Ice age and Progestogen: Difference between pages

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→‎Antiandrogen: Added a citation, better explained that cyproterone is antiandrogen BECAUSE it's a progestin (previously, the section merely contained one word)
 
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{{expert}}
{{Citations missing|date=May 2007}}
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{{dablink|For the prehistoric era commonly referred to as The Ice Age, see the [[Pleistocene]] and the [[Paleolithic]] epoch.}}
'''Progestagens''' (also spelled '''progestogens''' or '''gestagens''') are [[hormone]]s that produce effects similar to those of [[progesterone]], the only natural progestagen. All other progestogens are [[Chemical synthesis|synthetic]] and are often referred to as [[progestins]].
{{otheruses}}
[[Image:AntarcticaDomeCSnow.jpg|thumb|300px|Ice sheets expand during an ice age. This image is of the Antarctic ice sheet.]]
[[Image:Vostok-ice-core-petit.png|thumb|300px|Variations in temperature, CO<sub>2</sub>, and dust from the [[Vostok, Antarctica|Vostok]] ice core over the last 400,000 years]]
An '''ice age''' is a period of long-term reduction in the [[temperature]] of the [[Earth]]'s surface and atmosphere, resulting in an expansion of continental [[ice sheet]]s, polar ice sheets and alpine [[glacier]]s. [[Glaciology|Glaciologically]], ''ice age'' is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age (because the [[Greenland ice sheet|Greenland]] and [[Antarctic ice sheet]]s still exist). More colloquially, when speaking of the last few million years, ''ice age'' is used to refer to colder periods with extensive ice sheets over the [[North America]]n and [[Eurasia]]n continents: in this sense, the [[last glacial period|most recent ice age]] [[Last Glacial Maximum|peaked]] about 20,000 years ago. This article will use the term ''ice age'' in the former, glaciological, sense: ''[[glacial period|glacials]]'' for colder periods during ice ages and ''[[interglacials]]'' for the warmer periods.


All progestagens have antiestrogenic (counteracting the effects of [[estrogen]]s on the body) and antigonadotropic (inhibiting the production of [[sex steroid]]s by [[gonad]]s) properties.


Progestogens differ in their potency (affinity for [[progesterone receptor]]s) and side-effects. Such side-effects may be [[androgen]]ic (medroxyprogesterone and most C19 progestagens), [[antiandrogen]]ic (cyproterone acetate), [[estrogen]]ic, [[glucocorticoid]] (some C21 progestogens), or anti[[mineralocorticoid]] (progesterone).


==Uses==
==Evidence for ice ages=
===Birth control===


{{main|Hormonal contraception}}
Despite the difficulties, analyses of ice core and ocean sediment cores has shown periods of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.


*Progestogens are used alone in [[progestogen only pill]]s, or with an [[estrogen]] in [[combined oral contraceptive pill]]s, a [[contraceptive patch]], and a [[NuvaRing|contraceptive vaginal ring]].
==Major ice ages==
*Medroxyprogesterone acetate ([[Depo-Provera]]) and norethindrone enantate (Noristerat) are used by [[Injection (medicine)#Depot injection|depot injection]].
[[Image:EisrandlagenNorddeutschland.png|thumb|left|Ice age map of northern central Europe. Red: maximum limit of Weichselian ice age; yellow: Saale ice age at maximum (Drenthe stage); blue: Elster ice age maximum glaciation.]]
*Etonogestrol is released by [[subcutaneous]] implants ([[Implanon]]). [[Norplant]] and [[Jadelle]] implants release [[levonorgestrel]].
There have been at least four major ice ages in the Earth's past. Outside these periods, the [[Earth]] seems to have been ice-free even in high latitudes.{{Fact|date=May 2008}}
*Levonorgestrel is released by the [[IntraUterine System|intrauterine system]] IUS (Mirena).


