Greenhouse and icehouse Earth

Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: greenhouse and icehouse Earth. Both climate states last for millions of years and should not be confused with the much smaller glacial and interglacial periods, which occur as alternating phases within an icehouse period (known as an ice age) and tend to last less than one million years. There are five known icehouse periods in Earth's climate history, namely the Huronian, Cryogenian, Andean-Saharan (also known as Early Paleozoic), Late Paleozoic and Late Cenozoic glaciations.

The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric greenhouse gases such as carbon dioxide (CO₂) and less importantly methane (CH₄), changes in Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes from 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.

A "greenhouse Earth" is a period during which no continental glaciers exist anywhere 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. Earth has been in a greenhouse state for about 85% of its history.

The state should not be confused with a hypothetical runaway greenhouse effect, which is an irreversible tipping point that corresponds 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."

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 have traditionally been interpreted as feedback to Earth's climate rather than as an independent driver, but geologic drivers of CO₂ and climate change have been identified. Other phenomena have instead 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 tectonic shifts that result in the release of greenhouse gases (such as CO₂ and CH₄) via volcanic activity. Volcanoes emit massive amounts of CO₂ and methane into the atmosphere when they are active, which can trap enough heat to cause a greenhouse effect. In a greenhouse Earth, atmospheric concentrations of greenhouse gases like carbon dioxide (CO₂) and methane (CH₄) are higher, trapping solar energy in the atmosphere via the greenhouse effect. Methane, the main component of natural gas, is responsible for more than a third of the current global warming. While it has an 80-fold higher global warming potential than CO₂ in the 20 years after it has been introduced into the atmosphere (due to a stronger greenhouse effect), it has a relatively short half life in the atmosphere of 10-20 years. An increase in the solar constant increases the net amount of solar energy absorbed into Earth's atmosphere, and changes in Earth's obliquity and eccentricity increase the net amount of solar radiation absorbed into Earth's atmosphere.

Earth is now in an icehouse state, and ice sheets are present in both poles simultaneously. Climatic proxies indicate that greenhouse gas concentrations tend to lower during an icehouse Earth. Similarly, global temperatures are also lower under icehouse conditions. Earth then fluctuates between glacial and interglacial periods, and the size and the distribution of continental ice sheets fluctuate dramatically. The fluctuation of the ice sheets results in changes in regional climatic conditions that affect the range and the distribution of many terrestrial and oceanic species.

On scales ranging from thousands to hundreds of millions of years, the Earth's climate has transitioned from warm to chilly intervals within life-sustaining ranges. There have been three periods of glaciation in the Phanerozoic Eon (Ordovician, Carboniferous, and Cenozoic), each lasting tens of millions of years and bringing ice down to sea level at mid-latitudes. During these frigid "icehouse" intervals, sea levels were generally lower, CO₂ levels in the atmosphere were lower, net photosynthesis and carbon burial were lower, and oceanic volcanism was lower than during the alternate "greenhouse" intervals. Transitions from Phanerozoic icehouse to greenhouse intervals coincided with biotic crises or catastrophic extinction events, indicating complicated biosphere-hydrosphere feedbacks.^[39]

The glacial and interglacial periods tend to alternate in accordance with solar and climatic oscillation until Earth eventually returns to a greenhouse state.