Greenhouse and icehouse Earth

Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.

Overview of greenhouse Earth
A "greenhouse Earth" is a period in which there are no continental glaciers whatsoever on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. The Earth has been in a greenhouse state for about 85% of its history.

This state should not be confused with a hypothetical hothouse earth, which is an irreversible tipping point corresponding to the ongoing runaway greenhouse effect on Venus. The IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."

Causes of greenhouse Earth
There are several theories as to how a greenhouse Earth can come about. The geological record shows CO2 and other greenhouse gases are abundant during this time. Tectonic movements were extremely active during the more well-known greenhouse ages (such as 368 million years ago in the Paleozoic Era). Because of continental rifting (continental plates moving away from each other) volcanic activity became more prominent, producing more CO2 and heating up the Earth's atmosphere. Earth is more commonly placed in a greenhouse state throughout the epochs, and the Earth has been in this state for approximately 80% of the past 500 million years, which makes understanding the direct causes somewhat difficult.

Overview of icehouse Earth
An "icehouse Earth" is a period in which the Earth has at least two ice sheets, Arctic and Antarctic (on both poles); these sheets wax and wane throughout shorter times known as glacial periods (with other ice sheets in addition to the 2 polar ones) and interglacial periods (without). During an icehouse Earth, greenhouse gases tend to be less abundant, and temperatures tend to be cooler globally. The Earth is currently in an icehouse stage, that started 34 Ma with the ongoing Late Cenozoic Ice Age. Inside it, the last glacial, Würm, recently ended (110 to 12 ka), still has remnants of non-polar ice sheets (Alps, Himalaya, Patagonia). It will likely be soon followed by another interglacial, similar to the last one, Eemian (130 to 115 ka), when there were forests in North Cape and hippopotamus in the rivers Rhine and Thames. Then glacials and interglacials, of similar lengths as the recent ones, will continue to alternate until the end of the 2 pole ice sheets, meaning the end of the current Icehouse and the start of the next Greenhouse.

Causes of icehouse Earth
The causes of an icehouse state are much debated, because not much is really known about the transitions between greenhouse and icehouse climates and what could make the climate change. One important aspect is clearly the decline of CO2 in the atmosphere, possibly due to low volcanic activity.

Other important issues are the movement of the tectonic plates and the opening and closing of oceanic gateways. These seem to play a crucial part in icehouse Earths because they can bring cool waters from very deep water circulations that could assist in creating ice sheets or thermal isolation of areas. Examples of this occurring are the opening of the Tasmanian gateway 36.5 million years ago that separated Australia and Antarctica and which is believed to have set off the Cenozoic icehouse, and the creation of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica, though it was believed by other scientists that this did not come into effect until around 23 million years ago. The closing of the Isthmus of Panama and the Indonesian seaway approximately 3 or 4 million years ago may have been a major cause for our current icehouse state. For the icehouse climate, tectonic activity also creates mountains, which are produced by one continental plate colliding with another one and continuing forward. The revealed fresh soils act as scrubbers of carbon dioxide, which can significantly affect the amount of this greenhouse gas in the atmosphere. An example of this is the collision between the Indian subcontinent and the Asian continent, which created the Himalayan Mountains about 50 million years ago.

Glacials and interglacials
Within icehouse states, there are "glacial" and "interglacial" periods that cause ice sheets to build up or retreat. The causes for these glacial and interglacial periods are mainly variations in the movement of the earth around the Sun. The astronomical components, discovered by the Serbian geophysicist Milutin Milanković and now known as Milankovitch cycles, include the axial tilt of the Earth, the orbital eccentricity (or shape of the orbit) and the precession (or wobble) of the Earth's rotation. The tilt of the axis tends to fluctuate between 21.5° to 24.5° and back every 41,000 years on the vertical axis. This change actually affects the seasonality upon the earth, since more or less solar radiation hits certain areas of the planet more often on a higher tilt, while less of a tilt would create a more even set of seasons worldwide. These changes can be seen in ice cores, which also contain information showing that during glacial times (at the maximum extension of the ice sheets), the atmosphere had lower levels of carbon dioxide. This may be caused by the increase or redistribution of the acid/base balance with bicarbonate and carbonate ions that deals with alkalinity. During an icehouse, only 20% of the time is spent in interglacial, or warmer times. Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years, due to emissions - including complete deglaciation of the Northern Hemisphere.

Snowball earth
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 Joseph Kirschvink 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$2$ from atmosphere). Both these conditions might have caused a substantial drop in CO$2$ 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$2$. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO$2$ from the atmosphere. CO$2$ was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO$2$ 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$2$ 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 multi-cellular 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.

