Proxies of past CO2 and climate over the past 300 m.y. now reveal multiple global climate change events in unprecedented detail. Evidence for past CO2 spikes comes from expanded and refined stomatal index data of fossil Ginkgo and related leaves. New evidence for synchronous climatic change comes from paleosols in Montana, Utah, and neighboring states. Each CO2 spike was coeval with unusually clayey, red, and decalcified paleosols that can be traced throughout the Colorado Plateau. Spikes in atmospheric CO2 also were coeval with increases in paleosol alkali depletion as an indication of high temperature, and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation. In the Colorado Plateau, times of warmer climate were also more humid, perhaps due to the greater moisture potential of warmer air. Seasonality of climate did not increase during warm-wet spikes. The Mesozoic greenhouse was not persistently hot with cool spells, but warm with hot flashes. These data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature (MAT) and mean annual precipitation (MAP) due to rising CO2 in a mid-latitude, mid-continental region. The magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions.

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