Istid

Jordens middeltemperatur vist over forskellige tidsintervaller.
Jordens glaciation/temperatur som funktion af bl.a. excentricitet og præcession.

Istid er et glaciologisk begreb, der betegner, at store dele af Jordens overflade har arktisk eller subarktisk klima.

Under istiderne sker der ophobning af store mængder af vand i gletsjerne, hvilket får hav-niveauet til at falde (eustasi). Idet istiderne er globale fænomener, hersker der under istiderne kolde forhold også i områder tættere på ækvator, end vi for eksempel oplever nu. Istider er desuden karakteriseret ved, at der forekommer gletsjere nærmere ækvator, end vi kender det i dag, men ikke nødvendigvis at der findes tykke lag is.

Det er også væsentligt at huske, at afkølingen nedsætter fordampningen fra verdenshavene. Det medfører, at luften indeholder betydeligt mindre vanddamp, og derfor bliver nedbørsmængden mindre under istiderne. Da en stor del af nedbøren yderligere falder som sne, der ikke uden videre deltager i vandets kredsløb, bliver istiderne til markante tørkeperioder for de is-frie områder, f.eks. kan det vises, at øen Korsika, der ligger langt ude i Middelhavet, har været landfast med Europa i flere omgange under istiderne. Hver gang er der indvandret kulde- og tørketålende plantearter, der endnu i dag har populationer på de høje bjerge midt på øen.

Istidsperioderne er ikke gennemgående lige kolde, men temperaturerne kan, regionalt og globalt, variere meget. I de koldere perioder rykker iskapperne frem, mens de trækker sig tilbage i de lidt mildere perioder. Dermed kan et område blive dækket af is i flere omgange under en enkelt istid, f.eks. blev Danmark delvis dækket af is 3 til 4 gange under seneste istid, Weichsel-istiden, der fandt sted mellem 110.000 og 15.000 år før nu. Den sydvestlige del af Jylland formodes at have været isfrit under hele sidste istid.

I runde tal har der i de sidste omtrent 2,6 millioner år været ca. 15 kolde perioder (istider) hver med en længde på omkring 100.000 år[kilde mangler]. Istiderne var afbrudt af mellemistiderne (interglacialer) på ca. 10.000 til 15.000 års længde. I Danmark har man spor af 4 – muligvis 5 istider med isdække.

I Jordens nyere geologiske historie kaldes epoken med istider og mellemistider for Pleistocæn. Den nuværende mellemistid efter den sidste istid kaldes Flandern, og er synkron med epoken Holocæn. Tiden siden starten af de seneste istider (for 2,6 mio. år siden) kaldes Kvartær.

Der har gennem geologisk tid været flere glaciationer. I Sen Ordovicium dækkede den Hirnantiske Istid det nuværende Sahara. Der er indikationer af, at Jorden i løbet af Prækambrium har været helt dækket af is gennem længere tid, den såkaldte "Snowball Earth"-teori. Som klodens geografi er nu, udgør istiderne det normale mønster. De klimabetingelser, vi oplever i dag, hører til undtagelserne, da Jorden i 75 % af kvartærtiden har været istid.

Dette skyldes, at det arktiske verdenshav har så lille en forbindelse med de varme tropiske have. Derfor hænger istiderne sandsynligvis sammen med pladetektonikken, altså kontinenternes bevægelser på klodens overflade. Kun når kontinenternes placering hindrer en udveksling mellem koldt og varmt vand, vil der opstå istider. Og kun når kontinenterne ligger tæt nok på polerne, vil der dannes langvarige iskapper.

Pleistocæne istider og mellemistidsfaser

De angivne tidsintervaller er ca. tider for Danmark, men man skal være opmærksom på, at tiderne er behæftet med en betydelig usikkerhed – især for perioderne ældre end Eem. Man skal også være opmærksom på, at Danmark ingenlunde har været isdækket hele tiden i istiderne – store dele af istidsperioderne har Danmark været mere eller mindre isfrit men har haft et meget koldt klima med arktisk eller subarktisk vegetation og fauna.

