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 several icehouse periods in Earth's climate history, namely the Huronian, Cryogenian, Andean-Saharan (also known as Early Paleozoic), Late Paleozoic, Cretaceous, 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 () and, less importantly, methane (), 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.
Greenhouse Earth 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." Nor should it be confused with the "Hothouse Earth" of the most extreme projected extent of modern climate change.
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. 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.
Icehouse Earth
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.
The glacial and interglacial periods tend to alternate in accordance with solar and climatic oscillation until Earth eventually returns to a greenhouse state.
Earth's current icehouse state is known as the Quaternary Ice Age and began approximately 2.58 million years ago. However, an ice sheet has existed in Antarctica for approximately 34 million years. Earth is now in a clement interglacial period that started approximately 11,800 years ago. Earth will likely phase into another interglacial period such 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. 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 levels and an icehouse state, with evidence that the balance between geologic sources, weathering and carbon burial is responsible for secular transitions from Greenhouse and Icehouse climates and a clear role of ocean carbon storage changes in driving most of Quaternary and climate change. However, other factors such as solar, orbital and plate-tectonic forcing also influence climateWoodard, S. C., & Thomas, D. J. (2012). Oceanic and atmospheric response to climate change over varying geologic timescales. by Stella C. Woodard. [Texas A&M University]. and the global carbon cycle.
Potential drivers of previous icehouse states include the movement of the tectonic plates and the opening and the closing of oceanic gateways. They seem to play a crucial part in driving Earth into an icehouse state, as tectonic shifts result in the transportation of cool, deep water, which circulates to the ocean surface and assists in ice sheet development at the poles. Examples of oceanic current shifts as a result of tectonic plate dynamics include the opening of the Tasmanian Gateway 36.5 million years ago, which separated Australia and Antarctica, and 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 of the Indonesian seaway approximately 3 to 4 million years ago may also be a contributor to Earth's current icehouse state.
One proposed driver of the Ordovician Ice Age was the evolution of land plants. Under that paradigm, the rapid increase in photosynthetic biomass gradually removed from the atmosphere and replaced it with increasing levels of , which induced global cooling.
A proposed driver of the Quaternary Ice age is the collision of the Indian subcontinent with Eurasia to form the Himalayas and the Tibetan Plateau. Under that paradigm, the resulting continental uplift revealed massive quantities of unweathered silicate rock , which reacted with to produce (lime) and (silica). The 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 from the atmosphere.
Glacials and interglacials
Within icehouse states are "glacial" and "interglacial" periods, where ice sheets build up or retreat. During an icehouse period, only about 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 because of emissions, including the complete deglaciation of the Northern Hemisphere.
The main causes for glacial and interglacial periods are variations in the movement of 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 Earth, the orbital eccentricity (or shape of the orbit), and the precession (or wobble) of Earth's rotation. The tilt of the axis tends to fluctuate from 21.5° to 24.5° and back every 41,000 years on the vertical axis. The change actually affects the seasonality on Earth since a change in 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 change in physical ocean circulation, biological productivity and acid-base chemistry.
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-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 the low latitudes (Harland and Rudwick). That became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect) that once Earth enters the route of becoming a snowball Earth, it would never be able to leave that state.
In 1992, geologist Joseph Kirschvink brought up a solution to the paradox and how Earth could enter and exit the snowball period. Since the continents were then huddled at the low and the middle latitudes, there was less ocean water available to absorb the higher amount solar energy hitting the tropics, and there was also an increase in rainfall because more land exposed to higher solar energy might have caused chemical weathering, which would contribute to removal of CO from the atmosphere. Both conditions might have caused a substantial drop in CO atmospheric levels which resulted in cooling temperatures and increasing ice albedo (ice reflectivity of incoming solar radiation), which would further increase global cooling (a positive feedback). That might have been the mechanism of entering Snowball Earth state.