===Antiandrogen===
The earliest hypothesized ice age, called the [[Huronian]], was around 2.7 to 2.3 [[1000000000 (number)|billion]] years ago during the early [[Proterozoic]] Eon.
Progestinic compounds decrease [[Luteinizing_hormone|luteinizing hormone]] levels <ref>T.B. Hatlera, S.H. Hayesa, D.L. Raya, P.S. Reamesa and W.J. Silvia [http://dx.doi.org/10.1016/j.tvjl.2007.06.003 "Effect of subluteal concentrations of progesterone on luteinizing hormone and ovulation in lactating dairy cows"], ''[http://www.sciencedirect.com/science/journal/10900233 The Veterinary Journal]''</ref> and as such, will have [[Antiandrogen|antiandrogenic]] properties in [[Transwoman|trans-women]] and [[Cisgender|cisgender]] [[Male|males]] alike due to decreased [[Luteinizing_hormone|LH]] stimulation of the [[Testicle|testes]]. [[Cyproterone]] is a common example of a progestinic medication frequently used as an antiandrogen.


===Progestogen withdrawal bleeding===
The earliest well-documented ice age, and probably the most severe of the last 1 billion years, occurred from 850 to 630 million years ago (the [[Cryogenian]] period) and may have produced a [[Snowball Earth]] in which permanent ice covered the entire globe and was ended by the effects of the accumulation of [[greenhouse gas]]es such as CO2 produced by volcanoes. "The presence of ice on the continents and pack ice on the oceans would inhibit both silicate weathering and photosynthesis, which are the two major sinks for CO2 at present."<ref> [http://www.palaeos.com/Proterozoic/Neoproterozoic/Cryogenian/Snowballs.html Cryogenian Snowballs]</ref> It has been suggested that the end of this ice age was responsible for the subsequent [[Ediacaran]] and [[Cambrian Explosion]], though this theory is recent and controversial.[[Image:Five Myr Climate Change.png|thumb|400px|right|Sediment records showing the fluctuating sequences of glacials and interglacials during the last several million years.]]
In a normal [[menstrual cycle]], a sudden drop in progesterone levels triggers menstruation. [[Norethindrone acetate]] (brand name Aygestin) and [[medroxyprogesterone acetate]] (brand name Provera) may be used to artificially induce progestogen withdrawal bleeding.


[[Category:Progestagens]]
A minor ice age, the [[Andean-Saharan]], occurred from 460 to 430 million years ago, during the [[Late Ordovician]] and the [[Silurian]] period. There were extensive polar [[ice cap]]s at intervals from 350 to 260 million years ago, during the [[Carboniferous]] and [[Permian|early Permian]] Periods, associated with the [[Karoo Ice Age]].
[[Category:Sex steroids]]

While an ice sheet on Antarctica began to grow some 20 million years ago, the [[Quaternary glaciation|current ice age]] is said to have started about 2.58 million years ago. During the late [[Pliocene]] the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called [[glacial period|glacials]] (glacial advance) and [[interglacial]]s (glacial retreat). The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago. All that remains of the continental [[ice sheet]]s are the [[Greenland ice sheet|Greenland]] and [[Antarctic ice sheet]]s.

Ice ages can be further divided by location and time; for example, the names ''Riss'' (180,000–130,000 years [[before present|bp]]) and ''[[Würm glaciation|Würm]]'' (70,000–10,000 years bp) refer specifically to glaciation in the [[Alps|Alpine region]]. Note that the maximum extent of the ice is not maintained for the full interval. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage. It is possible that glacial periods other than those above, especially in the [[Precambrian]], have been overlooked because of scarcity of exposed rocks from high latitudes from older periods.

== Glacials and interglacials ==
{{see also|Interglacial}}
[[Image:Ice Age Temperature.png|right|thumb|300px|Shows the pattern of temperature and ice volume changes associated with recent glacials and interglacials]]

Within the ice ages (or at least within the last one), more temperate and more severe periods occur. The colder periods are called ''glacial periods'', the warmer periods ''interglacials'', such as the [[Eemian interglacial era]].

Glacials are characterized by cooler and drier climates over most of the Earth and large land and sea ice masses extending outward from the poles. [[Mountain]] glaciers in otherwise unglaciated areas extend to lower elevations due to a lower [[snow line]]. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the Earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an ''interglacial''.

