Natural Climate Variability

Spring 2012, Lecture 10

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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

• 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

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

experiences notably colder climatic conditions• During an ice age

The polar regions are cold There are large differences in temperature from

the equator to the pole Large, continental-size glaciers or ice sheets can

cover enormous regions of the earth

Previous Ice Ages

• The climate history of earth is under active investagation

• Two Precambrian ice ages are known 2000 MYBP 600 MYBP

Late Paleozoic ice age, about 250 MYBP Pleistocene ice age

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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 1.6 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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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

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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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

• We also need to remember what causes earth’s seasons

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What Causes Earth’s Seasons

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Orbital Influence on Climate

• 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

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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

141879-1958

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

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Orbital Variations

• Milankovitch proposed on theory of climate modification based on variations in incoming solar radiation, caused by orbital variationso EccentricityoObliquityo Precession

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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

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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 received solar energy in January than in July

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At Maximum Eccentricity

• When the Earth's orbit is most elliptical the amount of solar energy received 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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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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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 which 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 spoon

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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.

Length of Winter and Summer

• 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

• 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 Frenchman 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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James Croll

• Scotsman James 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

• For this reason, Milankovitch cycles are sometimes called Croll-Milankovitch cycles

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Milankovitch Hypothesis

• Milankovitch 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 seasons 34

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 could 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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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

• Insolation refers to the amount of solar energy received at a given point on earth’s surface

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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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