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Natural Climate Variability

Global Environmental Change – Lecture 4 Spring 2015

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Mammoth Teeth and Climate• A mammoth tooth is about 15 cm across, and just

as high. They are the biggest grinding teeth in all of the animal

kingdom, bar none, and wonderfully adapted to crush woody vegetable matter

• Such teeth were found in the 18th century in several places in Siberia, attached to frozen carcasses. Thus, their origin is very well known.

• Later, in North America, beautifully preserved skeletons were found in the Rancho La Brea Tar Pits, in Hancock Park in Los Angeles.

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Georges Cuvier• Cuvier discovered that the mammoth, whose

remains were found all over the temperate zones of Europe and Asia, was not identical to now existing elephants, but rather belonged to an extinct species.

• Established the geologically recent extinction of large mammals such as the woolly mammoth (Mammuthus primigenius) and the woolly rhinoceros (Coelodonta antiquitatis), but he also put forth the phenomenon of mass extinction

• Proposed cause: catastrophic changes on the surface of the earth. In particular, rapid sea-level changes, with inundation of

low-lying areas

French naturalist and zoologist, 1769-1832

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Discovery of an Ice Age

• Louis Agassiz, a Swiss-American scientist and physician, was the first to recognize evidence for an ice age

• Started his career as Cuvier’s assistant• Trained in medicine and natural

history, he was the first to propose, in 1837, that earth had been subjected to a past ice age

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• 1807 - 1873

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Agassiz's Theory• Agassiz proposed that, "The gigantic quadrupeds, the

Mastodons, Elephants, Tigers, Lions, Hyenas, Bears, whose remains are found in Europe from its southern promontories to the northernmost limits of Siberia and Scandinavia...may indeed be said to have possessed the earth in those days. But their reign was over. A sudden intense winter, that was also to last for ages, fell upon our globe; it spread over the very countries where these tropical animals had their homes, and so suddenly did it come upon them that they were embalmed beneath masses of snow and ice, without time even for the decay which follows death.”o Agassiz, L. Geological Sketches, Ticknor and Fields, Boston, 1866.

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Louis Agassiz

• Agassiz moved to the United States in 1846

• He became professor of zoology and geology at Harvard University, and founded the Museum of Comparative Zoology

• He became interested in the last glacial advance in North America, and studied it for the remainder of his life

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Agassiz was Wrong• The onset of the Ice Age had nothing whatever to do with the

extinction of the mammoth and the woolly rhinoceros On the contrary: the giant mammals were creatures of the period of ice ages,

not its victims.  They died at the end of the last ice age, and their demise is a riddle It has been suggested that humans, or abrupt climate change, or perhaps both,

were to blame for the loss of the megafauna• The one great idea that stood the test of time was Agassiz's insistence

that there had been a lot of ice around not too long ago This idea of ice ages emphasized that climate on Earth can change greatly,

and it set the stage for climate reconstruction in Earth history

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Ice Ages• Ice ages - are times when the entire Earth experiences

notably colder climatic conditions• During an ice age

The polar regions are cold Temperature differences between the equator and the pole

increase Large, continental-size glaciers or ice sheets can cover

enormous regions of the earth

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Multiple Ice Ages• Geologist Archibald Geikie found plant fragments between layers of glacial

till in Scotland, and argued for sustained intervals of warm climate between different glacial ages

• Geologists worked in the field several decades after Agassiz’s initial discovery to refine the actual history of that astounding period

• It was found that there had been several large glaciations, one following the other, separated by warm periodso The last one of these, though large, had not destroyed entirely the evidence for the

earlier ones, some of which were even largero From the ribbons of end-moraines left by the great ice sheets, and the deposits of

wind-blown glacial dust (called "loess") there emerged the concept of four or five large glaciations during the ice-age time, during a period called the "Pleistocene." 

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Previous Ice Ages

• Two Precambrian ice ages are known 2400-2100 MYBP - Huronian 800-635 MYBP - Cryogenian

Paleozoic 450-420 - Andean-Saharan 360-260 - Karoo

Cenozoic 2.58 MYBP to 11,700 YBP - Pleistocene ice ages

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LatePaleozoicGlaciation

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Plate Tectonics and Climate

• Glaciers can only form on land

• As plates move, evidence for a cold climate, in the form of glaciation, exists primarily when land masses are present at high latitudes

• Movement of land masses also alters the oceanic circulation pattern, a vital factor in determining climate conditions

