MioVeg1: A Global Middle Miocene Vegetation Reconstruction for Climate Modeling

Climate models require boundary condition information, such as vegetation and soil distributions because they influence the mean state climate, and feedbacks can significantly influence regional climate and climate sensitivity to CO₂ forcing. Information about past distributions comes primarily from the paleobotanical record, which is often supplemented by a vegetation model to fill data gaps. For recent past periods such as the Pliocene, a quantitative suitability assessment of these vegetation model simulations is sufficient. However, the Miocene Climate Optimum spanning 16.9–14.7 Ma was the warmest period on Earth over the last ∼25 million years and models struggle to reproduce those conditions for the range of paleogeographies and CO₂ concentrations tested, particularly at high latitudes. Here we bring together the Miocene modeling and data communities to update previous vegetation reconstructions used for climate modeling with a new regional approach that relaxes the requirement for a single model simulation to be used, blending instead simulations forced by different paleogeographies and CO₂ concentrations. This ensures the simulated vegetation is first, and foremost, consistent with the paleorecord and provides a baseline for future comparisons. The reconstruction shows global increases in forest cover at all latitudes as compared to today and extensive C₃ grasslands across the high northern latitudes. Data gaps at high latitudes are filled with vegetation models forced by higher CO₂ concentrations than were required at lower latitudes consistent with the inability of current models to simulate Miocene high latitude warmth.

Plain Language Summary: The Miocene Climate Optimum was globally the warmest period over the last 25 million years. As a result of this warmth, the vegetation distribution was quite different to today. Climate models need information about that vegetation distribution to simulate the climate of the Miocene but paleobotanical data contains gaps. Vegetation models can be used to fill these gaps but require climate information to run, usually taken from climate models that have difficulty reproducing the warmth of the Miocene seen in the data. To overcome this problem, we use a new more flexible approach to fill the data gaps whereby paleobotanical experts have identified the best models on a regional basis and there is no need for a single model to be used globally. The Miocene vegetation shows more forests than today and more extensive grasslands in the Northern Hemisphere.