KEYNOTE ADDRESS: AN UNFORTUNATE SERIES OF GLOBAL ATMOSPHERIC GREENHOUSE SPIKES

The Mesozoic era has long been thought to have been warm and wet, with dinosaurs, cycadeoids and tropical ferns living at high paleolatitudes. There is little evidence for ice caps in the Mesozoic, with the exception of Early Cretaceous dropped pebbles and opalized glendonites in the subantarctic seas of South Australia. The stomatal index (percent stomatal versus epidermal cells) of fossil Ginkgo leaves is generally low in the Mesozoic, confirming that this was a time of elevated atmospheric CO₂. The amount of CO₂ can be calculated using a transfer function derived from studies of herbarium specimens that lived through the industrial rise of CO₂ and from greenhouse experiments with living Ginkgo biloba. This Gingko-paleobarometer indicates that the general level of atmospheric CO₂ for the past 300 million years was only 400-1000 ppm, and that Neogene and early Permian glacial climates had near modern levels (280 ppmV). The Gingko-paleobarometer also reveals profound perturbations when atmospheric CO₂ levels rose well above 2000 ppmV for periods of only a few hundred thousand years. Most of these hot flashes coincide with mass extinctions in the fossil record: middle Miocene, latest Eocene, latest Paleocene, latest Cretaceous, mid-Cretaceous (Cenomanian-Turonian), early Cretaceous (Aptian), latest Jurassic, middle Jurassic (Bajocian), early Jurassic (Toarcian), latest Triassic, late Triassic (Carnian), early Triassic (Spathian), latest Permian, late Permian (Guadalupian), and middle Permian (Kungurian). These 13 hot flashes in the past 300 million years are also apparent from the paleoclimatic record of fossil soils, because each coincides with chemically desalinized soils indicating warm temperatures, and chemically base-depleted soils with deep and thick zones of carbonate nodules, indicating more humid and more seasonal climates than usual in aridlands. In the paleosol successions of Montana, Utah, and Texas, these paleoclimatic hot flashes are represented by thick, red, clayey paleosols that form regional marker beds within their successions of thin, calcareous, brown paleosols. The Mesozoic greenhouse was thus not torrid with cool spells but warm with hot flashes. These global warming spikes are an archive of past global disasters, and a warning for our future.