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, Cryogenian, 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 (CO
2), 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.
This state should not be confused with a hypothetical runaway greenhouse effect, 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. 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 returns 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 been 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. Earth will likely phase into another interglacial period such as the Eemian, which occurred between 130 and 115 kya, during which 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 CO
2 levels and an Icehouse state. However, this does not mean that decreasing atmospheric levels CO
2 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. The closing of the Isthmus of Panama and the Indonesian seaway approximately 3 to 4 million years ago may also be a contributor to the Earth's current icehouse state. One proposed driver of the Ordovician Ice Age was the evolution of land plants. Under this paradigm, the rapid increase in photosynthetic biomass gradually removed CO
2 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 CO
2 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 CO
2 from the atmosphere.
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 CO
2 emissions - including complete deglaciation of the Northern Hemisphere.
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 CO2 from atmosphere). Both these conditions might have caused a substantial drop in CO2 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 CO2. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO2 from the atmosphere. CO2 was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO2 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, CO2 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 populous bi-lateral organisms and a greater diversity and mobility in all multicellular 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.
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".
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.
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
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.
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