For de korresponderende istider i Alperne og Nord-Amerika skal man være opmærksom på, at der kun er tale om et delvist sammenfald. Der er tale om korrespondance i rækkefølgen, mens tidspunkterne, for hvornår Alperne og især Nord-Amerika blev ramt af istiderne, kan variere ganske betydeligt. I alle tilfælde skal man være opmærksom på, at der er tale om ændringer i klimaet, der sker over tusinder af år. De angivne perioder er dem, der er definerende for perioden – de er ikke nødvendigvis udtryk, for hvornår istiden præcis ramte (eller ikke ramte) et givent område.[1]

BetegnelsePeriode (år f.v.t)
NordeuropaAlperneNordamerika
WeichselWürmWisconsinistid (seneste)115.000 - 9.600
Eemmellemistid128.000 - 115.000
SaaleRissIllinoisistid386.000 - 128.000
HolstenYarmouthmellemistid418.000 - 386.000
ElsterMindelKansasistid465.000 - 418.000
Cromermellemistid850.000 - 465.000
Bavel-komplekset-1.030.000 - 850.000
MenapGünzNebraskanistid1,2 - 1,03 mio.
Waalmellemistid1,45 - 1,2 mio.
Eburonistid1,8 - 1,45 mio.
Tegelenmellemistid2,4 - 1,8 mio.
Prætegelenistid2,588 - 2,4 mio.

For Holsten til Prætegelen er anvendt tider fra List of geochronologic names, tider der stemmer godt overens med "Naturen i Danmark – Geologien"[2]. For Saale stemmer tiden kun med "Naturen i Danmark – Geologien".

Kilder/referencer

  1. ^ http://www.quaternary.stratigraphy.org.uk/charts/chartversions/POSTERSTRAT_v2010(2).jpg
  2. ^ Larsen, Gunnar & Sand-Jensen, Kaj: "Naturen i Danmark – Geologien", 2006, Gyldendals Forlag, ISBN 87-02-03027-6

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Milankovitch Variations.png
Forfatter/Opretter: This image was produced by Robert A. Rohde from publicly available data, and is incorporated into the Global Warming Art project., Licens: CC BY-SA 3.0
This figure shows the variations in Earth's orbit, the resulting changes in solar energy flux at high latitude, and the observed glacial cycles.

According to Milankovitch Theory, the precession of the equinoxes and the apsides, variations in the tilt of the Earth's axis (obliquity) and changes in the eccentricity of the Earth's orbit are responsible for causing the observed 100 kyr cycle in ice ages by varying the amount of sunlight received by the Earth at different times and locations, particularly high northern latitude summer. These changes in the Earth's orbit are the predictable consequence of interactions between the Earth, its moon, and the other planets.

The orbital data shown are from Quinn et al. (1991). Principal frequencies for each of the three kinds of variations are labeled. The solar forcing curve (aka "insolation") is derived from July 1st sunlight at 65 °N latitude according to Jonathan Levine's insolation calculator [1]. The glacial data are from Lisiecki and Raymo (2005) and gray bars indicate interglacial periods, defined here as deviations in the 5 kyr average of at least 0.8 standard deviations above the mean.
Iceage time-slice hg.png
Forfatter/Opretter: Hannes Grobe/AWI, Licens: CC BY 3.0

The prelude, initiation and progression of the current ice age is shown in six different time slices of temperature change (180 Mio yr, 800 kyr, 150 kyr, 18 kyr, 1 kyr, 120 yr). The grey shaded box is the extracted time slice given in the following graf in a higher resolution. Start reading from upper right to upper left:

  • The decrease in temperature during the last 35 million years is due to changes in ocean current systems controlled by the movement and distribution of the continents (plate tectonic). This long-term cooling is the prelude to the ice age of the Quaternary.
  • Climate variations (glacial/interglacial cycles) during the ice age of the last 2 Million years are controlled by Milankovitch cycles in the earth orbit around the sun (excentricity, obliquity, precission).
  • The last glacial/interglacial cycles show a saw tooth shape - with a steep increase in temperature at the termination of a glacial and a slow cooling towards the following glacial.
  • The last glacial ended at about 18 kyr (21 calendar kiloyears before present), followed by a temperature increase of some degree up to the Holocene climate optimum, interrupted by a short cooling event (Younger Dryas).
  • The medival warm period is followed by the little ice age presumably caused by changes in the radiation of the sun.
  • During the last 100 years, a prominent temperature increase starting at the end of the 20th century calls for influence of mankind on climate caused by burning of fossil fuel.