The Earth has been in an interglacial period known as the [[Holocene]] for more than 11,000 years. It was conventional wisdom that "the typical interglacial period lasts about 12,000 years," but this has been called into question recently. For example, an article in ''Nature''<ref>{{cite journal
| title=Eight glacial cycles from an Antarctic ice core
| author=EPICA community members
| journal=Nature
| date=[[2004-06-10]]
| doi=10.1038/nature02599
| url=http://www.up.ethz.ch/people/flueckiger/publications/epica04nat.pdf
| volume=429
| pages=623
}}</ref> argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in [[orbital forcing]] suggest that the next glacial period would not begin before about 50,000 years from now, even in absence of man-made [[global warming]] <ref name=BergerLoutre>{{cite web |url=http://www.sciencemag.org/cgi/content/full/297/5585/1287 |title=CLIMATE: An Exceptionally Long Interglacial Ahead? |accessdate=2007-03-11 |year=2002 |publisher=Science}}</ref> (see [[Milankovitch cycles]]). Moreover, anthropogenic forcing from increased [[greenhouse gas]]es might outweigh orbital forcing for as long as intensive use of fossil fuels continues<ref>{{cite web |url=http://www.sciencedaily.com/releases/2007/08/070829193436.htm |title=Next Ice Age Delayed By Rising Carbon Dioxide Levels |accessdate=2008-02-28 |year=2007 |publisher=ScienceDaily}}</ref>.

== Positive and negative feedbacks in glacial periods ==

Each glacial period is subject to [[positive feedback]] which makes it more severe and [[negative feedback]] which mitigates and (in all cases so far) eventually ends it.

=== Processes which make glacial periods more severe ===

Ice and snow increase the Earth's [[albedo]], i.e. they make it reflect more of the sun's energy and absorb less. Hence, when the air temperature decreases, ice and snow fields grow, and this continues until an equilibrium is reached. Also, the reduction in [[Boreal forest|forests]] caused by the ice's expansion increases albedo.

Another theory has hypothesized that an ice-free [[Arctic Ocean]] leads to increased snowfall at high latitudes.{{Fact|date=September 2008}} When low-temperature ice covers the Arctic Ocean there is little evaporation or [[Sublimation (physics)|sublimation]] and the polar regions are quite dry in terms of precipitation, comparable to the amount found in mid-latitude [[desert]]s. This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes ''and'' more southerly latitudes, reducing the temperatures over land by increased albedo as noted above. (Current projected consequences of [[global warming]] include a largely ice-free Arctic Ocean within 15-20 years.) Additional fresh water flowing into the North Atlantic during a warming cycle may also reduce the [[Thermohaline circulation|global ocean water circulation]] (see ''[[Shutdown of thermohaline circulation]]''). Such a reduction (by reducing the effects of the [[Gulf Stream]]) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer. It has also been suggested that during an extensive ice age glaciers may move through the [[Gulf of Saint Lawrence]], extending into the North Atlantic ocean to an extent that the Gulf Stream is blocked.

=== Processes which mitigate glacial periods ===

Ice sheets that form during glaciations cause erosion of the land beneath them. After some time, this will reduce land below sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the lowering in [[sea level]] that accompanies the formation of ice sheets.

Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances.

== Causes of ice ages ==

The causes of ice ages remain controversial for both the large-scale ice age periods and the smaller ebb and flow of glacial–interglacial periods within an ice age. The consensus is that several factors are important: [[Earth's atmosphere|atmospheric composition]] (the concentrations of [[carbon dioxide]], [[methane]]); changes in the Earth's orbit around the [[Sun]] known as [[Milankovitch cycles]] (and possibly the [[Milky Way#Sun's location|Sun's orbit around the galaxy]]); the motion of [[Plate tectonics|tectonic plates]] resulting in changes in the relative location and amount of continental and oceanic crust on the Earth's surface, which could affect [[wind]] and [[ocean currents|ocean]] [[atmospheric circulation|currents]]; variations in [[Solar activity|solar output]]; the orbital dynamics of the Earth-Moon system; and the impact of relatively large [[meteorite]]s, and volcanism including eruptions of [[supervolcano]]es.

Some of these factors influence each other. For example, changes in Earth's atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which [[weathering]] removes CO<sub>2</sub>).

[[William Ruddiman]], Maureen Raymo, and others propose that the Tibetan and [[Colorado Plateau]]s are immense CO<sub>2</sub> "scrubbers" with a capacity to remove enough CO<sub>2</sub> from the global atmosphere to be a significant causal factor of the 40 million year [[Cenozoic#Climate|Cenozoic Cooling]] trend. They further claim that approximately half of their uplift (and CO<sub>2</sub> "scrubbing" capacity) occurred in the past 10 million years.<ref>Ruddiman, W.F. and J.E. Kutzbach. 1991. Plateau Uplift and Climate Change. Scientific American 264:66-74</ref><ref>
Raymo, M.E., W.F. Ruddiman, and P.N. Froelich (1988) Influence of late Cenozoic mountain building on ocean geochemical cycles. Geology, v. 16, p. 649-653. </ref>

===Changes in Earth's atmosphere===

There is evidence that [[greenhouse gas]] levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and vulcanism.

