Palaeoclimate: Snowball versus slushball


The reaction of the carbon cycle during rapidly cooling conditions could have prevented the Earth from completely freezing over during a critical period of its history. An increased rate of remineralization of a massive pool of dissolved organic carbon in the ocean kept atmospheric carbon dioxide levels in the past just high enough to prevent a ‘snowball Earth’; allowing photosynthesis to continue.

The snowball Earth hypothesis proposes that the Earth was fully covered with ice during a series of glaciations around 700 million years ago. Evidence from the rock record and from oxygen isotopes suggests that glaciers could have reached the Equator, with profound effects on photosynthesis and the subsequent evolution of life itself. Other theories suggest that there may have been open water in equatorial regions, but the issue remains controversial.

W. Richard Peltier and colleagues present a coupled model of the carbon cycle and climate system during the Neoproterozoic, which shows that the carbon cycle could have acted as a buffer to prevent complete snowball conditions. They find that more oxygen is taken up by a cooling ocean, which converts organic into inorganic carbon more efficiently. This creates a negative feedback loop that stabilizes low carbon dioxide levels in the atmosphere and prevents further cooling.

Many uncertainties remain and the model needs to be developed and tested further. The authors suggest that it may have been the carbon cycle and not the physical climate system that was operating in an extreme mode just before the Cambrian explosion of life.

CONTACT
W. Richard Peltier (University of Toronto, Ontario, Canada)
Tel: +1 416 978 2938; E-mail: peltier@atmosp.physics.utoronto.ca

Alan J. Kaufman (University of Maryland, College Park, MD, USA) N&V Author
Tel: +1 301 405 0395; E-mail: kaufman@geol.umd.edu