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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.
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Evidence for past temperatures comes mainly from isotopic considerations (especially δ18O); the Mg/Ca ratio of foram tests, and alkenones, are also useful. Often, many are used in conjunction to get a multi-proxy estimate for the temperature. This has proven crucial in studies on glacial/interglacial temperature. [1]
The last 3 million years have been characterized by cycles of glacials and interglacials within a gradually deepening ice age. Currently, the Earth is in an interglacial period, beginning about 20,000 years ago (20 kya).
The cycles of glaciation involve the growth and retreat of continental ice sheets in the Northern Hemisphere and involve fluctuations on a number of time scales, notably on the 21 ky, 41 ky and 100 ky scales. Such cycles are usually interpreted as being driven by predictable changes in the Earth orbit known as Milankovitch cycles. At the beginning of the Middle Pleistocene (0.8 million years ago, close to the Brunhes–Matuyama geomagnetic reversal) there has been a largely unexplained switch in the dominant periodicity of glaciations from the 41 ky to the 100 ky cycle.
The gradual intensification of this ice age over the last 3 million years has been associated with declining concentrations of the greenhouse gas carbon dioxide, though it remains unclear if this change is sufficiently large to have caused the changes in temperatures. Decreased temperatures can cause a decrease in carbon dioxide as, by Henry's Law, carbon dioxide is more soluble in colder waters, which may account for 30ppmv of the 100ppmv decrease in carbon dioxide concentration during the last glacial maximum.
Similarly, the initiation of this deepening phase also corresponds roughly to the closure of the Isthmus of Panama by the action of plate tectonics. This prevented direct ocean flow between the Pacific and Atlantic, which would have had significant effects on ocean circulation and the distribution of heat. However, modeling studies have been ambiguous as to whether this could be the direct cause of the intensification of the present ice age.
This recent period of cycling climate is part of the more extended ice age that began about 40 million years ago with the glaciation of Antarctica.
In the earliest part of the Eocene period, a series of abrupt thermal spikes have been observed, lasting no more than a few hundred thousand years. The most pronounced of these, the Paleocene-Eocene Thermal Maximum (PETM) is visible in the figure at right. These are usually interpreted as caused by abrupt releases of methane from clathrates (frozen methane ices that accumulate at the bottom of the ocean), though some scientists dispute that methane would be sufficient to cause the observed changes.[ citation needed ] During these events, temperatures in the Arctic Ocean may have reached levels more typically associated with modern temperate (i.e. mid-latitude) oceans.[ citation needed ] During the PETM, the global mean temperature seems to have risen by as much as 5–8 °C (9–14 °F) to an average temperature as high as 23 °C (73 °F), in contrast to the global average temperature of today at just under 15 °C (60 °F). Geologists and paleontologists think that during much of the Paleocene and early Eocene, the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle, while much of the continental United States had a sub-tropical environment. [5]
During the later portion of the Cretaceous, from 100 to 66 million years ago, average global temperatures reached their highest level during the last ~200 million years. [6] This is likely to be the result of a favorable configuration of the continents during this period that allowed for improved circulation in the oceans and discouraged the formation of large scale ice sheet.[ citation needed ]
The Phanerozoic eon, encompassing the last 542 million years and almost the entire time since the origination of complex multi-cellular life, has more generally been a period of fluctuating temperature between ice ages, such as the current age, and "climate optima", similar to what occurred in the Cretaceous. Roughly 4 such cycles have occurred during this time with an approximately 140 million year separation between climate optima. In addition to the present, ice ages have occurred during the Permian-Carboniferous interval and the late Ordovician-early Silurian. There is also a "cooler" interval during the Jurassic and early Cretaceous, with evidence of increased sea ice, but the lack of continents at either pole during this interval prevented the formation of continental ice sheets and consequently this is usually not regarded as a full-fledged ice age. In between these cold periods, warmer conditions were present and often referred to as climate optima. However, it has been difficult to determine whether these warmer intervals were actually hotter or colder than occurred during the Cretaceous optima.
The Neoproterozoic era ( 1,000 to 538.8 million years ago), provides evidence of at least two and possibly more major glaciations. The more recent of these ice ages, encompassing the Marinoan & Varangian glacial maxima (about 560 to 650 million years ago), has been proposed as a snowball Earth event with continuous sea ice reaching nearly to the equator. This is significantly more severe than the ice ages during the Phanerozoic. Because this ice age terminated only slightly before the rapid diversification of life during the Cambrian explosion, it has been proposed that this ice age (or at least its end) created conditions favorable to evolution. The earlier Sturtian glacial maxima (~730 million years) may also have been a snowball Earth event though this is unproven.
The changes that lead to the initiation of snowball Earth events are not well known, but it has been argued that they necessarily led to their own end. The widespread sea ice prevents the deposition of fresh carbonates in ocean sediment. Since such carbonates are part of the natural process for recycling carbon dioxide, short-circuiting this process allows carbon dioxide to accumulate in the atmosphere. This increases the greenhouse effect and eventually leads to higher temperatures and the retreat of sea ice. [8]
Direct combination of these interpreted geological temperature records is not necessarily valid, nor is their combination with other more recent temperature records, which may use different definitions. Nevertheless, an overall perspective is useful even when imprecise. In this view time is plotted backwards from the present, taken as 2015 CE. It is scaled linear in five separate segments, expanding by about an order of magnitude at each vertical break. Temperatures in the left-hand panel are very approximate, and best viewed as a qualitative indication only. [9] Further information is given on the graph description page.
