Transcript Milancovitch Cycles

Milankovitch Cycles and
the Big Chill
CGF3MI
Friday, April 1, 2016
Milankovitch’s Big Chill
Milankovitch cycles are cycles in the Earth’s
orbit that influence the amount of solar
radiation striking different parts of Earth at
different times of the year.
To explain this we are interested in three
types of variations in Earth’s orbital patterns:
eccentricity of the orbit,
obliquity (axial tilt),
and precession.
Milankovitch’s Big Chill
So what is The Milankovitch Theory?
Milankovitch’s Big Chill
The Milankovitch or astronomical theory of climate
change is an explanation for changes in the
seasons which result from changes in the earth's
orbit around the sun.
The theory is named for Serbian
astronomer Milutin Milankovitch(18791958) , who calculated the slow
changes in the earth's orbit by careful
measurements of the position of the
stars, and through equations using the
gravitational pull of other planets and
stars.
Milankovitch’s Big Chill
Milankovitch used the
results of previous
scientists to compute the
geographic and seasonal
distribution of sunlight for
the past million years and
showed that the effect of
the tilt on climate is very
important.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Milankovitch’s Big Chill
Joseph Alphonse Adhemar (1797 — 1862) was a French
mathematician. He was the first to suggest that ice ages
were controlled by astronomical forces in his 1842 book
Revolutions of the Sea.
The essence of his theory was that the amount of
radiation received by the southern hemisphere was
less than that received in the north, thus accounting
for the Antarctic ice sheet.
Adhemar's theory was initially rejected, but later
further developed and greatly modified, by James
Croll.
Milankovitch’s Big Chill
James Croll (1821-1890)
was largely self-educated
became an apprentice wheelwright,
but gave that up due to ill-health.
a tea merchant… manager of a
temperance hotel…insurance agent
He was appointed as a janitor in the
museum at Glasgow, in 1859.
Milankovitch’s Big Chill
Croll’s theory built on Adhemar’s and considered of the
effects of variations of the earth's orbit on climate cycles.
idea was that decreases in winter sunlight would
favour snow accumulation
first to couple this to the idea of a positive icealbedo feedback to amplify the solar variations.
Suggested ice ages occur on 22,000 year cycles in
each hemisphere, and alternate between southern and
northern hemispheres, lasting approximately 10,000
years each.
This is wrong, but that was not known then!
Milankovitch’s Big Chill
Feedback Loop: Reinforcement which either accelerates
(positive feedback) or retards (negative feedback) a
process.
The accumulation of glacial snow and ice increasing the
albedo of the surrounding region, thus cooling the air,
thus accelerating glacier growth.
http://www.homepage.montana.edu/~geol445/hyperglac/glossary.htm#feedback_loop
Milankovitch’s Big Chill
By the end of the 19th century, Croll's theory was widely
disbelieved.
However, the basic idea of orbitally-forced insolation
variations influencing terrestrial temperatures was further
developed by Milankovitch.
Milankovitch’s Big Chill
Milankovitch determined that the earth "wobbles" in its orbit.
The earth's "tilt" is what causes seasons, and changes in
the tilt of the earth change the strength of the seasons.
The seasons can also be accentuated or modified by the
eccentricity (degree of roundness) of the orbital path around
the sun.
Then the precession effect, which affects the position of the
solstices in the annual orbit.
http://www.ngdc.noaa.gov/paleo/milankovitch.html
Milankovitch’s Big Chill
And now the theory….
Milankovitch’s Big Chill
Eccentricity of the Orbit
Earth’s orbit around the
sun is a flattened
ellipse, so there are
times when it is closer
to the sun (perihelion)
and times when it is
farther from the sun
(aphelion). Currently,
perihelion occurs in
January and aphelion in
July.
Earth
Sun
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Milankovitch’s Big Chill
Eccentricity of the Orbit
When e=0, the ellipse is a circle. When e is very near 1, the
ellipse is very long and skinny.
Milankovitch’s Big Chill
Eccentricity of the Orbit
Milankovitch’s Big Chill
Eccentricity of the Orbit
Variation in orbit affects the amount of energy different
areas on Earth receive by season.
Over time, the elliptical shape of the orbit is more
circular, or flattens.
The change in the ellipse increases the effects the variation.
Milankovitch’s Big Chill
Eccentricity of the Orbit
When orbit is highly elliptical, one hemisphere will
have hot summers and cold winters.
The other hemisphere will have warm
summers and cool winters.
When orbit is nearly circular, both hemispheres will
have similar seasonal variations in temperature.
Milankovitch’s Big Chill
Eccentricity of the Orbit
The amount of variation in energy receipt will be less than
about 0.2%, but has an important impact on expansion or
melting of ice sheets.
The variation in the shape of the orbit occurs on the
order of every 100,000 years and is known as Orbital
Eccentricity.
Milankovitch’s Big Chill
Eccentricity of the Orbit
Variation in the
eccentricity of the Earth's
orbit over the last
750,000 years. The blue
line traces the
eccentricity. The orange
line shows today's value
for comparison. The data
are from Berger and
Loutre (1991).
Periodicity: ~100,000 years
http://www.museum.state.il.us/exhibits/ice_ages/why_4_cool_periods.html
Milankovitch’s Big Chill
Obliquity of the Axis
The second part of
the theory deals with
the tilt of Earth’s axis.
Over time, the tilt
varies. Today it is
about 23.5 degrees,
but ranges from 21.6
to 24.5 degrees.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
What happens if the tilt decreases?
Milankovitch’s Big Chill
Obliquity of the Axis
Closely linked with
Precession of Equinoxes,
causing up to 15% variation
of energy in high latitudes
that greatly affects growth
and melting of ice sheets.
Milankovitch’s Big Chill
Obliquity of the Axis
Changes in tilt cause large
changes in seasonal distribution
of energy at high latitudes, and
variations in length of polar
night.
Has very little effect on low
latitudes
Milankovitch’s Big Chill
Obliquity of the Axis
Periodicity: ~41,000 years
http://www.museum.state.il.us/exhibits/ice_ages/why_4_cool_periods.html
Milankovitch’s Big Chill
Precession of the Equinoxes
The third part of the theory deals
with Precession of the Equinoxes.
Equinox is the time when the sun is
directly over the equator.
March 21 and September
21 at present.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Milankovitch’s Big Chill
Precession of the Equinoxes
Over time, Earth’s axis
wobbles like a spinning top,
and this changes the timing of
the equinoxes.
Changing timing of wobble
mixed with changes in
timing of perihelion and
aphelion affect the seasonal
balance of energy receipt.
Milankovitch’s Big Chill
Precession of the Equinoxes


