Greenhouse and icehouse Earth

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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 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. [2] 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. [1]

Contents

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

Greenhouse Earth

An illustration of ice age Earth at its glacial maximum. IceAgeEarth.jpg
An illustration of ice age Earth at its glacial maximum.

A "greenhouse Earth" is a period during which no continental glaciers exist anywhere on the planet. [6] 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. [7] Earth has been in a greenhouse state for about 85% of its history. [6]

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. [8] 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." [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. [1] However, it is important to recognize that high CO2 levels are interpreted as an indicator of Earth's climate, rather than as an independent driver. Other phenomena have instead likely played a key role in influencing global climate by altering oceanic and atmospheric currents [10] and increasing the net amount of solar radiation absorbed by Earth's atmosphere. [11] 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, [12] 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. On 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 quarter of the current global warming. It is a formidable pollutant with an 80-fold higher global warming potential than CO2 in the 20 years after it has been introduced into the atmosphere. An increase in the solar constant increases the net amount of solar energy absorbed into Earth's atmosphere, [11] and changes in Earth's obliquity and eccentricity increase the net amount of solar radiation absorbed into Earth's atmosphere. [11]

Icehouse Earth

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 Quaternary Ice Age and began approximately 2.58 million years ago. [17] However, an ice sheet has existed in Antarctica for approximately 34 million years. [17] Earth is now in a clement interglacial period that started approximately 11,800 years ago. [17] 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. [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. [19] 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. [7] 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, [20] [21] and the opening of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica, [21] 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. [22] 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 CO2 from the atmosphere and replaced it with increasing levels of O2, which induced global cooling. [23] One proposed driver of the Quaternary Ice age is the collision of the 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

Within icehouse states are "glacial" and "interglacial" periods that cause ice sheets to build up or to retreat. The main causes for glacial and interglacial periods are variations in the movement of Earth around the Sun. [24] 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 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

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. However, Joseph Kirschvink  [ de ] brought up a solution to the paradox in 1992. 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 CO2 from the atmosphere. Both conditions might have caused a substantial drop in CO2 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. 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 but accumulate atmospheric CO2. At the same time, global ice cover would prevent chemical weathering (particularly hydrolysis), responsible for removal of CO2 from the atmosphere. CO2 therefore accumulated in the atmosphere. Once the atmosphere accumulation of CO2 reached a threshold, temperature would rise enough for ice sheets to start melting. That would in turn reduce the ice albedo effect, which would in turn further reduce the ice cover and allow an exit from Snowball Earth. At the end of Snowball Earth, before the equilibrium "thermostat" between volcanic activity and the by then slowly resuming chemical weathering was reinstated, CO2 in the atmosphere had accumulated enough to cause temperatures to peak to as much as 60 °C, thrusting the Earth into a brief moist greenhouse state. Around the same geologic period of Snowball Earth (it is debated if it was the cause or the result of Snowball Earth), the Great Oxygenation Event (GOE) was occurring. The event known as the Cambrian Explosion followed and produced the beginnings of populous bilateral organisms, as well as a greater diversity and mobility in multicellular life. [26] However, some biologists claim that a complete snowball Earth could not have happened since 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 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

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

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.[ citation needed ]

List of icehouse and greenhouse periods

Modern conditions

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.

See also

Related Research Articles

<span class="mw-page-title-main">Eocene</span> Second epoch of the Paleogene Period

The Eocene is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek Ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

<span class="mw-page-title-main">Ice age</span> Period of long-term reduction in temperature of Earths surface and atmosphere

An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and greenhouse periods during which there are no glaciers on the planet. Earth is currently in the ice age called Quaternary glaciation. Individual pulses of cold climate within an ice age are termed glacial periods, and intermittent warm periods within an ice age are called interglacials or interstadials.

<span class="mw-page-title-main">Snowball Earth</span> Worldwide glaciation episodes during the Proterozoic eon

The Snowball Earth is a geohistorical hypothesis that proposes during one or more of Earth's icehouse climates, the planet's surface became nearly entirely frozen with no liquid oceanic or surface water exposed to the atmosphere. The most academically mentioned period of such a global ice age is believed to have occurred some time before 650 mya during the Cryogenian period, which included at least two large glacial periods, the Sturtian and Marinoan glaciations.

<span class="mw-page-title-main">Climate variability and change</span> Change in the statistical distribution of climate elements for an extended period

Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. Climate change may refer to any time in Earth's history, but the term is now commonly used to describe contemporary climate change, often popularly referred to as global warming. Since the Industrial Revolution, the climate has increasingly been affected by human activities.

<span class="mw-page-title-main">Paleoclimatology</span> Study of changes in ancient climate

Paleoclimatology is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to understand natural variation and the evolution of the current climate.

