Atmospheric CO 2 concentrations in the latest Carboniferous period ( ∼300–310 Ma) have recently been empirically estimated ( 17) to fluctuate wildly between ∼150 and 700 ppm in response to variations of Earth’s orbital parameters. Indeed, the major part of Earth’s coal deposits was formed in this time period ( 15, 16). In addition to tectonic factors, the high burial rate of organic carbon ( 13) was the major process for the observed drawdown of atmospheric CO 2 during this late Palaeozoic ice age ( 14). Proxy records ( 12) reveal a minimum in atmospheric CO 2 ∼300 Ma close to the boundary between the Carboniferous (359–299 Ma) and Permian (299–252 Ma) periods. The only other epoch with similarly low values of atmospheric CO 2 since these global glaciations occurred in the Palaeozoic era (541– 252 Ma)-except for the most recent past, for which the steadily increasing solar luminosity ( 11) makes global glaciation ever more difficult. These findings highlight the importance of orbital cycles for the climate and carbon cycle during the late Paleozoic ice age and the climatic significance of the fossil carbon stored in Earth’s coal deposits. Global glaciation occurs at CO 2 concentrations <40 ppm, suggesting a rather narrow escape from a fully glaciated Snowball Earth state given the low levels and large fluctuations of atmospheric CO 2. The coldest orbital configurations are characterized by large axial tilt and small eccentricities of Earth’s elliptical orbit, whereas the warmest configuration occurs at minimum tilt, maximum eccentricity, and a perihelion passage during Northern hemisphere spring. Here, I explore the sensitivity of the climate around the Carboniferous/Permian boundary to changes in Earth’s orbital parameters and in atmospheric CO 2 using a coupled climate model. A recent analysis of a high-resolution record reveals large orbitally driven variations in atmospheric CO 2 concentration between ∼150 and 700 ppm for the latest Carboniferous and very low values of 100 ± 80 ppm for the earliest Permian. The high burial rate of organic carbon correlates with a significant drawdown of atmospheric carbon dioxide (CO 2) at that time. The bulk of Earth’s coal deposits used as fossil fuel today was formed from plant debris during the late Carboniferous and early Permian periods.
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