lecture 7

Paleoclimate andthe Ice Ages

Ruth Valley Glacier Alaska--Theglacier occupying Yosemite Valleyprobably looked similar.

Over the past 2 millionyears, the earth has hadice sheets which havevaried in size, causingalternations between iceages and interglacialperiods. This entire timeperiod is known as thePleistocene.

Glaciers also exist athigh altitudes in thetropics. The annualaccumulation of snow,which provides climateinformation at yearlyresolution, is clearlyvisible in this photographof a tropical glacier.

Key definitionscryospheresnow cover

glacierice sheet

Where are the ice sheets in thepresent-day climate?In the northern hemisphere, Greenlandis covered by an enormous ice sheetwhich is a few kilometers thick andmore than five times larger than thestate of California in area. If this icesheet were to melt, global sea levelwould rise by about 7 meters.This is all that remains of the huge icesheets that covered North Americaand Eurasia during the last glacialmaximum, about 20,000 years ago.

In the southern hemisphere, the Antarctic continent iscovered with an even larger ice sheet. The Antarcticcontinent is about 25% larger than the United States. Ifthis ice sheet were to melt entirely, global sea level wouldrise by about 70 meters. About 80% of the planet’sfreshwater is lockedup in this ice sheet.The Greenland icesheet contains about9% of the planet’sfreshwater, with theremainder beingcontained mostly inlakes and mountainglaciers.

The extent of thenorthernhemisphere icesheets at thelast glacialmaximum. Theexpansion ofthese ice sheetsresulted in a sealevelapproximately130 meterslower thantoday’s.

Deformation of the land surface,such as glacial moraines, areevidence of ice sheet extent

Because of the lowered sea level, the coastlinesduring the last ice age differed significantly fromtoday’s. North America was connected to Eurasia bya land bridge where the Bering strait is today, and theMediterranean basin was cut off from the Atlantic.

The climate in North America was significantly different...

It is possible to reconstruct anapproximate chronology of theice ages by measuring theoxygen isotopes in oceansediment cores. As marineorganisms perish, theirskeletons are deposited on thebottom of the sea floor. Thelayers of deposited skeletonsprovide information aboutclimate.

OXYGEN ISOTOPESThe chemical elements are often found in more thanone form. The number of protons in the atomicnucleus is constant for any given element, but thenumber of neutrons in the nucleus can vary. Thechemical behavior of isotopes of the same element isvery similar, the main difference being their weight.Oxygen primarily comes in two isotopes: Oxygen 16and Oxygen 18. Oxygen 18 has two extra neutrons,making it about 10% heavier than Oxygen 16.

ICE VOLUME EFFECT ON OXYGEN ISOTOPESThe ratio of the heavier to the lighter oxygen isotope variesin the ambient seawater because of the variations in totalice volume. The water molecules containing the heavierisotope are less likely evaporate and be incorporated intothe ice sheet. So as total ice volume increases, the oceanbecomes increasingly enriched in the heavier isotope ofoxygen.

TEMPERATURE EFFECT ON OXYGEN ISOTOPESWhen temperatures are colder, the organismsincorporate more of the heavier isotope of oxygen intotheir skeletons. Measuring the ratio of the heavier tothe lighter oxygen isotope in the skeletons thereforeindicates how warm the water was when the organismwas alive.

OXYGEN ISOTOPE SIGNATURE IN SEDIMENT CORESSo when ocean temperatures are cold and ice volume islarge, the skeletons should be enriched in the heavierisotope. Presumably temperature and ice volume aretightly correlated, so an examination of the oxygen isotoperecord in ocean sediment cores gives us a clear picture ofwhen the ice ages occurred.

This is the oxygen isotope record from the Pacific Oceanoff the coast of South America. It reveals that the ice agesoccurred approximately once every 100,000 years.

Variations in theearth’s orbit

The orbits of the planets are ellipses (from Kepler’s law). Ifyou take any two points (called foci) in space, you can definean ellipse as the collection of points whose combined distanceto the two points is a constant. In the case of the planets’ orbit,the sun is at one of the two foci of the elliptical orbit.

The point in the orbitwhen the a planet isclosest to the sun iscalled perihelion. Thepoint when the planet isfurthest from the sun iscalled the aphelion.

In the case of earth’s orbit, perihelion occurs inearly January, while aphelion occurs in early July.The planet gets more sunshine at perihelion, andless at aphelion, because of the inverse square law.

The planet orbits fasterwhen it is closest to thesun. So perihelion is thepoint in the orbit whenthe planet is traveling thefastest. This is why ourcalendar months haveirregular lengths.

Pope Gregory XIII hired someastronomers to create theGregorian calendar (1582) toguarantee the solstices andequinoxes would always fallon the 21st of Dec, Mar, Jun,and Sep.

The eccentricity of the earth’s orbit varies between 0 an0.06 with a period of about 100,000 years and 400,000years. This variation is due to the gravitational pull ofother planets.

The axis about which theearth rotates is tiltedrelative to the plane ofthe earth’s orbit. This tilt,called obliquity, is thereason why our planethas seasons. Currentlythe earth’s obliquity is23.5 degrees. If theobliquity were 0 (i.e. notilt), the northernhemisphere would getabout the same amountof sunshine all yearround.

The tilt of a spinning object can vary; earth isno exception. The obliquity of the earth variesbetween about 22 and 24 degrees with a periodof about 41,000 years.

The direction of theearth’s spin axis alsoprecesses. Thismeans that theearth’s spin axistraces out a circle.Like obliquity,precession occurson very long timescales. It takes about20,000 years for thespin axis to trace outa circle once.

The situation now.Aphelion occurs inearly July andcoincides with northernhemisphere summer.

The situation about10,000 years ago inthe opposite phase ofthe earth’sprecessional cycle.Aphelion occurs inearly January andcoincides withsouthern hemispheresummer.

Summary: how orbital variations affect sunshineObliquity When obliquity is high, seasonality is enhanced. Inmid and high latitudes, more sunshine comes in summer, andless comes in winter. Averaged over the whole year, the highlatitudes receive more sunshine, and the low latitudes receiveless.Precession When perihelion occurs at summer solstice, thecontrast between summer and winter is enhanced. Whenperihelion occurs at winter solstice, seasonality is weakened.Precession has little effect on annual mean sunshine.Eccentricity Changes in the eccentricity of the earth’s orbithave very little effect on the annual mean sunshine anywhere.However, they do have a large impact on the amplitude of theprecessional cycle. If the earth’s orbit is very eccentric, thetiming of perihelion in the calendar year becomes more critical.

The periodicities associated with variations in the earth’sorbit are dominant in chronologies of ice volume!

It is possible to quantify how muchvariability is associated with everytime scale using a statisticaltechnique called spectrum analysis.We can apply this technique to theice volume time series, and measurehow much variability occurs at theorbital time scales.

Spectrum of ice volume history

Note the clear peaks in the spectrum at the time scales associated withorbital variability. In fact over 95% of the variability occurs at these timescales. The idea that orbital variations might be responsible for climatevariability is called the Milankovitch theory of climate.

Milutin Milankovitch (1879-1958)

So how might orbital variations cause ice ages?One idea is that since the ice sheets melt the mostin the summertime, the sunshine during this timeof year might be important for determining icevolume.But exactly how orbital variability is translatedinto the huge observed climate changes remainsone of the major unsolved problems of climate.Any satisfactory explanation will probably involvethe dynamics of the ice sheets themselves.

Another major unsolved problem: why do the greenhousegases vary in phase with the ice ages?