This glossary of climate change is a list of definitions of terms and concepts relevant to climate change, global warming, and related topics.

A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate between glacial periods. The Last Glacial Period ended about 15,000 years ago. The Holocene is the current interglacial. A time with no glaciers on Earth is considered a greenhouse climate state.

The geologic temperature record are changes in Earth's environment as determined from geologic evidence on multi-million to billion (109) year time scales. The study of past temperatures provides an important paleoenvironmental insight because it is a component of the climate and oceanography of the time.

The Huronian glaciation was a period where at least three ice ages occurred during the deposition of the Huronian Supergroup. Deposition of this largely sedimentary succession extended from approximately 2.5 to 2.2 billion years ago (Gya), during the Siderian and Rhyacian periods of the Paleoproterozoic era. Evidence for glaciation is mainly based on the recognition of diamictite, that is interpreted to be of glacial origin. Deposition of the Huronian succession is interpreted to have occurred within a rift basin that evolved into a largely marine passive margin setting. The glacial diamictite deposits within the Huronian are on par in thickness with Quaternary analogs.

<span class="mw-page-title-main">Quaternary glaciation</span> Series of alternating glacial and interglacial periods

The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma and is ongoing. Although geologists describe this entire period up to the present as an "ice age", in popular culture this term usually refers to the most recent glacial period, or to the Pleistocene epoch in general. Since Earth still has polar ice sheets, geologists consider the Quaternary glaciation to be ongoing, though currently in an interglacial period.

<span class="mw-page-title-main">Late Paleozoic icehouse</span> Ice age

The late Paleozoic icehouse, also known as the Late Paleozoic Ice Age (LPIA) and formerly known as the Karoo ice age, was an ice age that began in the Late Devonian and ended in the Late Permian, occurring from 360 to 255 million years ago (Mya), and large land-based ice sheets were then present on Earth's surface. It was the second major icehouse period of the Phanerozoic, after the Late Ordovician Andean-Saharan glaciation.

<i>Plows, Plagues and Petroleum</i> 2005 book by William Ruddiman

Plows, Plagues and Petroleum: How Humans Took Control of Climate is a 2005 book published by Princeton University Press and written by William Ruddiman, a paleoclimatologist and Professor Emeritus at the University of Virginia. He has authored and co-authored several books and academic papers on the subject of climate change. Scientists often refer to this period as the "Anthropocene" and define it as the era in which humans first began to alter the Earth's climate and ecosystems. Ruddiman contends that human induced climate change began as a result of the advent of agriculture thousands of years ago and resulted in warmer temperatures that could have possibly averted another ice age; this is the early anthropocene hypothesis.

<span class="mw-page-title-main">Carbon dioxide in Earth's atmosphere</span> Atmospheric constituent and greenhouse gas

In Earth's atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of three main greenhouse gases in the atmosphere of Earth. The concentration of carbon dioxide in the atmosphere reach 427 ppm (0.04%) in 2024. This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity.

<span class="mw-page-title-main">Azolla event</span> Hypothetical geoclimatic event

The Azolla event is a paleoclimatology scenario hypothesized to have occurred in the middle Eocene epoch, around 49 million years ago, when blooms of the carbon-fixing freshwater fern Azolla are thought to have happened in the Arctic Ocean. As the fern died and sank to the stagnant sea floor, they were incorporated into the sediment over a period of about 800,000 years; the resulting draw-down of carbon dioxide has been speculated to have helped reverse the planet from the "greenhouse Earth" state of the Paleocene-Eocene Thermal Maximum, when the planet was hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.

This is a list of climate change topics.

<span class="mw-page-title-main">Greenhouse gas</span> Gas in an atmosphere with certain absorption characteristics

Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).

<span class="mw-page-title-main">Atmospheric methane</span> Methane in Earths atmosphere

Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing (RF) of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.

Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum, the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.

<span class="mw-page-title-main">Atmospheric carbon cycle</span> Transformation of atmospheric carbon between various forms

The atmospheric carbon cycle accounts for the exchange of gaseous carbon compounds, primarily carbon dioxide, between Earth's atmosphere, the oceans, and the terrestrial biosphere. It is one of the faster components of the planet's overall carbon cycle, supporting the exchange of more than 200 billion tons of carbon in and out of the atmosphere throughout the course of each year. Atmospheric concentrations of CO2 remain stable over longer timescales only when there exists a balance between these two flows. Methane, Carbon monoxide (CO), and other human-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle.

<span class="mw-page-title-main">Late Cenozoic Ice Age</span> Ice age of the last 34 million years, in particular in Antarctica

The Late Cenozoic Ice Age, or Antarctic Glaciation, began 34 million years ago at the Eocene-Oligocene Boundary and is ongoing. It is Earth's current ice age or icehouse period. Its beginning is marked by the formation of the Antarctic ice sheets.

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