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The snowball Earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the Neoproterozoic era, in a self-enhanced glaciation caused by the high albedo of the ice-covered planet. In a hard-snowball picture, the subsequent rapid unfreezing resulted from an ultra-greenhouse event attributed to the buildup of volcanic carbon dioxide (CO2) during glaciation. High partial pressures of atmospheric CO2 (; from 20,000 to 90,000?p.p.m.v.) in the aftermath of the Marinoan glaciation (?635?Myr ago) have been inferred from both boron and triple oxygen isotopes. These values are 50 to 225 times higher than present-day levels. Here, we re-evaluate these estimates using paired carbon isotopic data for carbonate layers that cap Neoproterozoic glacial deposits and are considered to record post-glacial sea level rise. The new data reported here for Brazilian cap carbonates, together with previous ones for time-equivalent units, provide estimates lower than 3,200?p.p.m.v.-and possibly as low as the current value of ?400?p.p.m.v. Our new constraint, and our re-interpretation of the boron and triple oxygen isotope data, provide a completely different picture of the late Neoproterozoic environment, with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard-snowball Earth. [ABSTRACT FROM AUTHOR]