Greenhouse and icehouse Earth

Source: Wikipedia, the free encyclopedia.
"Climate state" redirects here. For the different levels of climate due to climate change, see Equilibrium climate sensitivity.

Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: greenhouse and icehouse Earth.[1] Both 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.[2] There are five known Icehouse periods in Earth's climate history, which are known as the Huronian, Cryogenian, Andean-Saharan, Late Paleozoic, and Late Cenozoic glaciations.[1] The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide (CO2), changes in Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes from tectonic plate dynamics.[3] 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.[4][5]

Timeline of the five known great icehouse periods, shown in blue. The periods in between depict greenhouse conditions.

Greenhouse Earth

An illustration of the last ice age Earth at its glacial maximum.
An illustration of ice age Earth at its glacial maximum.

A "greenhouse Earth" is a period during which no continental

sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions.[7] Earth has been in a greenhouse state for about 85% of its history.[6]

The state should not be confused with a hypothetical

IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."[9]

Causes

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.

obliquity and eccentricity increase the net amount of solar radiation absorbed into Earth's atmosphere.[11]

Icehouse Earth

See also: Ice age and Timeline of glaciation

Earth is now in an icehouse state, and ice sheets are present in both poles simultaneously.[6] Climatic proxies indicate that greenhouse gas concentrations tend to lower during an icehouse Earth.[13] Similarly, global temperatures are also lower under Icehouse conditions.[14] Earth then fluctuates between glacial and interglacial periods, and the size and the distribution of continental ice sheets fluctuate dramatically.[15] 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.[4][5][16] 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.[15]

Earth's current icehouse state is known as the

Eemian, which occurred between 130,000 and 115,000 years ago, during which evidence of forest in North Cape, Norway, and hippopotamus in the Rhine and Thames Rivers can be observed.[16]
Earth is expected to continue to transition between glacial and interglacial periods until the cessation of the Quaternary Ice Age and will then enter another greenhouse state.

Causes

It is well established that there is strong correlation between low CO2 levels and an icehouse state.[18] However, that does not mean that decreasing atmospheric levels CO2 is a primary driver of a transition to the icehouse state.[11][18] Rather, it may be an indicator of other solar, geologic, and atmospheric processes at work.[18][10][11]

Potential drivers of previous icehouse states include the movement of the tectonic plates and the opening and the closing of oceanic gateways.

Indian Subcontinent with Eurasia to form the Himalayas and the Tibetan Plateau.[17] Under that paradigm, the resulting continental uplift revealed massive quantities of unweathered silicate rock CaSiO
3, which reacted with CO2 to produce CaCO
3 (lime) and SiO
2 (silica). The CaCO
3 was eventually transported to the ocean and taken up by plankton, which then died and sank to the bottom of the ocean, which effectively removed CO2 from the atmosphere.[17]

Glacials and interglacials

Main articles: Glacial period and Interglacial

Within icehouse states are "

solar radiation hits certain areas of the planet more often on a higher tilt, and a lower tilt creates a more even set of seasons worldwide. The changes can be seen in ice cores, which also contain evidence that during glacial times (at the maximum extension of the ice sheets), the atmosphere had lower levels of carbon dioxide. That may be caused by the increase or the redistribution of the acid-base balance with bicarbonate and carbonate ions that deals with alkalinity. During an icehouse period, only 20% of the time is spent in interglacial, or warmer times.[24] Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years because of CO2 emissions, including the complete deglaciation of the Northern Hemisphere.[25]

Snowball Earth

Main article: Snowball Earth

A "snowball Earth" is the complete opposite of greenhouse Earth in which 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-

photosynthetic life would not have survived under many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica[citation needed]. Most scientists[citation needed] now 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 considered to be 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 got out of its state and whether a snowball or a slushball Earth occurred.[27]

Transitions

Causes

The Eocene, which occurred between 56.0 and 33.9 million years ago, was Earth's warmest temperature period for 100 million years.[28] However, the "super-greenhouse" period had eventually become an icehouse period by the late Eocene. It is believed that the decline of CO2 caused the change, but mechanisms of positive feedback may have contributed to the cooling.

The best available record for a transition from an icehouse to greenhouse period in which plant life existed is for the

period, which occurred around 300 million years ago. A major transition took place 40 million years ago and caused Earth to change from a moist, icy planet in which rainforests covered the tropics to a hot, dry, and windy location in which little could survive. Professor Isabel P. Montañez of University of California, Davis, who has researched the time period, found the climate to be "highly unstable" and to be "marked by dips and rises in carbon dioxide."[29]

Impacts

The Eocene-Oligocene transition was the latest and occurred approximately 34 million years ago. It resulted in a rapid global cooling, the glaciation of Antarctica, and a series of biotic extinction events. The most dramatic species turnover event associated with the time period is the

Grande Coupure, a period that saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.[30]

Research

methane hydrates, the trigger for which remains a mystery. The increase of atmospheric methane, 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.[citation needed
]

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 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 during which the entire Earth was at times 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 million 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 million 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, Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in

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, and the later constriction of the Tethys and Central American Seaways is more important in explaining the observed Cenozoic cooling.[34]
Scientists have tried to compare the past transitions between icehouse and greenhouse, and vice versa, to understand what type of climate state Earth will have next.

Without the human influence on the greenhouse gas concentration, a

anthropogenic greenhouse gas emissions mean the next climate state will be a greenhouse Earth period.[33]
Permanent ice is actually a rare phenomenon in the history of Earth and occurs only in coincidence with the icehouse effect, which has affected about 20% of Earth's history.

See also

References

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