A "greenhouse earth" or "hothouse 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.
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 becomes 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.
An "icehouse earth" is the earth as it experiences an ice age. Unlike a greenhouse earth, an icehouse earth has ice sheets present, and these sheets wax and wane throughout times known as glacial periods and interglacial periods. 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, as ice sheets are present on both poles and glacial periods have occurred at regular intervals over the past million years.
The causes of an icehouse state are much debated, because not much is really known about the transition periods between greenhouse to icehouse climates and what could make the climate so different. 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 forth 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.
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 spin. 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 contains information that shows 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.
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 Joe Kirschvink brought up a solution to the paradox. It is believed that since the continents at this time were huddled at the low and mid-latitudes that there was a great cooling event by planetary albedo, or reflection of the earth’s surface. Kirschvink explained that the way to get out of the snowball could be connected to carbon dioxide, since volcanic activity would not halt, and that the buildup and lack of "scrubbing" of this carbon dioxide in the atmosphere, that the earth would return to a greenhouse state. Some scientists believe that the end of the snowball Earth caused an event known as the Cambrian Explosion, 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, it has been observed that, even under meters of thick ice around Antarctica, sunlight shows through. Most scientists today believe that a "hard" Snowball Earth, one completely covered by ice, is probably impossible. However, a "slushball earth", with points of openings 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.
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" soon became an icehouse by the late Eocene. It was believed that the decline of CO2 caused this change, though there are possible positive feedbacks, or added influence that contributes to the cooling.
The best record we have for a transition from an icehouse to a greenhouse period where plant life exists is during the Permian epoch that occurred around 300 million years ago. In 40 million years 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".
Whenever earth transitions from either greenhouse or icehouse to the other, mass extinctions occur.
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 (fossilized pollen) and isotope analysis (such as Radiometric dating), scientists can create models of past climate. From such models, scientists have determined that the atmospheric carbon dioxide of the Earth could have been up at least 350 times higher than our modern day levels. 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 carbon isotopic composition of global inorganic carbon pool by 2.5 parts per million. A hypothesis noted for this negative drop of isotopes could be 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.
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. 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 instead heading toward a greenhouse earth period. Permanent ice is actually a rare phenomenon in the history of the Earth, occurring only during the 20% of the time that the planet is under an icehouse effect.