LAUSR

Abstract The accuracy of CO2 hindcasting using fossil Ginkgo stomatal index is ripe for revision for three reasons: exponential rise in atmospheric CO2 over the past decade, discovery of a Kew herbarium specimen of Ginkgo picked in 1754, and increased sophistication of a pedogenic CO2 paleobarometer as an independent parallel record. Past mass extinctions coincide with revised CO2 spikes of 1500 ppm or more. Increases such as the middle Miocene level of 640 ± 71 ppm expected before the year 2100 resulted in biome shifts, with expansion of tropical forests northward, and of grasslands into deserts. Deep time records from paleosols and from stomatal index reveal that CO2 levels less than 180 ppm and more than 1500 ppm are toxic to the biosphere.