Greenhouse and icehouse Earth
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]
"Climate state" redirects here. For the different levels of climate due to climate change, see Equilibrium climate sensitivity.Transitions[edit]
Causes[edit]
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 Permian 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[edit]
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[edit]
Paleoclimatology is a branch of science that attempts to understand the history of greenhouse and icehouse conditions over geological time. 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 allows scientists to create models of Earth's past energy budgets and the 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 today's levels, up to 2,000 parts per million.[29] Studies on lake sediments suggest that the "hothouse" or "super-greenhouse" Eocene was in a "permanent El Niño state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Niño-Southern Oscillation.[31] 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.[32] A hypothesis posed for this drop of isotopes was the increase of 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.
Modern conditions[edit]
Currently, 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.[33] Some processes that may have led to the 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, 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 glacial period would be the next climate state. 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[35] (see Milankovitch cycles), but the ongoing 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.