At present the Earth passes
through the perihelion
during the northern winter
(January), with the Earth's
proximity to the Sun during
this period lessening
somewhat the harsh winter
cold in this hemisphere.
the Earth reaches its
aphelion during the
northern summer (July),
the greater distance
between the planet and
Sun at this time softens the
summer heat.
Milankovitch’s Big Chill
Precession of the Equinoxes
Milankovitch’s Big Chill
Precession of the Equinoxes
Periodicity: ~23,000 years
http://www.museum.state.il.us/exhibits/ice_ages/why_4_cool_periods.html
Milankovitch’s Big Chill
Putting it all together…
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Adding up the “little bits”
Orbital
Eccentricity
100,000 yrs
variation in
energy receipt
~0.2%,
Obliquity of
the Axis
41,000 yrs
15% variation
of energy in
high latitudes
Precession of
the Equinoxes
23,000 yrs
Slight
variation in
seasonal
intensity
6
4
2
0
0
200000
400000
600000
800000
1000000
-2
-4
-6
Precession of the Equinoxes
Eccentricity of the Orbit
Obliquity of the Axis
6
4
2
0
0
200000
400000
600000
800000
1000000
-2
-4
-6
Precession of the Equinoxes
Eccentricity of the Orbit
Obliquity of the Axis
6
4
2
0
0
200000
400000
600000
800000
1000000
-2
-4
-6
Precession of the Equinoxes
Eccentricity of the Orbit
Obliquity of the Axis
Milankovitch’s Big Chill
Putting it all together…
In discussion with Koppen
(climate distribution over
Earth) and Wegener (plate
tectonics), he found that
summer snow melt is most
affected by changes.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf

A 1976 study, published in the
journal Science examined deep-sea
sediment cores and found that
Milankovich's theory corresponded to
periods of climate change. Indeed,
ice ages had occurred when the
earth was going through different
stages of orbital variation.
Milankovitch’s Big Chill
Putting it all together…
Any decrease in intensity of summer sunlight would inhibit
melting, making the annual snow budget positive and leading
to growth of the ice sheets.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Milankovitch’s Big Chill
Putting it all together…
Formulated a mathematical relationship between
summer radiation and the altitude of snowline
(elevation at which there is at least some snow year
round).
Determined the increase in snow cover that would result
from any given change in summer radiation.
http://www.geo.ucalgary.ca/~wu/TUDelft/IceAgeIceModel.pdf
Milankovitch’s Big Chill
Putting it all together…
Summer insolation at 65 degrees North is the most
important latitude and season to model
Great ice sheets grew
near this latitude and
that cooler summers
might reduce summer
snowmelt, leading to
a positive annual
snow budget and ice
sheet growth.
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_3.html
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
Theory suggests that the primary driver of ice ages is the
total summer radiation received in northern latitude zones
where major ice sheets have formed in the past, near 65
degrees north.
Past ice ages correlate well to 65N summer insolation
(Imbrie 1982).
Milankovitch’s Big Chill
Putting it all together…
Astronomical calculations show that 65 degree north summer
insolation should increase gradually over the next 25,000
years
No declines in 65 degree north summer insolation sufficient
to cause an ice age are expected in the next 100,000 years
(Hollan 2000).
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
For about 50 years, Milankovitch's theory was largely ignored.
1976, a study published in the journal Science examined
deep-sea sediment cores and found that Milankovitch's
theory did in fact correspond to periods of climate
change (Hays et al. 1976).
the authors were able to extract the record of
temperature change going back 450,000 years
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_3.html
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
Oxygen isotope data from deep sea cores confirm the
existence of the astronomical cycles with periods of 100, 43,
24, and 19 thousand years.
These correspond to peaks in
eccentricity, tilt and two in precession.
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
They found that major
variations in climate were
closely associated with changes
in the geometry (eccentricity,
obliquity, and precession) of
Earth's orbit.
Indeed, ice ages had occurred when the
Earth was going through different
stages of orbital variation!
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
Then, what does Milankovitch Theory say about
future climate change (and consequently, our
future!)?
Orbital changes occur over thousands of years, and
the climate system may also take thousands of years
to respond to orbital forcing.
Milankovitch’s Big Chill
Putting it all together…accepting the theory.
It would appear that we are
in an interglacial rather than
a post glacial world.
http://www.learn360.com/ShowVideo.aspx?ID=348514