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Pleistocene Glaciation

• Began about 2.58 MYBP

• There were at least 4 glacial advances

• Climate cooled 5-10ºC during glacial episodes, warming in between

• Last episode peaked 18,000 years ago, ice covering about 30% of the earth’s surface

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Midwestern U.S. Names

• T.C. Chamberlain and F. Leveratt gave the following divisions of glacials and interglacials

Glacial Preceding Interglacial

Wisconsin, 12-60 TYBP Sangamon, 115-140 TYBP

Illinoian, 140-350 TYBP Yarmouth, 440-500 TYBP

Kansan, 500-640 TYBP Aftonian, 640-700 TYBP

Nebraskan, 780-900 TYBP

•Acronym - "Never Kiss In Winter -- After You've Sneezed"

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North American Ice Cover

• Figure shows the extent of ice cover from 18,000 to 8000 years ago

• White is ice, blue is glacial meltwater lakes

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Climate Questions

• What causes the onset of glacial conditions?

• What caused the alternation of glacial and interglacial conditions during the Pleistocene?

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Possible Causes

• CO2 concentration changes

• Changes in the brightness of the sun The earth’s orbit around the sun, modified by its

interaction with other bodies in the solar system, and rotation around its own axis, influence climate Eccentricity Obliquity Precession

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Natural Variability

• The earth’s climate has a fairly large natural variability

• Before we examine how much man is changing climate, we need to understand what contributes to natural variability

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Eccentricity

• Eccentricity is the shape of the Earth's orbit around the Sun

• Orbital shape ranges between more and less elliptical (0 to 5% ellipticity) – the drawing actually exaggerates the effect for clarity

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Cause of Eccentricity

• Eccentricity if caused by perturbations of earth’s orbit due to other bodies

• Venus, the closest planet to earth, has the largest effect

• Jupiter, because it is so massive, has a sizable effect

• Eccentricity shows peaks every 95,000 years, but superimposed on those are larger peaks at 125,000 and 400,000 years

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Eccentricity Effects

• These oscillations, from more elliptic to less elliptic, are of prime importance to glaciation

• The oscillation alters the distance from the Earth to the Sun, thus changing the distance the Sun's short wave radiation must travel to reach Earth

• This reduces or increases the amount of radiation received at the Earth's surface in different seasons Insolation is the rate of delivery of direct solar radiation per unit of

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Solar Energy Received by Earth

• At present, a difference of only about 3 percent occurs between aphelion (farthest point) and perihelion (closest point)

• The present eccentricity is near the minimum possible, so heating is almost uniform around the globe

• This 3 percent difference in distance means that Earth experiences a 6 percent increase in insolation in January than in July

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At Maximum Eccentricity

• When the Earth's orbit is most elliptical the insolation at the perihelion would be in the range of 20 to 30 percent more than at aphelion

• Continually altering the amounts of received solar energy around the globe will result in large changes in the Earth's climate and glacial regimes 

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Obliquity(Axial Tilt)

• Refers to the tilt of the earth’s axis

• The present value is 23.44°, but the value can range from 22.1° to 24.5°

• The obliquity largely accounts for the earth’s annual seasons

• The period of the obliquity is 41,000 years

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Obliquity Animation• The animation illustrates the

revolution of the earth around the sun combined with the obliquity

• This effects the insolation

• If the orbit is eccentric, instead of circular as shown, the effect increases insolation in one hemisphere while reducing it in the other

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Minimum Axial Tilt

• When the axial tilt is at a minimum, the variation between seasons is reduced

• Winter is warmer, summer is cooler

• However, reduced tilt means solar radiation is less evenly distributed between equatorial and polar regions

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Insolation Variance with Angle• Sh, the amount of solar energy received

per unit time on a unit horizontal surface at the top of the atmosphere is proportional to the cosine of θs, the solar zenith distance which is defined as the angle between the solar rays (Sr) and the normal to the Earth's surface at any particular point.o Sh=Sr cos θs

• The total amount of energy received by the perpendicular surface (SrA1 on figure 2.9) is distributed across a larger surface (SrA1 = ShA2 = (ShA1)/(cos θs)).

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Insolation Variation

• Diagram illustrates the different amounts o energy received at the equator and the poles

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Atmospheric Depth Effects

• Increasing the distance radiation passes through the atmosphere also increases scatter and absorption

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Response to Minimum Tilt

• As a reaction to a smaller degree of axial tilt, it is hypothesized that ice sheets would grow

• Warmer winter mean warmer air, which holds more moisture

• More moisture in the air would lead to a greater amount of snowfall

• Cooler summer temperatures would result in less melting of the winter's snow accumulation

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Precession

• Precession is the Earth's slow wobble as it spins on axis

• This top-like wobble, or precession, has a periodicity of 23,000 years

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Precession Video

• Top precessing in the bowl of a spoon32

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Where is Earth’s Axis Pointing?

