Climatologists know that Earth has naturally cycled through several ice ages in the past. Interestingly, there is a theory that at least on four different occasions, Earth has been completely held within the grip of an ice age—referred to as "snowball Earth"—only to come out of that and switch to a tropical state—referred to as "hothouse Earth." Snowball Earth theory was first proposed by Brian Harland, a Cambridge geologist in 1964, when he discovered glacial deposits near the equator. Later it was hypothesized by Paul Hoffman and Dan Schrag, professors in the Department of Earth and Planetary Sciences at Harvard University, when they proposed that nearly 700 million years ago Earth's climate cooled and the polar ice caps expanded. As the ice advanced, more sunlight was reflected back into space, cooling off Earth even more. The process gained momentum until Earth was completely covered in ice 0.6 mile (1 km) thick. Hoffman and Schrag have supported their hypothesis by collecting supporting evidence from sites all around the world. For example, in a lecture Hoffman delivered at the University of Texas at Austin in April 2006, he cited the following field evidence they had obtained in support of their snowball Earth hypothesis:
• carbon isotopic data that shows significant changes in the carbon content of the sea water bracketing the snowball/glaciations periods
• paleomagnetic data that depicts globally distributed glacial deposits in the mid and low latitudes occurring at the same time snowball Earth existed
(opposite page) Snowball/hothouse Earth alternations. Global temperatures fall and ice packs form, reflecting solar energy back into space. The atmosphere cools, and global temperatures plummet. The Earth becomes entombed in ice. With no rainfall, volcanic carbon dioxide accumulates in the atmosphere, and the planet begins to warm and melt the sea ice. An intense greenhouse effect begins, melting the ice, and forcing the Earth into a hothouse condition.
Snowball Earth at Its Coldest
Snowball Earth Prologue
Snowball Earth as It Thaws
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• tidal bundles that indicate a shallow marine depositional environment and the formation of an iceline feature close to the equator
• banded iron formations, which are found only in glacial marine strata along with the existence of ice-covered continents
• deposition of postglacial "cap carbonates," which are sedimentary structures found on coastal margins, formations that exist when a "hothouse" environment occurs after a snowball Earth episode
According to scientists at the National Oceanic and Atmospheric Administration's (NOAA) Paleoclimatology Research, hothouse Earth followed snowball Earth causing a great swing in climate.
Approximately 600 million years ago, the first snowball Earth gripped the planet, burying the Tropics in ice. Even the bottoms of the oceans froze, despite the heat in Earth's molten core, because temperatures were extremely cold—down to -58°F (-50°C). All life on the planet was killed except for the most simplistic organisms. The only disturbances on the vast, dead landscape were the volcanoes spewing carbon dioxide, the gas that would slowly accumulate and melt the ice through a global warming process. Surprisingly, it took only a few centuries to put the Earth into a brutal hothouse environment, with temperatures climbing to 122°F (50°C). Many scientists believe this cycle occurred four times between 580 and 750 million years ago. The figure on page 17 illustrates how this process may have occurred.
Hoffman and Schrag have found evidence worldwide to support the existence of these occurrences, such as glacial deposits in tropical areas, mixed with iron-rich rocks that could only have formed in an environment with little or no atmosphere, topped with deposits of carbonate rock formed in warm water. The carbon levels in the rock also indicate that a scarce level of life existed through these episodes.
These ice ages were perpetuated by their high albedo to such a point that it became a runaway response. When ice formed at the tropical latitudes, Earth's albedo rose at a faster rate because the sunlight was striking a larger surface area head on, equaling a huge amount of lost energy. Once it got to this point, surface temperatures dropped rapidly, allowing the planet to freeze over.
One theory is that these episodes occurred because at the time the Sun was about 6 percent weaker than it is today. Other theories have suggested the geographic distributions of the continents played a factor. Other scientists strongly oppose the existence of snowball and hothouse Earth, arguing that if ice and snow completely covered the planet, it would have been impossible for the ice to melt and what these snowball Earth episodes warn scientists of today is that Earth is capable of experiencing abrupt climate change and the current activity of humans on the environment with global warming needs to be taken seriously.
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