About 800 to 1,800 million years ago, there was a period of climate stasis, also known as the Boring Billion. During this period there was hardly any tectonic activity, no glaciations and the atmosphere composition remained stable. It is bordered by two different oxygenation and glacial events.
Temperature reconstructions based on oxygen and silicon isotopes from rock samples have predicted much hotter Precambrian sea temperatures. [10] [11] These predictions suggest ocean temperatures of 55–85 °C during the period of 2,000 to 3,500 million years ago, followed by cooling to more mild temperatures of between 10-40 °C by 1,000 million years ago. Reconstructed proteins from Precambrian organisms have also provided evidence that the ancient world was much warmer than today. [12] [13]
However, other evidence suggests that the period of 2,000 to 3,000 million years ago was generally colder and more glaciated than the last 500 million years.[ citation needed ] This is thought to be the result of solar radiation approximately 20% lower than today. Solar luminosity was 30% dimmer when the Earth formed 4.5 billion years ago, [14] and it is expected to increase in luminosity approximately 10% per billion years in the future. [15]
On very long time scales, the evolution of the sun is also an important factor in determining Earth's climate. According to standard solar theories, the sun will gradually have increased in brightness as a natural part of its evolution after having started with an intensity approximately 70% of its modern value. The initially low solar radiation, if combined with modern values of greenhouse gases, would not have been sufficient to allow for liquid oceans on the surface of the Earth. However, evidence of liquid water at the surface has been demonstrated as far back as 4,400 million years ago. This is known as the faint young sun paradox and is usually explained by invoking much larger greenhouse gas concentrations in Earth's early history, though such proposals are poorly constrained by existing experimental evidence.
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.
The Pleistocene is the geological epoch that lasted from c. 2.58 million to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in 2009 by the International Union of Geological Sciences, the cutoff of the Pleistocene and the preceding Pliocene was regarded as being 1.806 million years Before Present (BP). Publications from earlier years may use either definition of the period. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology. The name is a combination of Ancient Greek πλεῖστος (pleîstos) 'most' and καινός 'new'.
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.
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.
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.
Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing. Some press reports in the 1970s speculated about continued cooling; these did not accurately reflect the scientific literature of the time, which was generally more concerned with warming from an enhanced greenhouse effect.
The Cryogenian is a geologic period that lasted from 720 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.
There have been five or six major ice ages in the history of Earth over the past 3 billion years. The Late Cenozoic Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation, in progress since 2.58 million years ago.
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.
Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to ancient ocean temperature which in turn reflects ancient climate. Cycles in the ratio mirror climate changes in the geological history of Earth.
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.
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.
The 100,000-year problem of the Milankovitch theory of orbital forcing refers to a discrepancy between the reconstructed geologic temperature record and the reconstructed amount of incoming solar radiation, or insolation over the past 800,000 years. Due to variations in the Earth's orbit, the amount of insolation varies with periods of around 21,000, 40,000, 100,000, and 400,000 years. Variations in the amount of incident solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of initiation and termination of glaciations.
Marine Isotope Stage 11 or MIS 11 is a Marine Isotope Stage in the geologic temperature record, covering the interglacial period between 424,000 and 374,000 years ago. It corresponds to the Hoxnian Stage in Britain.
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 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, which are 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, 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.
The Marinoan glaciation, sometimes also known as the Varanger glaciation, was a period of worldwide glaciation. Its beginning is poorly constrained, but occurred no earlier than 654.5 Ma. It ended approximately 632.3 ± 5.9 Ma during the Cryogenian period. This glaciation possibly covered the entire planet, in an event called the Snowball Earth. The end of the glaciation was caused by volcanic release of carbon dioxide and dissolution of gas hydrates and may have been hastened by the release of methane from equatorial permafrost.
The Middle Pliocene Warm Period (mPWP), also known as the Mid-Piacenzian Warm Period or the Pliocene Thermal Maximum, was an interval of warm climate during the Pliocene epoch that lasted from 3.3 to 3.0 million years ago (Ma).
Lorraine Lisiecki is an American paleoclimatologist. She is a professor in the Department of Earth Sciences at the University of California, Santa Barbara. She has proposed a new analysis of the 100,000-year problem in the Milankovitch theory of climate change. She also created the analytical software behind the LR04, a "standard representation of the climate history of the last five million years".
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.
The Mid-Pleistocene Transition (MPT), also known as the Mid-Pleistocene Revolution (MPR), is a fundamental change in the behaviour of glacial cycles during the Quaternary glaciations. The transition occurred gradually, taking place approximately 1.25–0.7 million years ago, in the Pleistocene epoch. Before the MPT, the glacial cycles were dominated by a 41,000-year periodicity with low-amplitude, thin ice sheets, and a linear relationship to the Milankovitch forcing from axial tilt. Because of this, sheets were more dynamic during the Early Pleistocene. After the MPT there have been strongly asymmetric cycles with long-duration cooling of the climate and build-up of thick ice sheets, followed by a fast change from extreme glacial conditions to a warm interglacial. This led to less dynamic ice sheets. Interglacials before the MPT had lower levels of atmospheric carbon dioxide compared to interglacials after the MPT. One of the MPT's effects was causing ice sheets to become higher in altitude and less slippery compared to before. The MPT greatly increased the reservoirs of hydrocarbons locked up as permafrost methane or methane clathrate during glacial intervals. This led to larger methane releases during deglaciations. The cycle lengths have varied, with an average length of approximately 100,000 years.
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