• This means the axis points to different places in the sky over a 23,800 year period

• The precession of Earth wobbles from pointing at Polaris (North Star) to pointing at the star Vega

• When this shift to the axis pointing at Vega occurs, Vega would then be considered the North Star

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Affect of Precession

• This means that the Northern Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun

• This coincidence will result in greater seasonal contrasts

• At present, the Earth is at perihelion very close to the winter solstice – perihelion is currently January 3

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• When the axis is tilted towards Vega the positions of the Northern Hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively.

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Orbital Travel Times

• The sun is not the center of the ellipse

• This means that it takes the earth longer to travel from the vernal equinox to the autumnal equinox than from the autumnal to the vernal equinox

• This figure shows a large difference between perihelion and aphelion, but it makes the point clear

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Length of Winter and Summer

• Northern Hemisphere winter now is shorter than the Southern Hemisphere winter

• In 12,900 years, the North will have longer winters and shorter summers

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Antarctic Ice Sheet

• In whichever hemisphere winter is longer, snow will be more likely to accumulate, leading to ice sheet growth

• This was first suggested in 1842 by French mathematician Joseph Alphonse Adhémar

• He used the massive ice sheet in Antarctica as evidence, since the Southern Hemisphere currently has longer winter and shorter summer

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Croll’s Proposal

• Scotsman James Croll proposed: That changes in the orbit of Earth would result in

changes in the severity of winters, which would generate appropriate changes in snowfall

Feedback from reflection of sunlight off the snow during the rest of the year

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James Croll

• Croll combined the eccentricity of the orbit and the precession and in the 1860s and 1870s presented his ideas on the effects of the cycles and how they might influence climate, especially the colder winters when they correspond with the aphelion

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Milutin Milankovitch• Milutin Milankovitch was a Serbian

astrophysicist best known for developing one of the most significant theories relating Earth motions and long-term climate change

• He attended the Vienna Institute of Technology and graduated in 1904 with a doctorate in technical sciences

401879-1958

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Milankovitch Theory• After five years of work as a civil engineer, he accepted a

faculty position in applied mathematics at the University of Belgrade in 1909, a position he held for the remainder of his life

• During WWI, he was interned by the Austro-Hungarian army

• While interned in Budapest, he was allowed use of the library of the Hungarian Academy of Sciences

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First Publication• By the end of the war he published, in 1920, a paper whose

translated title is “Mathematical theory of thermal phenomena caused by solar radiation”

• He dedicated his career to developing a mathematical theory of climate based on the seasonal and latitudinal variations of solar radiation received by the Earth

• This idea is now known as the Milankovitch Theory

• The resulting cycles are sometimes known as the Croll-Milankovich cycles

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Milankovitch Hypothesis• Milankovitch proposed on theory of climate modification based on

variations in incoming solar radiation, caused by orbital variations of eccentricity, obliquity, and precession

• He combined all three cycles in a mathematical formulation that predicted their combined effect on climate fluctuations of the Pleistocene

• For this reason, he usually gets all the credit

• The three factors have almost no effect on the total amount of solar energy reaching the earth

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MilankovitchCycles

• The effect of the various cycles is to change the contrast between seasons44

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Effects of the Milankovitch Hypothesis

• Milder winters in high latitudes lead to climate warming, and greater snowfall

• Cooler summers would reduce snowmelt

• Combined, this might trigger ice formation, and lead to ice sheet formation

• Coupled with positive feedbacks, like the ice-albedo effect, this and trigger an ice age

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Glacial to Interglacial and Return

• As orbital cycles progress, the Milankovitch forcing will change, and climate will start to warm

• Positive feedbacks will amplify the warming

• This can explain the alternating glacial-interglacial effects seen in the Pleistocene

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Milankovitch Cycles Animation

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Acceptance of the Milankovitch Hypothesis

• Milankovitch enjoyed a considerable reputation as the result of his paper

• He drew a curve of insolation at the earth’s surface as part of his paper

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Final Acceptance

• In 1924, the great meteorologist and climatologist Wladimir Köppen, together with his son-in-law Alfred Wegener, introduced the curve in their work, entitled Climates of the geological past

• This led to wide-spread acceptance of Milankovitch’s ideas

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