Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 22

ing global albedo further, (c) the water vapor concentration will be high due to the
fact that the vapor pressure of water increases with temperature, enhancing the
greenhouse effect (although the effect of clouds is dicult to evaluate), and
(d) globally warm conditions are likely to increase emissions of greenhouse gases
CO 2 and CH 4 from deposits on land and sea (as evidenced by past variations in
greenhouse gas concentrations across ice age-interglacial transitions).
Therefore, if the Earth can enter a hothouse Earth state where the polar ice is
all melted and greenhouse gas concentrations are high, it could conceivably remain
that way stably for some time. Only a drop in greenhouse gas concentrations
would restore polar ice. Geological evidence suggests that during the Cambrian
period (about 570 to 510 million years ago) the Earth was a hothouse with essen-
tially no polar or high-altitude glaciers. It is conjectured that there were much
higher concentrations of greenhouse gases in the atmosphere compared with the
present day. During this period, most of the continents as we know them today
were either underwater or part of the so-called Gondwana ''supercontinent.'' The
oceans were all interconnected, bringing warm water to polar areas. About 85%
or more of the Earth's surface was covered with water (compared with approxi-
mately 70% at present) and there was a lack of significant topographic relief.
Chemical weathering was at a minimum because the landmasses were minimal
and they were all conjoined. Toward the end of the Cambrian period, this super-
continent began to break up—dispersing the landmasses—and the greenhouse gas
concentration dropped markedly, leading to a cooling trend.
The theory of snowball Earth first originated when Harland and Ruddick
(1964) observed that glacial deposits from 600 mybp were widely distributed on
almost every continent. Magnetic orientation of mineral grains in these glacial
deposits indicated that the continents were clustered together near the equator in
this era. They therefore suggested that there might have been a great global ice age
at the time.
Around the same time that Harland publicized his theory, Budyko (1969)
developed a mathematical energy-climate model that explained how tropical
glaciers could form. Budyko estimated that if Earth's climate were to cool, and ice
were to form at lower latitudes, the planetary albedo would rise at a faster rate
because there is more surface area per degree of latitude as one approaches the
Equator (Hoffman and Schrag, 2002). It was found that at the critical latitude of
30 north or south, the positive feedback became overwhelming, and once having
passed through 30 the freeze becomes irreversible, yielding an entirely frozen
Earth.
Walker (2003) published a book that provides an excellent summary of the
scientific rationale for snowball Earth, if one can get past the interminable descrip-
tions of the personalities of the players and the travelog descriptions of geological
localities that are interspersed with the science. First, why might a snowball Earth
occur at all? Walker (2003) pointed out that magnetic traces in rocks suggest that
at the time period suggested for snowball Earth, the continents drifted into a band
near the Equator. As Walker emphasized: ''tropics soak up most of the heat that
arrives on Earth from the Sun. Because land is more reflective than ocean, putting