“SNOWBALL EARTH”
THE FIRST ICE AGES
2.2 billion years ago
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| “Snowball Earth.” There is evidence that the Earth may have undergone periods of extreme cold, with ice covering much or even all of the planet multiple times in the ancient past. These periods have been called “Snowball Earth,” or in less extreme models, “Slushball Earth.” Credit: Image by University of Bristol/NASA. |
Imagine a world so cold that all of the surface water of the oceans froze solid, the entire earth covered with ice. The average temperature would have been 74 degrees below zero, with temperatures at the equator as cold as Antarctica is today. This may have happened at least three times in the earth’s history, first at around 2.2 billion years ago, then at 710 million years ago, and again about 640 million years ago. Simple forms of life could have survived in the seas below the ice layer. These super-cold phases may have been caused by a periodic reduction in “greenhouse gases” (carbon dioxide and methane) in the atmosphere (such gases tend to warm the planet, so a reduction of these gases would cool the planet), combined with the fact that the sun was also dimmer in the past. Such extreme super-cold phases may have prevented more advanced forms of life to evolve during the first few billion years of the earth’s history.
There are different models for how the greenhouse gases got depleted enough to trigger such a snowball event. One model focuses on carbon dioxide and a reduction in atmospheric CO2 when plate tectonics happened to place much of the continental mass right in the tropical zone where accelerated weathering of silicate rocks would have ‘gobbled up’ available carbon dioxide. The loss of methane from the atmosphere is also modeled as a powerful trigger to snowball events, as the warming power of methane was lost when it combined with the oxygen that early photosynthesis was pumping into the atmosphere. Evidence has also been suggested that the later “snowball” events happened when the Earth passed through and interstellar cloud of dust in the Milky Way which reflected solar radiation away from the planet.
It is thought that once these “snowball earth” events got underway, they were somewhat self-perpetuating for a time, as the ice on the surface of the Earth would have reflected so much of the solar radiation back into space rather than absorb it into the land and oceans. Thus ice cover that may have begun in more northern latitudes would have been able to spread toward the equator. With the most extreme ice cover, scientists have deduced that the mean temperature of the Earth could have been about -74 degrees F, and as cold as -10 degrees even at the equator. Substantial glaciers would have developed, and tremendous glacial flows would have moved across the surface of the Earth, dragging up debris from the Earth’s crust and depositing it in glacial deposits of sediment, such as glacial moraines and tills. Evidence for the latest of these events is found on virtually every continent, but the signs of the earlier ones are more limited, since subsequent tectonic plate movement as well as weathering and erosion have ‘scrubbed’ large portions of the earth of such evidence. Such icy periods may well have eventually helped bring about their own ends, as the cold, icy climate would have slowed consumption of CO2 in the silicate weathering process, and, meanwhile, volcanic activity would have continued to pump more CO2 into the atmosphere.
HOW DO WE KNOW?
Massive deposits of tillite (gravels representing the outwash of melting glaciers) are known to have been produced around the world at the end of these “Snowball Earth” periods. Furthermore, various glacial deposits and striations are sometimes found in areas that evidently would have been in tropical latitudes during the time they were deposited, indicating profoundly climate over much of the Earth at that time.
WHY SHOULD I CARE?
It is likely that the harsh conditions of these Snowball Earths prevented life from becoming much more complex than single-celled organisms such as bacteria. With the end of the last Snowball Earth around 635 million years ago, these warmer conditions created the environment that ushered in the first known multicellular forms (plants and animals), and one of these animals would ultimately lead to us.
WEB RESOURCES
Very detailed information on the Snowball Earth Theory, evidence and arguments:
https://www.snowballearth.org/
An overview of the Snowball Earth Theory:
https://en.wikipedia.org/wiki/Snowball_Earth
A timeline of the different periods of glaciations in the Earth’s history:
https://en.wikipedia.org/wiki/Timeline_of_glaciation
BOOKS
Macdougall, Doug. 2004. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press.
Walker, Gabrielle. 2003. Snowball Earth: The Story of the Great Global Catastrophe That Spawned
Life As We Know It. New York: Crown Publishers.
BOOKS (on the history of the earth)
Palmer, Douglas. 2003. Prehistoric Past Revealed: The Four Billion Year History of Life on Earth. Berkeley: University of California Press.
Palmer, Douglas. 2009. Evolution: The Story of Life. Berkeley: University of California Press.
VIDEO
How the Earth Was Made. 2007. History Channel.
Miracle Planet. Episode 2: Snowball Earth. 2006. Ambrose Video.
Earth: The Biography. The Story of our World. 2008. BBC Video.
