nolan et al. Science. 31 Aug 2018. DOI: 10.1126/science.aan5360 