Dr. Warren will discuss the Snowball Earth events in Earth history, when the ocean apparently froze all the way to the Equator. These events were qualitatively different from the recent ice ages of the Pleistocene, in which ice sheets advanced over large parts of the northern continents. On Snowball Earth, by contrast, Warren will argue that most of the action was in the ocean.
After a transient period with sea ice, the dominant ice type on the tropical ocean would be "sea glaciers", similar to modern ice shelves but not dependent on continental glaciation. As they flowed equatorward into the tropical region of net sublimation, their surface snow and subsurface firn would sublimate away, exposing bare glacier ice to the atmosphere and to solar radiation. This ice would be freshwater (meteoric) ice, which originated from snow and firn, so it would contain numerous air bubbles, which determine the albedo.
The modern surrogate for this type of ice (glacier ice exposed by sublimation, which has never experienced melting), are the bare-ice surfaces of the Antarctic Ice Sheet near the Trans-Antarctic Mountains. These areas have been well mapped because of their importance in the search for meteorites.
Dr. Warren will discuss recent fieldwork in Antarctica during which he carried out a transect across an icefield to sample ice of different ages that has traveled to different depths en route to the sublimation front, near the Allan Hills.
Dr Warren will show how this short transect could represent a north-south transect across many degrees of latitude on the snowball ocean. Surfaces on the transect transitioned through the sequence: new snow - old snow - firn - young white ice - old blue ice. Albedo parameterizations were developed as a function of density for three broad wavelength bands commonly used in GCMs: visible, near-infrared, and total solar.
On the equatorial ocean of Snowball Earth, climate models predict thick ice, or thin ice, or open water, depending largely on their albedo parameterizations; the measured albedos appear to be within the range that favors ice hundreds of meters thick.