The [[Snowball Earth]] hypothesis maintains that the severe freezing in the late [[Proterozoic]] was ended by an increase in CO<sub>2</sub> levels in the atmosphere, and some supporters of Snowball Earth argue that it was caused by a reduction in atmospheric CO<sub>2</sub>. The hypothesis also warns of future Snowball Earths.

[[William Ruddiman]] has proposed the [[early anthropocene]] hypothesis, according to which the [[anthropocene]] era, as some people call the most recent period in the Earth's history when the activities of the human race first began to have a significant global impact on the Earth's climate and ecosystems, did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the [[Milankovitch cycles]]. In his [[overdue-glaciation]] hypothesis Ruddiman claims that an incipient ice age would probably have begun several thousand years ago, but the arrival of that scheduled ice age was forestalled by the activities of early farmers.

===Position of the continents===
The geological record appears to show that ice ages start when the continents are in [[Continental drift|positions]] which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase the Earth's [[albedo|reflectivity]] and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a [[positive feedback]] loop. The ice age continues until the reduction in weathering causes an increase in the [[greenhouse effect]].

There are three known configurations of the continents which block or reduce the flow of warm water from the equator to the poles:
* A continent sits on top of a pole, as [[Antarctica]] does today.
* A polar sea is almost land-locked, as the [[Arctic Ocean]] is today.
* A supercontinent covers most of the equator, as [[Rodinia]] did during the [[Cryogenian]] period.

Since today's Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth will continue to endure glacial periods in the geologically near future.

Some scientists believe that the [[Himalayas]] are a major factor in the current ice age, because these mountains have increased Earth's total rainfall and therefore the rate at which CO<sub>2</sub> is washed out of the atmosphere, decreasing the greenhouse effect. The Himalayas' formation started about 70 million years ago when the [[Indo-Australian Plate]] collided with the [[Eurasian Plate]], and the Himalayas are still rising by about 5 mm per year because the Indo-Australian plate is still moving at 67 mm/year. The history of the Himalayas broadly fits the long-term decrease in Earth's average temperature since the [[Eocene|mid-Eocene]], 40 million years ago.

Other important aspects which contributed to ancient climate regimes are the '''ocean currents''', which are modified by continent position as well as other factors. They have the ability to cool (e.g. aiding the creation of Antarctic ice) and the ability to warm (e.g. giving the British Isles a temperate as opposed to a boreal climate). The closing of the Isthmus of Panama about 3 million years ago may have ushered in the present period of strong glaciation over North America by ending the exchange of water between the tropical Atlantic and Pacific Oceans.<ref>[http://findarticles.com/p/articles/mi_m1511/is_n4_v17/ai_18107917 We are all Panamanians] - formation of Isthmus of Panama may have started a series of climatic changes that led to evolution of hominids</ref>

===The Uplift of the Tibetan Plateau and surrounding Mountain Areas above the Snowline===

[[Matthias Kuhle|Matthias Kuhle's]] geological theory of Ice Age development was suggested by the existence of an ice sheet covering the [[Tibetan plateau]] during the Ice Ages ([[Last Glacial Maximum]] ?). The plate-tectonic uplift of Tibet past the snow-line has led to a c. 2.4 million km² ice surface with a 70% greater [[albedo]] than the bare land surface. The reflection of energy into space resulted in a global cooling, triggering the [[Pleistocene]] Ice Age. Because this highland is at a subtropical latitude, with 4 to 5 times the insolation of high-latitude areas, what would be Earth's strongest heating surface has turned into a cooling surface.