Please keep in mind: All grafs are principle scetches and do NOT reflect the most recent knowledge of climate change in detail!

Detailed original description:

(1) Mean global temperature through the last 180 million years, derived from oxygen isotope analyses of various marine and terrestrial deposits (from L.A. Frakes, Climates Through Geologic Time, Elsevier, Amsterdam, 1979). The present (ca. 1900) condition, for reference, is shown as a horizontal line. Of note are (1) a global cooling trend since the time of the Cretaceous, when global surface temperatures were 8-10°C warmer than today, and (2) the onset of a continuing series of deeper, periodic glacial/interglacial oscillations in the latest, Quaternary period. Also shown (dark band) is the range of modeled surface temperature based on a doubling of atmospheric CO2, projecting an increase from present values of about 2-5°C (Crowley, 1990 doi:10.1175/1520-0442(1990)003<1282:ATASGA>2.0.CO;2). Note that a different linear time scale is used for each of the three geologic divisions.

(2) Surface temperature through the last 850,000 years, derived from measurements of the ratio of 16O to 18O in fossil plankton which had settled to the sea floor and were recovered in a deepsea core from the equatorial Pacific Ocean (Shackleton and Opdyke, 1973 doi:10.1016/0033-5894(73)90052-5). The changes mainly reflect variations in global ice volume; the scale used here was added to show schematically the probable associated changes in global average surface temperature, based on a model-derived difference of 4-6°C between full glacial and full interglacial conditions (Clark, Carbon Dioxide Review, Oxford University Press, New York, 1982). The reference line at 15°C corresponds to surface temperatures of the modern era. The glacial/interglacial oscillations, characteristic of the Pleistocene epoch, are now thought to be induced by periodic variations in the orbit of the earth and in its axis of inclination (the Milankovitch effect), which act together to bring about systematic changes in the seasonal distribution of sunlight over the surface of the planet.

(3) Air temperature over Antarctica, expressed as a difference from the modern surface temperature value. These estimates are derived from hydrogen/deuterium ratios measured in an ice core from the Vostok station in Antarctica (Jouzel et aI., 1987 doi:10.1038/329403a0). Of note are the present (Holocene) and the preceding, somewhat warmer "Eemian" interglacial periods, each characterized by a rapid onset to an early interglacial maximum temperature,and a subsequent, slower decline. The glacial period between, called the Wisconsin glaciation in the Americas, is itself characterized by significant variations in temperature that fall systematically to a coldest extreme (maximum glaciation) about 20,000 years before the present (B.P.).

(4) Variations in surface temperature, estimated from a variety of sources, principally isotope ratios from Greenland ice cores, for the last 18,000 years. The onset and subsequent character of the present interglacial or Holocene epoch are depicted. Of note are century-scale oscillations in temperature, identified in the Greenland record and in certain European lakes, during the period of deglaciation between about 15,000 and 10,000 years B.P., and a broad Holocene maximum about 5000-6000 years B.P., when summer temperatures may have been 1-2°C warmer than the present era. At these expanded scales, the temperature excursions depicted in this and the subsequent graph are the most conjectural of the set (modified from J.T. Houghton et aI., Climate Change: The IPCC Assessment, Cambridge University Press, Cambridge, 1990).

(5) Variations in surface air temperature estimated from a variety of sources, including temperature-sensitive tree growth indices and written records and accounts of various kinds, largely from western Europe and eastern North America. Of note is a possible protracted global warming through the Medieval period, when surface temperatures may have averaged about 0.3°C warmer than the A.D. 1900 reference. It was followed by a longer period of much colder conditions, loosely termed the Little Ice Age, when the estimated global mean temperature may have fallen about 0.6°C below the reference norm, reflecting global temperatures almost 1°C lower than the values attained during the middle of the current century (modified from a not-to-be-taken-literally schematic in Houghton et al., 1990).

(6) Globally averaged, direct measurements of the combined sea surface temperature and air temperature over the land, shown in this case relative to 1951-80. A stepped warming of about 0.6°C is evident, qualified in the consensus 1990 IPCC Report as 0.3-0.6°C to reflect uncertainties in the data used (from J.T.Houghton et aI., 1990).