Natural Climate Variability

Natural Climate Variability
Spring 2012, Lecture 10
Discovery of an Ice Age
• Louis Agassiz, a SwissAmerican scientist and
physician, was the first to
recognize evidence for an ice
• Trained in medicine and
natural history, he was the first
to propose, in 1837, that earth
had been subjected to a past ice
• 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
Ice Ages
• Ice ages - are times when the entire Earth
experiences notably colder climatic
• 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
• Two Precambrian ice ages are known
 2000 MYBP
 600 MYBP
 Late Paleozoic ice age, about 250 MYBP
 Pleistocene ice age
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
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
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?
Natural Variability
• The earth’s climate has a fairly large natural
• 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
What Causes Earth’s Seasons
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
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
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 AustroHungarian army
• While interned in Budapest, he was allowed use of
the library of the Hungarian Academy of Sciences
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
• This idea is now known as the Milankovitch Theory
Orbital Variations
• Milankovitch proposed on theory of climate
modification based on variations in incoming
solar radiation, caused by orbital variations
o Eccentricity
o Obliquity
o Precession
• Eccentricity is the shape of the Earth's orbit around the
• Orbital shape ranges between more and less elliptical
(0 to 5% ellipticity) – the drawing actually exaggerates
the effect for clarity
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
• Eccentricity shows peaks every 95,000 years,
but superimposed on those are larger peaks at
125,000 and 400,000 years
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
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
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
(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
• The period of the obliquity is 41,000 years
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
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
• 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
Precession Video
• Top precessing in the bowl of a spoon
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
• When this shift to the axis pointing at Vega
occurs, Vega would then be considered the
North Star
Affect of
• 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
• When the axis is tilted towards
• At present, the Earth is at
Vega the positions of the
perihelion very close to the
Northern Hemisphere winter
winter solstice – perihelion
and summer solstices will
is currently January 3
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
• Northern Hemisphere winter now is shorter
than the Southern Hemisphere winter
• In 12,900 years, the North will have longer
winters and shorter summers
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
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
Milankovitch Hypothesis
• Milankovitch combined all three cycles in a
mathematical formulation that predicted their
combined effect on climate fluctuations of the
• 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
• The effect of the various cycles is to change the
contrast between seasons
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 icealbedo effect, this could trigger an ice age
Glacial to Interglacial and Return
• As orbital cycles progress, the Milankovitch
forcing will change, and climate will start to
• Positive feedbacks will amplify the warming
• This can explain the alternating glacialinterglacial effects seen in the Pleistocene
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
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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