Snowball Earth

Evidence is fairly strong that, at various times from 600 million years ago to one billion years ago, the Earth underwent several glaciations that enveloped the entire planet.  Think about it the next time it snows.  Ice and snow are reflective, casting much more light back into space than seawater or forest.  One of the reasons the Arctic is melting so quickly today is because the feedback loop goes in the opposite direction now...more dry land begets higher temperatures, melting more ice.  I observe the same phenomenon in miniature every warm summer day in Chicago...it only gets "nice" here during the winter when the snow cover is gone.  Chicago winters arc ice ages in miniature, including the threat that, someday, spring, or an interglacial, will never come again.  It is not increasing snowfall that promotes glaciation, it is those patches of snow that never melt in spring.  Each is potentially the nucleus of a glacier.

     It turns out that, when the ice sheets combine with sea ice to cover latitudes lower 

than 30 degrees, nothing can stop them.  They rush to the poles and lock the world in ice.  The Marinoan glaciation might have been such an event, lasting for a million years or longer.  The Sturian was probably a series of lesser events, all severe enough to lock the ancient supercontinent of Rodinia in ice, maybe, we do not know for sure about any of this.  There are, however, eerie dropstones from sea ice carrying wedges of rock, broken off terrestrial cliffs and then moved to sea by glaciers, at latitudes very close to the equator during this time.

    From an evolutionary point of view, the mass extinctions inevitably brought by this make a certain amount of sense....there is a strange lack of transitional biota between the old, hydrothermal vent and anoxia remnants of the archaean, and this brave new world of multicellular life that came about in the wake of Icehouse Earth.  In fact, repeated glaciations, and crazy-hot interglacials that followed, as CO2 from volcanos brought about an end to the icehouse, may have reorganized the biosphere to favor increasingly complex protists, multicellular life, and us.  

  This round of glaciers is every bit as extreme, or more so, than the ones of the Permian, Devonian, Cambrian.  A million years from now, an icebox episode could happen again.

 

Snowball Period
(millions of years ago)
A recent estimate of the timing and duration of Proterozoic glacial periods. Note that great uncertainty surrounds the dating of pre-Gaskiers glaciations. The status of the Kaigas is not clear; its dating is very insecure and many workers do not recognise it as a glaciation. From Smith (in press).[66]