Model-dependence of the CO₂ threshold for melting the hard Snowball Earth

• Main Authors: W. R. Peltier, F. Ding, J. Yang, Y. Hu
• Format: Article
• Language: English
• Published: Copernicus Publications 2011-01-01
• Series: Climate of the Past
• Online Access: http://www.clim-past.net/7/17/2011/cp-7-17-2011.pdf

One of the critical issues of the Snowball Earth hypothesis is the CO₂ threshold for triggering the deglaciation. Using Community Atmospheric Model version 3.0 (CAM3), we study the problem for the CO₂ threshold. Our simulations show large differences from previous results (e.g. Pierrehumbert, 2004, 2005; Le Hir et al., 2007). At 0.2 bars of CO₂, the January maximum near-surface temperature is about 268 K, about 13 K higher than that in Pierrehumbert (2004, 2005), but lower than the value of 270 K for 0.1 bar of CO₂ in Le Hir et al. (2007). It is found that the difference of simulation results is mainly due to model sensitivity of greenhouse effect and longwave cloud forcing to increasing CO₂. At 0.2 bars of CO₂, CAM3 yields 117 Wm⁻² of clear-sky greenhouse effect and 32 Wm⁻² of longwave cloud forcing, versus only about 77 Wm⁻² and 10.5 Wm⁻² in Pierrehumbert (2004, 2005), respectively. CAM3 has comparable clear-sky greenhouse effect to that in Le Hir et al. (2007), but lower longwave cloud forcing. CAM3 also produces much stronger Hadley cells than that in Pierrehumbert (2005). <br><br> Effects of pressure broadening and collision-induced absorption are also studied using a radiative-convective model and CAM3. Both effects substantially increase surface temperature and thus lower the CO₂ threshold. The radiative-convective model yields a CO₂ threshold of about 0.21 bars with surface albedo of 0.663. Without considering the effects of pressure broadening and collision-induced absorption, CAM3 yields an approximate CO₂ threshold of about 1.0 bar for surface albedo of about 0.6. However, the threshold is lowered to 0.38 bars as both effects are considered.