Kuhle explains the [[interglacial]] periods by the 100 000-year cycle of radiation changes due to variations of the earth's orbit. This comparatively insignificant warming, when combined with the lowering of the Nordic inland ice areas and Tibet due to the weight of the superimposed ice-load, has led to the repeated complete thawing of the inland ice areas. <ref>Kuhle, M.(1988): The Pleistocene Glaciation of [[Tibet]] and the Onset of Ice Ages- An Autocycle Hypothesis. GeoJournal 17 (4, Tibet and High-Asia. Results of the Sino-German Joint Expeditions (I), 581-596.</ref><ref>Kuhle, M. (2004): The High Glacial (Last Ice Age and LGM) ice cover in High and Central Asia. Development in Quaternary Science 2c (Quaternary Glaciation - Extent and Chronology, Part III: South America, [[Asia]], Africa, Australia, Antarctica, Eds: Ehlers, J.; Gibbard, P.L.), 175-199. (Elsevier B.V., Amsterdam)</ref><ref>Kuhle, M. (2007): The Past Ice Stream Network in the Himalayas and the Tibetan Ice Sheet during the Last Glacial Period and its glacial-isostatic, eustatic and climatic consequences. Tectonophysics 445 (1-2), 116-144</ref>

=== Variations in Earth's orbit (Milankovitch cycles)===
The [[Milankovitch cycles]] are a set of cyclic variations in characteristics of the Earth's orbit around the sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.

It is very unlikely that the Milankovitch cycles can start or end an ice age (series of glacial periods):
* Even when their effects reinforce each other they are not strong enough.
* The "peaks" (effects reinforce each other) and "troughs" (effects cancel each other) are much more regular and much more frequent than the observed ice ages.

In contrast, there is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial periods within an ice age. The present ice ages are the most studied and best understood, particularly the last 400,000 years, since this is the period covered by [[ice core]]s that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milanković orbital forcing periods is so close that orbital forcing is generally accepted. The combined effects of the changing distance to the Sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which affect the intensity of [[seasons]]. For example, the amount of solar influx in July at 65 degrees north [[latitude]] varies by as much as 25% (from 400 [[watt|W]]/m² to 500 W/m², see graph at [http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html]). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter. Some workers believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like CO<sub>2</sub> may explain this mismatch.

While Milankovitch forcing predicts that cyclic changes in the Earth's [[Orbit#Orbital parameters|orbital parameters]] can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial–interglacial periods. In particular, during the last 800,000 years, the dominant period of glacial–interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's [[eccentricity (orbit)|eccentricity]] and orbital [[inclination]]. Yet this is by far the weakest of the three frequencies predicted by Milankovitch. During the period 3.0–0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000-year period of changes in Earth's [[obliquity]] (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.

The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000-year cycle over the last 8 cycles. [[Richard A. Muller]] and Gordon J. MacDonald [http://www.pnas.org/cgi/content/full/94/16/8329] [http://muller.lbl.gov/pages/glacialmain.htm]
[http://muller.lbl.gov/papers/sciencespectra.htm] and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100,000-year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in [[insolation]], as the earth moves in and out of known dust bands in the solar system. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Jose Antonio Rial [http://pangea.stanford.edu/Oceans/GES290/Rial1999.pdf].

Another worker, [[William Ruddiman]], has suggested a model that explains the 100,000-year cycle by the modulating effect of eccentricity (weak 100,000-year cycle) on precession (23,000-year cycle) combined with greenhouse gas feedbacks in the 41,000- and 23,000-year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41,000-year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the second or third cycle triggers an ice age. This would imply that the 100,000-year periodicity is really an illusion created by averaging together cycles lasting 80,000 and 120,000 years. This theory is consistent with the existing uncertainties in dating, but not widely accepted at present (Nature 434, 2005, [http://web.mit.edu/~phuybers/www/Doc/Obliquity_HuybersWunsch.pdf]).

===Variations in the Sun's energy output===
There are at least two types of variation in the Sun's energy output:
* In the very long term, astrophysicists believe that the sun's output increases by about 10% per billion (10<sup>9</sup>) years. In about one billion years the additional 10% will be enough to cause a runaway [[greenhouse effect]] on Earth—rising temperatures produce more water vapour, water vapour is a greenhouse gas (much stronger than CO<sub>2</sub>), the temperature rises, more water vapour is produced, etc.{{Fact|date=June 2008}}
* Shorter-term variations, some possibly caused by ''[[Hunting (engineering)|hunting]]''. Since the [[Sun]] is huge, the effects of imbalances and [[negative feedback]] processes take a long time to propagate through it, so these processes overshoot and cause further imbalances, etc.—"long time" in this context means thousands to millions of years.{{Fact|date=June 2008}}

The long-term increase in the Sun's output cannot be a cause of ice ages.

