Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 23

all available land in the tropics could reflect more of the incoming sunlight, and
help the planet to cool.'' Walker provided an additional argument. The concen-
tration of CO 2 has an important effect on global climate. This concentration is
reached as an equilibrium between sources (volcanoes) and sinks (burial of
carbonates in rocks). When there are continents at fairly high latitudes, they act in
opposition to the spread of ice toward the equator because as the ice spreads
southward, it covers land and removes that land from the realm that acquires
CO 2 into carbonate rocks. As a result, the CO 2 concentration in the atmosphere
builds up and the ensuing warming counteracts the further spreading of ice. When
the continents are grouped around the Equator, this effect will not oppose south-
ward spreading of ice as the Earth cools. Whether this qualitative argument can be
backed up by convincing quantitative analysis remains unclear. Budyko's analysis
(as well as several other more recent studies) suggests that a snowball Earth or at
least a slushball Earth may be theoretically possible.
A good deal of Walker's topic is concerned with the efforts of several
geologists to find evidence that a snowball Earth actually occurred. She points out
how rocks transported by ice are distinguished, either by uniform scratches or by
their anomalous placement. When the theory of continental drift was established,
some argued that these rocks might have been deposited when their continents had
been in polar regions, thus contradicting the snowball Earth hypothesis. However,
when rocks are formed, they often carry a magnetic signature that is horizontal
near the Equator and vertical at high latitudes, following the field lines of the
Earth's magnetic field. The evidence from many ice-deposited rocks indicated that
they were equatorial in origin.
Next, the question arises that if the snowball Earth formed, how did it
disappear? The answer provided by Kirschvink (1992) is that during a global
glaciation, shifting tectonic plates would continue to build volcanoes and supply
the atmosphere with carbon dioxide that would leak out into the atmosphere
through fissures in the ice. If the Earth were completely frozen over, the processes
that remove carbon dioxide from the atmosphere via acid rain forming carbonate
rocks would essentially cease, allowing carbon dioxide to build up in the atmo-
sphere to extremely high levels. Eventually, this would produce so much heating
that it would reverse a snowball Earth. Once melting begins, the ice-albedo
feedback would be reversed and this, combined with the extreme greenhouse
atmosphere, would drive surface temperatures rapidly upward. The warming
would proceed rapidly because the change in albedo begins in the tropics, where
solar irradiance and surface area are maximal. With the resumption of evapora-
tion, the addition of water vapor to the atmosphere would add dramatically to the
greenhouse effect. CO 2 would remain in the atmosphere for many thousands of
years. Calculations cited by Hoffman and Schrag (2002) suggest that tropical sea
surface temperatures would reach almost 50 C in the aftermath of a snowball
Earth, driving an intense hydrologic cycle. Sea ice hundreds of meters thick
globally would disappear within a few hundred years.
Another intriguing aspect of a snowball Earth is the question of how life
survived through this period. Another related question is whether the rapid trans-