Geologic temperature record
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This article is devoted to temperature changes in Earth's environment as determined from geologic evidence on multi-million to billion (109) year time scales.
For articles related describing temperature changes on other time scales, please see: Temperature record.
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[edit] Description of the temperature record
[edit] Recent past
The last 3 million years have been characterized by cycles of glacials and interglacials within a gradually deepening ice age. These cycles involve the growth and retreat of continental ice sheets in the Northern Hemisphere and involve fluctuations on 40,000 and 100,000 year time scales. Such cycles are usually interpreted as being driven by predictable changes in the Earth orbit known as Milankovitch cycles. 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.
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 ~40 million years ago with the glaciation of Antarctica.
[edit] Initial Eocene thermal maxima
In the earliest part of the Eocene period, a series of abrupt thermal spikes have been observed, lasting no more than a few 100,000 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. During these events, temperatures in the Arctic Ocean may have reached levels more typically associated with modern temperate (i.e. mid-latitude) oceans.
[edit] Cretaceous thermal optimum
During the later portion of the Cretaceous (65-100 million years ago), average global temperatures reached their highest level during the last ~200 million years. This is likely 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. Perhaps the visible anecdotal evidence of high temperatures during this period was the occurrence of deciduous forests extending all the way to the poles.
[edit] Fluctuations during the remainder of the Phanerozoic
The Phanerozoic eon, encompassing the last 542 million years and almost the entire time since the origination of complex multi-celluar 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 in the 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 period, 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.
[edit] Late Proterozoic ice ages
The Neoproterozoic era (542-1000 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 (~560 - ~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 age 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 lead 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.
[edit] An Overall View
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 rather imprecise. (Here the time scale is reversed; with time increasing to the right to match the shorter-term temperature record plotting convention.)
[edit] Other temperature changes in Earth's past
Prior to the Neoproterozoic, evidence of temperature changes and glaciation is usually too scattered and sporadic to draw firm conclusions though it seems likely to temperature fluctuations were also substantial during this period.
Some evidence does exist however that the period of 2000-3000 million years ago was very generally colder and more glaciated than the last 500 million years. This is usually believed to have resulted from solar radiation approximately 20% lower than today. See next section for further discussion.
[edit] Processes affecting global temperatures
On geologically short time scales, greenhouse gases are the most important process influencing temperature. By trapping solar radiation, carbon dioxide, water vapor, methane, and some other gases, contribute to a greenhouse effect essential to maintaining the Earth's temperature.
However, modern reconstructions of greenhouse gas concentrations do not show fluctuations over geologically long periods that match to the observed changes in temperatures and ice volumes. This has led most observers to believe that changes in continent configurations are much more important in determine long-term temperature. In particular, the lack of land masses at the poles inhibits the formation of large ice sheets. Similarly, the presence of expansive sea ways, especially near the equator, aids in distributing heat across the globe and leads to higher temperature on average.
Recently, Shaviv has proposed that the motion of the solar system through the galaxy has an effect on global climate. As the solar system moves in and out of the spiral arms of the galaxy, the flux of cosmic rays will vary by approximately a factor of 3. Since such cosmic rays play an important role in ionizing the atmosphere, it is plausible that such variations can influence climate, and may be able to explain the 140 million year quasi-periodicity observed in temperatures during the Phanerozoic. The biggest present weakness in this theory is that the time period between encounters with galactic spiral arms is only poorly constrained by existing astronomical observations.
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 till at least 3500 million years ago. This is known as the faint young sun paradox and is usually explained by invoking much much larger greenhouse gas concentrations in Earth's early history, though such proposals are poorly constrained by existing experimental evidence.