The best known shorter-term variations are [[sunspot|sunspot cycles]], especially the [[Maunder minimum]], which is associated with the coldest part of the [[Little Ice Age]]. Like the Milankovitch cycles, sunspot cycles' effects are too weak and too frequent to explain the start and end of ice ages but very probably help to explain temperature variations within them.

===Volcanism===

It is theoretically possible that undersea volcanoes could end an ice age by causing global warming. One suggested explanation of the [[Paleocene-Eocene Thermal Maximum]] is that undersea volcanoes released [[methane]] from [[clathrate]]s and thus caused a large and rapid increase in the [[greenhouse effect]]. There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.

It is challenging to see how volcanism could cause an ice age, since its cooling effects would have to be stronger than and to outlast its warming effects. This would require dust and [[Particulate|aerosol]] clouds which would stay in the upper atmosphere blocking the sun for thousands of years, which seems very unlikely. Undersea volcanoes could not produce this effect because the dust and aerosols would be absorbed by the sea before they reached the atmosphere.

==Recent glacial and interglacial phases==
{{main|Timeline of glaciation}}
{{Expand-section|Recent glacial and interglacial phases in other areas outside North America|date=March 2008}}

=== Glacial stages in North America ===<!-- This section is linked from [[Seattle, Washington]]. Changed the Seattle link to current name. Jo, 2008-01-30 -->
[[Image:Northern icesheet hg.png|thumb|Northern hemisphere glaciation during the last ice ages. The set up of 3 to 4 km thick ice sheets caused a [[sea level rise|sea level lowering]] of about 120 m.]]

The major glacial stages of the current ice age in North America are the [[Illinoian Stage|Illinoian]], [[Last glacial period|Wisconsin]] and [[Sangamon interglacial|Sangamon]] stages. The use of the Afton, Nebraskan, Kansan, and Yarmouth stages to subdivide the ice age in North America have been discontinued by Quaternary geologists and geomrophologists. These stages have all been merged into the [[Pre-Illinoian Stage]] in the 1980s<ref name="Hallberg1">Hallberg, G.R., 1986, ''Pre-Wisconsin glacial stratigraphy of the Central Plains region in Iowa, Nebraska, Kansas, and Missouri.'' Quaternary Science Reviews. vol. 5, pp. 11-15.</ref><ref name="RichmondOther1">Richmond, G.M. and D.S. Fullerton, 1986, ''Summation of Quaternary glaciations in the United States of America.'' Quaternary Science Reviews. vol. 5, pp. 183-196.</ref><ref name="GibbardOthers2007">Gibbard, P.L., S. Boreham, K.M. Cohen and A. Moscariello, 2007, [http://www.quaternary.stratigraphy.org.uk/correlation/POSTERSTRAT_v2007b_small.jpg ''Global chronostratigraphical correlation table for the last 2.7 million years v. 2007b.''], jpg version 844 KB. Subcommission on Quaternary Stratigraphy, Department of Geography, University of Cambridge, Cambridge, England</ref>

During the most recent North American glaciation, during the latter part of the [[Last Glacial Maximum|Wisconsin Stage]] (26,000 to 13,300 years ago), ice sheets extended to about 45 degrees north latitude. These sheets were 3 to 4 km thick.<ref name="RichmondOther1">Richmond, G.M. and D.S. Fullerton, 1986, ''Summation of Quaternary glaciations in the United States of America.'' Quaternary Science Reviews. vol. 5, pp. 183-196.</ref>

This Wisconsin glaciation left widespread impacts on the North American landscape. The [[Great Lakes (North America)|Great Lakes]] and the [[Finger Lakes]] were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old [[Teays River]] drainage system was radically altered and largely reshaped into the [[Ohio River]] drainage system. Other rivers were dammed and diverted to new channels, such as the [[Niagara Falls|Niagara]], which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall, at the present [[Clark Reservation State Park]] near [[Syracuse, New York]], is now dry.

The area from [[Long Island]] to [[Nantucket]] was formed from glacial [[till]], and the plethora of lakes on the [[Canadian Shield]] in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of [[loess]] many dozens of feet thick in the [[Missouri River|Missouri Valley]]. [[Isostatic rebound]] continues to reshape the [[Great Lakes]] and other areas formerly under the weight of the ice sheets.

