Equable end Mesoproterozoic climate in the absence of high CO2

Abstract

The Proterozoic Eon (2500–542 Ma) appears to have been a warm period bookended by glaciations, despite a 5%–18% reduction in solar output compared to modern during this interval. Radiative-convective climate models suggest that glaciation could have been avoided if p CO 2 were 30–300× preindustrial atmospheric levels (PIAL, 280 ppmv). Constraints from late Mesoproterozoic (ca. 1.2–1.0 Ga) microfossil calcification sheaths and paleosol mass balance, however, suggest that p CO 2 may have been no higher than 10× PIAL. In the lower oxygen Mesoproterozoic atmosphere, an increased CH 4 flux from methanogenic bacteria may have contributed additional greenhouse warming. We use a fully coupled atmosphere-ocean general circulation model (the U.S. National Center for Atmospheric Research Community Earth System Model, CESM) to test whether these p CO 2 constraints are consistent with the absence of widespread glaciation inferred from the geologic record. We vary p CO 2 and p CH 4 between 1400 and 2800 ppmv and 3.5 and 140 ppmv, respectively, using a reconstructed 1.0 Ga paleogeography and solar output reduced by 9%. Our simulations suggest that ice-free conditions can be maintained at 10× PIAL CO 2 when CH 4 is 140 ppmv. When CH 4 is lowered to 28 ppmv at 10× PIAL CO 2 , or if p CO 2 is lowered to 5× PIAL, permanent land snow cover at high and middle latitudes suggests that glaciation would be more extensive than preindustrial conditions, but with warm tropical regions. Global glaciation occurs if p CO 2 is reduced below 5× PIAL. Overall, our simulations suggest that an ice-free climate for the Mesoproterozoic (1.6–1.0 Ga) is consistent with the relatively low p CO 2 implied from proxies if CH 4 or other greenhouse gas concentrations were sufficiently elevated.