スノーボールアース
A "snowball earth" is the complete opposite of greenhouse Earth, in which the earth's surface is completely frozen over; however, a snowball earth technically does not have continental ice sheets like during the icehouse state. "The Great Infra-Cambrian Ice Age" has been claimed to be the host of such a world, and in 1964, the scientist W. Brian Harland brought forth his discovery of indications of glaciers in low latitudes (Harland and Rudwick). This became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect) that, once the earth enters the route of becoming a snowball earth, it would never be able to leave that state. However, in 1992 brought up a solution to the paradox. Since the continents at this time were huddled at the low and mid-latitudes, there was less ocean water available to absorb the higher amount solar energy hitting the tropics, and at the same time, increased rainfall due to more land mass exposed to higher solar energy might have caused chemical weathering (removing CO from atmosphere). Both these conditions might have caused a substantial drop in CO atmospheric levels resulting in cooling temperatures, increasing ice albedo (ice reflectivity of incoming solar radiation), further increasing global cooling (a positive feedback). This might have been the mechanism of entering Snowball Earth state. Kirschvink explained that the way to get out of Snowball Earth state could be connected again to carbon dioxide. A possible explanation is that during Snowball Earth, volcanic activity would not halt, accumulating atmospheric CO. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO from the atmosphere. CO was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO would reach a threshold, temperature would rise enough for ice sheets to start melting. This would in turn reduce ice albedo effect which would in turn further reduce ice cover, exiting Snowball Earth state. At the end of Snowball Earth, before reinstating the equilibrium "thermostat" between volcanic activity and the by then slowly resuming chemical weathering, CO in the atmosphere had accumulated enough to cause temperatures to peak to as much as 60° Celsius, before eventually settling down. Around the same geologic period of Snowball Earth (debated if caused by Snowball Earth or being the cause of Snowball Earth) the Great Oxygenation Event (GOE) was occurring. The event known as the Cambrian Explosion followed, which produced the beginnings of populous bi-lateral organisms and a greater diversity and mobility in all multicellular life. However some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived underneath many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica. Most scientists today believe that a "hard" Snowball Earth, one completely covered by ice, is probably impossible. However, a "slushball earth", with points of opening near the equator, is possible.