CENOZOIC VEGETATION, CO2, AND PALEOCLIMATE: IMPLICATIONS FOR FUTURE TERRESTRIAL HABITATS

Comparison of paleobotanical records of Cenozoic vegetation with proxy-based reconstructions of Cenozoic CO₂ reveals long-term terrestrial ecosystem changes related to greenhouse gases and other climatic forcing factors. The last 65 million years include intervals characterized by elevated CO₂ concentrations (1000 – 1500 ppm) and other periods with preindustrial levels (280 ppm). Recent research documenting Cenozoic vegetational patterns during intervals of elevated and near-modern CO₂ concentrations is reviewed, as are the direct and indirect impacts of changing greenhouse gas concentrations on terrestrial ecosystems. During the late Paleocene and Eocene, a time of elevated CO₂ concentrations (≥1000 ppm), humidity was greater, and temperate to subtropical forests occupied polar latitudes. The Oligocene was characterized by progressive decreases in CO₂ concentrations, reaching near modern levels (~300 ppm) at the Oligocene-Miocene boundary. Vegetation structure changed during this time, as grasslands expanded globally. During the Miocene - middle Pliocene, when CO₂ values were ~300-400 ppm, latitudinal boundaries between biomes shifted pole-ward, and boreal forest extended to nearly 80˚ N latitude. These boundaries abruptly shifted equator-ward ~2.7 Ma, when CO₂ concentrations decreased to pre-industrial levels, northern hemisphere glaciation intensified, and mean annual temperatures decreased. Uncertainties remain about the relative timing of changes in vegetation, temperature, CO₂ and other drivers that structured terrestrial habitats of the Cenozoic. High-resolution, multiproxy analysis of additional sites is needed to reduce uncertainty on the relative roles of these forcing factors and to improve our capability to model terrestrial ecosystem response under a range of scenarios.