Paleoclimate: ?Snowball Earth?
|What would the hydrological cycle of the Earth be like if the seas were frozen from equator to pole ("Snowball Earth")? This diagram shows the exchange of water between atmosphere, ocean, and ice according to a simplified climate model. Sea ice in our model is hundreds of meters thick everywhere, and flows glacially under its own weight from pole to equator.|
Raymond T. Pierrehumbert (University of Chicago)
Geological evidence suggests that 700 million years ago, the Earth might have been globally glaciated, with frozen oceans from pole to equator (Hoffman et al, 1998). However, the geological evidence is disputed (Christie-Blick et al, 1998), and numerical climate simulations (Poulsen et al, 2001; Peltier, 2001) do not definitively tell us whether such a state was likely. So, was the Earth totally ice-covered or not? I have approached the problem by identifying unusual physical processes that ought to occur in a ?snowball? climate, and attempting to test their similarity to the geological observations.
In particular, in a sufficiently cold climate, floating sea ice will become hundreds of meters thick, allowing it to flow glacially under its own weight. These ?sea glaciers? (Goodman and Pierrehumbert, 2003) are unheard-of in more temperate climates, and may present a unique fingerprint of true global glaciation. I have performed modeling studies of the global hydrological cycle and flow of these sea glaciers, incorporating elements of ice rheology, thermal conduction, and optical properties of the ice to determine its thickness, flow rate, and the degree of isolation of the ocean from atmosphere (Goodman, 2006). This modeling effort demonstrated that a previously-hypothesized zone of stable thin ice near the equator (Pollard and Kasting, 2005) cannot exist: the ice is everywhere hundreds of meters thick. This is important because such a thick ice layer would cut off sunlight from the liquid ocean, making the survival of photosynthetic organisms problematic. The fact that they patently did survive presents a stumbling block for proponents of the Snowball Earth hypothesis.
Bodiselitsch et al (2005) have attempted to explain an enigmatic spike in platinum-group elements occurring in seafloor sediments at the end of the Snowball era by noting that platinum-element-rich interplanetary dust will continuously settle onto the ice-covered ocean during a multi-million-year-long Snowball episode. Thus when this ice melts at the glacial termination, a large slug of platinum elements will be rapidly deposited in seafloor sediments. My model considers in detail the hydrological cycle of an ice-covered planet, and finds that despite the motion of the ice, basal melting and freezing, and surface snowfall and sublimation, the oceanic and atmospheric hydrological cycles remain totally isolated from one another. The ocean exchanges mass with the base of the ice sheet, and the atmosphere with the surface ice, but snowfall-derived ice (and the dust it contains) never melts into the ocean. Thus, my model supports the viability of Koeberl et al?s hypothesis.
Another outstanding question in the Snowball Earth debate is, ?if the planet froze over completely, how did we escape the deep-freeze?? A cold planet covered with high-albedo snow and ice is a very stable climate state. Existing explanations for an escape from Snowball Earth, involving geological carbon cycle feedbacks, have been called into question (Christie-Blick et al, 1999). One possibility that has not been investigated is the effect of a giant meteorite impact, which might disperse dust and water vapor in ways that could conceivable promote deglaciation. Such an impact could also explain the spike in platinum elements noted above, and might not be a rare event on the long timescales under consideration. I am beginning to collaborate with Christian Koeberl, an expert in ancient impacts, to attempt to combine impact modeling with atmospheric climate models to study this possibility.