Recent studies may have again complicated the idea of a snowball earth. In October 2011, a team of French researchers announced that the carbon dioxide during the last speculated "snowball earth" may have been lower than originally stated, which provides a challenge in finding out how Earth was able to get out of its state and if it were a snowball or slushball.

Causes
The Eocene, which occurred between 53 and 49 million years ago, was the Earth's warmest temperature period for 100 million years. However, this "super-greenhouse" eventually became an icehouse by the late Eocene. It is believed that the decline of CO2 caused this change, though it is possible that positive feedbacks contributed to the cooling.

The best record we have for a transition from an icehouse to greenhouse period where that plant life existed during the Permian period that occurred around 300 million years ago. 40 million years ago, a major transition took place, causing the Earth to change from a moist, icy planet where rainforests covered the tropics, into a hot, dry and windy location where little could survive. Professor Isabel P. Montañez of University of California, Davis, who has researched this time period, found the climate to be "highly unstable" and "marked by dips and rises in carbon dioxide".

Impacts
The Eocene-Oligocene transition, the latest transition, occurred approximately 34 million years ago, resulting in a rapid global temperature decrease, the glaciation of Antarctica and a series of biotic extinction events. The most dramatic species turnover event associated with this time period is the Grande Coupure, a period which saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.

Research
The science of paleoclimatology attempts to understand the history of greenhouse and icehouse conditions over geological time. Through the study of ice cores, dendrochronology, ocean and lake sediments (varve), palynology, (paleobotany), isotope analysis (such as Radiometric dating and stable isotope analysis), and other climate proxies, scientists can create models of Earth's past energy budgets and resulting climate. One study has shown that atmospheric carbon dioxide levels during the Permian age rocked back and forth between 250 parts per million (which is close to present-day levels) up to 2,000 parts per million. Studies on lake sediments suggest that the "Hothouse" or "super-Greenhouse" Eocene was in a "permanent El Nino state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Nino-Southern Oscillation. A theory was suggested for the Paleocene–Eocene Thermal Maximum on the sudden decrease of the carbon isotopic composition of the global inorganic carbon pool by 2.5 parts per million. A hypothesis posed for this drop of isotopes was the increase of methane hydrates, the trigger for which remains a mystery. This increase of methane in the atmosphere, which happens to be a potent, but short-lived, greenhouse gas, increased the global temperatures by 6 °C with the assistance of the less potent carbon dioxide.

List of Icehouse and Greenhouse Periods

 * A greenhouse period ran from 4.6 to 2.4 billion years ago.
 * Huronian Glaciation – an icehouse period that ran from 2.4 billion years ago to 2.1 billion years ago
 * A greenhouse period ran from 2.1 billion to 720 million years ago.
 * Cryogenian – an icehouse period that ran from 720 to 635 million years ago, at times the entire Earth was frozen over
 * A greenhouse period ran from 635 million years ago to 450 million years ago.
 * Andean-Saharan glaciation – an icehouse period that ran from 450 to 420 million years ago
 * A greenhouse period ran from 420 million years ago to 360 million years ago.
 * Late Paleozoic Ice Age – an icehouse period that ran from 360 to 260 million years ago
 * A greenhouse period ran from 260 million years ago to 33.9 million years ago
 * Late Cenozoic Ice Age – the current icehouse period which began 33.9 million years ago

Modern conditions
Currently, the Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in Antarctica; the ice sheets in the Arctic did not start forming until 2 million years ago. Some processes that may have led to our current icehouse may be connected to the development of the Himalayan Mountains and the opening of the Drake Passage between South America and Antarctica but climate model simulations suggest that the early opening of the Drake Passage played only a limited role, while the later constriction of the Tethys and Central American Seaways is more important in explaining the observed Cenozoic cooling. Scientists have been attempting to compare the past transitions between icehouse and greenhouse, and vice versa, to understand where our planet is now heading.

Without the human influence on the greenhouse gas concentration, the Earth would be heading toward a glacial period. Predicted changes in orbital forcing suggest that in absence of human-made global warming, the next glacial period would begin at least 50,000 years from now (see Milankovitch cycles), but due to the ongoing anthropogenic greenhouse gas emissions, the Earth is heading towards a greenhouse Earth period. Permanent ice is actually a rare phenomenon in the history of the Earth, occurring only in coincidence with the icehouse effect, which has affected about 20% of Earth's history.