The [[Driftless Zone]], a portion of western and southwestern Wisconsin along with parts of adjacent [[Minnesota]], [[Iowa]], and [[Illinois]], was not covered by glaciers.
{{See also|Glacial history of Minnesota}}

== Effects of glaciation ==
Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out landscape in Canada, Greenland, northern Eurasia and Antarctica. The erratic boulders, till, drumlins, eskers, kettle lakes, moraines, cirques, horns, etc., are typical features left behind by the glaciers.

The weight of the ice sheets was so great that they deformed the earth's crust and mantle. After the ice sheets melted, the ice-covered land rebounded (see [[Post-glacial rebound]]). Due to the high viscosity of the Earth, the flow of mantle rocks which controls the rebound process is very slow &ndash; at a rate of about 1 cm/year near the center of rebound today.

During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level drops by about 120 meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During deglaciation, the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed [[ice dam]]s resulting in [[salinity|salination]] of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale. It can even cause temporary reglaciation. This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the Baltic and Scandinavian regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe.

The redistribution of ice-water on the surface of the Earth and the flow of mantle rocks causes the gravitational field and the Moment of Inertia of the Earth to change. Changes in the moment of inertia result in a change in the rotational motion of the Earth. The redistribution of surface mass induced stress within the Earth and caused earthquakes (see [[Post-glacial rebound]]), according to some scientists. However, many mainstream geologists are doubtful that the effect on rotational motion, at least at the end of the last glacial maximum, was sufficient to create significant earthquake effect. That does not remove the possibility that the rebound itself generated regional tectonic effects.

==See also==
* [[Geology]]
* [[International Union for Quaternary Research]]
* [[Irish Sea Glacier]]
* [[Last Glacial Maximum]]
* [[Little ice age]]
* [[Post-glacial rebound]]
* [[Timeline of glaciation]]

==References==
{{reflist|1}}


==External links==
==External links==
*[http://www.millionwomenstudy.org/index2.html The Million Women Study]
{{commonscat|Ice age}}
*[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11384629&dopt=Abstract The Women's HOPE Study (PubMed Abstract)]
*[http://www.pbs.org/wgbh/nova/ice/ Cracking the Ice Age] from PBS
*[http://jama.ama-assn.org/cgi/content/abstract/280/7/605 HERS Study (JAMA Abstract)]
*http://www.globalchange.umich.edu/globalchange1/current/lectures/samson/climate_patterns/
*[http://www.maureenraymo.com/uplift_overview.php/ Raymo - uplift hypothesis]

{{Greenhouse and Icehouse Earth}}


{{Sex hormones}}
[[Category:Ice ages|*]]
[[Category:History of climate]]
[[Category:Glaciology]]


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Revision as of 04:50, 10 October 2008

Progestagens (also spelled progestogens or gestagens) are hormones that produce effects similar to those of progesterone, the only natural progestagen. All other progestogens are synthetic and are often referred to as progestins.

All progestagens have antiestrogenic (counteracting the effects of estrogens on the body) and antigonadotropic (inhibiting the production of sex steroids by gonads) properties.

Progestogens differ in their potency (affinity for progesterone receptors) and side-effects. Such side-effects may be androgenic (medroxyprogesterone and most C19 progestagens), antiandrogenic (cyproterone acetate), estrogenic, glucocorticoid (some C21 progestogens), or antimineralocorticoid (progesterone).

Uses

Birth control

Main article: Hormonal contraception

Antiandrogen

Progestinic compounds decrease luteinizing hormone levels [1] and as such, will have antiandrogenic properties in trans-women and cisgender males alike due to decreased LH stimulation of the testes. Cyproterone is a common example of a progestinic medication frequently used as an antiandrogen.

Progestogen withdrawal bleeding

In a normal menstrual cycle, a sudden drop in progesterone levels triggers menstruation. Norethindrone acetate (brand name Aygestin) and medroxyprogesterone acetate (brand name Provera) may be used to artificially induce progestogen withdrawal bleeding.

External links

Template:Sex hormones

  1. ^ T.B. Hatlera, S.H. Hayesa, D.L. Raya, P.S. Reamesa and W.J. Silvia "Effect of subluteal concentrations of progesterone on luteinizing hormone and ovulation in lactating dairy cows", The Veterinary Journal
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