Atmospheric CO2 exerts a robust and well-documented control on Earth’s climate, but the timing of glaciation during the late Paleozoic Ice Age (LPIA; ca. 360–260 Ma) is inconsistent with pCO2 reconstructions, hinting at another factor. Stratospheric volcanic aerosols produce a large but temporary negative radiative forcing under modern conditions. Here we examine explosive volcanism over 200 m.y. of Earth history to show that the LPIA corresponded with a sustained increase in volcanism in both tropical and extratropical latitudes. A major peak in explosive volcanism at ca. 300 Ma likely corresponded to stratospheric sulfur-injecting eruptions at least three to eight times more frequent than at present. This level of volcanism created a steady, negative radiative forcing potentially sufficient to initiate and, most critically, sustain icehouse conditions, even under increasing levels of pCO2, and helps resolve discrepancies between glacial timing and CO2 records. Accounting for the radiative forcing effects of CO2 and sulfate indicates that both are required to explain the LPIA, with sulfate producing an especially strong effect at peak icehouse ca. 298–295 Ma. Frequent explosive volcanism would have increased atmospheric acidity, enhancing the reactivity of iron in abundant volcanic ash and glacially generated mineral dust, thus strengthening the climate impact of volcanism through a marine biological pump further primed by feedback with glaciation.

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