Understanding Snowball Earth Deglaciation

1. AGU P11B-1826 3 December 2012 Understanding Snowball Earth Deglaciation Dorian S. Abbot; abbot@uchicago.edu; http://geosci.uchicago.edu/~abbot/ Motivation Dust-induced deceased albedo could cause deglaciation Lack of clouds causes FOAM to be 8-12K colder than other GCMs Earth, a normally clement planet comfortably in its star's habitable zone, suffered global or nearly global glaciation at least twice during the Neoproterozoic era (at about 635 and 710 million years ago). The mechanism stabilizing Earth’s climate must have broken during these glaciations, making understanding them critical for planetary habitability more generally. Furthermore, the dramatic effect of the Snowball Earth events on Earth’s development can be seen through their link to rises in atmospheric oxygen and evolutionary innovations. The Snowball Earth hypothesis is only viable if there is a plausible deglaciation mechanism. The traditional deglaciation mechanism is a massive build up of CO2 due to reduced weathering during Snowball Earth events until tropical surface temperatures reach the melting point.In pioneering work, Pierrehumbert [2004, 2005] was the first to test Snowball deglaciation in a global climate model (GCM). He found that FOAM was far too cold to deglaciate even at 20% CO2, which is too high to be consistent with geological data. This raised a potential challenge to the Snowball Earth hypothesis. Tropical and Annual Mean CAM Surface Air Temperature Continent Ice CO2=0.1 CO2=10-4 Temp[°C] Dust Layer FOAM GENESIS LMDz ECHAM CAM SP-CAM Less parameterized cloud scheme FOAM January Surface Temperatures The FOAM cloud condensate scheme is diagnostic and forms very little cloud condensate in cold conditions. This scheme is the source of a cold bias in cold regions in the modern climate. CO2would have to be decreased by a factor of 10-100 in other models to make them as cold as FOAM. When we artificially set cloud radiative forcing to zero in CAM, the surface temperature is similar to that in FOAM. GCMs can produce clouds because of strong Hadley circulation ascent Results from SP-CAM, which is a GCM with an embedded sophisticated cloud-resolving scheme. Contour spacing 50x109 kg s-1 Conclusions Model results suggest that either tropical dust concentration, clouds, or some combination of the two would be sufficient to deglaciate a Snowball Earth at a CO2 of 0.01-0.1 bar, but not at lower CO2, consistent with geological data. This eliminates difficulty deglaciating as a potential objection to the Snowball Earth hypothesis. The concentration of dust in the tropics would lower the albedo LongwaveShortwave TotalCERES Modern Obs. Abbot, D.S. and R.T. Pierrehumbert (2010), Mudball: Surface dust and Snowball Earth deglaciation, Journal of Geophysical Research, 115, D03104. Abbot, D.S., A. Voigt, M. Branson, R.T. Pierrehumbert, D. Pollard, G. Le Hir, D.D.B. Koll (2012), Clouds and Snowball EarthDeglaciation, Geophysical Research Letters, 39, L20711. Solid: CO2=10-4 Dotted: CO2=0.1