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Introduction Paragraph
Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: Greenhouse and Icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur as alternate phases within an Icehouse period, and tend to last less than 1 million years. There are five known Icehouse periods in Earth's climate history; known as the Huronian, Cryogenia n, Andean-Saharan, Late Paleozoic, and Late Cenozoic glaciations. The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide (CO2), 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 played key roles in the evolution of life on earth by directly and indirectly forcing biotic adaptation and turnover at various spatial scales across time.

Overview of Greenhouse Earth
A "Greenhouse Earth" is a period in which there are no continental glaciers whatsoever on the planet. Additionally, 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. It is important to note that the term "Greenhouse" should not be confused with a hypothetical "Hothouse Earth", which is an irreversible tipping point corresponding to the ongoing runaway greenhouse effect on Venus.

Causes of Greenhouse Earth
There are several theories as to how a Greenhouse Earth can come about. Geologic climate proxies indicate that there is a strong correlation between a greenhouse state and high CO2 levels. However, it is important to recognize that high CO2 levels are interpreted as an indicator of Earth's climate rather than an independent driver. Instead other phenomena have likely played a key role in influencing global climate by altering oceanic and atmospheric currents and increasing the net amount of solar radiation absorbed by Earth's atmosphere. Such phenomena may include but are not limited to: 1. Tectonic shifts that result in the release of greenhouse gases (such as CO2 and CH4) via volcanic activity, 2. An increase in the solar constant that increase the net amount of solar energy absorbed into the Earth's atmosphere, and 3. Changes in Earth obliquity and eccentricity that increase the net amount of solar radiation absorbed into Earth's atmosphere.

Overview of Icehouse Earth
The Earth is in an Icehouse state when ice sheets are present in both poles simultaneously. Climatic proxies indicate that greenhouse gas concentrations tend to lower when the Earth is in this state. Similarly, global temperatures are also lower under Icehouse conditions. In this climatic state Earth fluctuates between glacial and interglacial periods where the size and distribution of continental ice sheets fluctuate dramatically. The fluctuation of these ice sheets result in changes in regional climatic conditions that affect the range and distribution of many terrestrial and oceanic species. These glacial and interglacial periods tend to alternate in accordance with solar and climatic oscillation until Earth eventually return to a Greenhouse state.

Earth is currently in an Icehouse state known as the Quaternary Ice Age that began approximately 2.58 million years ago. However, an ice sheet has be present on the antarctic continent for approximately 34 million years. At this time, Earth is in a clement interglacial period that started approximately 11.8 kya. During the last interglacial period, known as the Eemian (130-115 kya), evidence of forests in North Cape, Norway as well as hippopotamus in the Rhine and Thames rivers can be observed. The Earth is expected to continue to transition between glacial and interglacial periods until the cessation of the Quaternary Ice Age where it will enter another Greenhouse state.

Causes of Icehouse Earth
It is well-established that there is strong correlation between low CO2 levels and an Icehouse state. However, this does not mean that decreasing atmospheric levels CO2 is a primary driver of a transition to the Icehouse state. Rather, it may be an indicator of other solar, geologic, and atmospheric processes at work.

Potential drivers of previous Icehouse States include the movement of the tectonic plates and the opening and closing of oceanic gateways. These seem to play a crucial part in driving Earth into an Icehouse state as such tectonic shifts result in the transportation of cool deep water circulations to the ocean surface that assist in ice sheet development at the poles. Examples of this oceanic current shifts as a result of tectonic plate dynamics include the opening of the Tasmanian gateway 36.5 million years ago that separated Australia and Antarctica as well as the opening of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica - both of which are believed to have allowed for the development of the Antarctic Ice sheet. A proposed driver of the Ordovician Ice Age was the evolution of land plants. Under this paradigm, the rapid increase in photosynthetic biomass gradually removed CO2 from the atmosphere and replaced it with increasing levels of O2 inducing overall climate cooling. One proposed driver of the Quaternary Ice age is the collision of the Indian subcontinent with Eurasia- forming the Himalayas and the Tibetan Plateau. Under this paradigm the resulting continental uplift revealed massive quantities of unweathered silicate rock (CaSiO3) which reacts with CO2 which then produces CaCO3 (lime) and SiO2 (silica). The CaCO3 is eventually transported to the ocean and taken up by plankton which then die and sink to the bottom of the ocean- effectively removing CO2 from the atmosphere.

Glacials and Interglacials
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Snowball Earth
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Transitions
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