The Heartbeat of Climate

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The Heartbeat of Climate

• Earths climate engine is the Sun
• Small variations in solar radiation can have
  large climate effects
• Feedbacks can amplify or reduce the insolation
  changes

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The Heartbeat of Climate

• Daily Cycle

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The Heartbeat of Climate

• Daily Cycle
• Annual cycle

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What determines seasons on Earth?
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The Annual Cycle
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The Heartbeat of Climate

• Daily Cycle
• Annual cycle
• Milankovitch cycles

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Milankovitch Variations
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Milankovitch Variations

• Precession - Earths Wobble
• Obliquity - Earths Tilt
• Eccentricity - Earths Elliptical Orbit

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Precession - Earths Wobble
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Precession - 20,000 years
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Precession of the Equinoxes
Present
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Precession of the Equinoxes
11,000 years from now
Northern Winter
Northern Summer
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Precession of the Equinoxes
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Precession of the Equinoxes

• Earths wobble on its axis and the rotation of
  Earths orbit around the sun together change the
  season of closest approach to the sun.
• These two effects have periodicities of 23,000
  and 19,000 years.

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Obliquity - 40,000 years
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Axial Tilt

• Tilt of Earths axis varies between 22.1º and
  24.5º.
• Tilt changes with a periodicity of 41,000 years.
• Tilt affects the poles, primarily. Low tilt
  makes the poles milder.

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Obliquity - 40,000 years
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Eccentricity - 100,000 years and 400,000 years
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The Orbital Parameters

• Axial Tilt (41,000 year period)
• Precession (19,000 and 23,000 year periods)
• Eccentricity (100,000 and 400,000 year periods)

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First Attempt

• Joseph Adhémar in 1842 made the first formal
  attempt at a theory of how Earths orbit affected
  climate.
• Adhémar focused on precession of the equinoxes,
  and he argued that changes in total insolation
  caused glacial-interglacial oscillations.
• Precession does not affect total insolation, so
  Adhémars ideas were rejected.

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James Croll

• In 1864, James Croll invoked combined effects of
  eccentricity and precession to explain
  glacial-interglacial oscillations.
• Croll was the first to propose that the seasonal
  distribution of insolation could produce changes
  in global climate.

http//www.ngdc.noaa.gov/paleo/slides/images/base/
iceage07.jpg
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More Croll

• Croll predicted a 22,000 year cycle of ice ages
  during glacial epochs.
• Based on the assumed importance of eccentricity,
  Croll argued that last ice age ended 80,000 years
  ago.
• Crolls emphasis on seasonality caused him to
  conclude that ice ages should be out of phase
  between hemispheres.

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Precession of the Equinoxes
Present
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Precession of the Equinoxes
11,000 years from now
Northern Winter
Northern Summer
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Not Quite, Croll

• Crolls carefully crafted hypothesis was
  rejected.
• Last glacial maximum was only 18,000 years ago,
  not 80,000 years ago.
• Glacial periods occur synchronously (at the same
  time) in the northern and southern hemispheres.

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Milutin Milankovitch

• Through the early part of the 20th century,
  Serbian mathematician Milutin Milankovitch worked
  on a complete orbital theory, including all three
  orbital parameters.
• Milankovitch focused on summer insolation.

http//www.ngdc.noaa.gov/paleo/ctl/images/iceage11
.jpg
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Milankovitch Theory

• Milankovitch emphasised summer because he
  thought glacial melting was the key to
  persistence of ice.
• Milankovitch argued that major periodicities were
  19,000, 23,000, and 41,000 years.
• Eccentricity was not believed to be important.

http//www.geo.arizona.edu/palynology/geos462/mmil
anko.gif
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Testing Milankovitch

• Milankovitch produced detailed latitude-specific
  predictions of insolation as a function of time.
• To test Milankovitch theory, scientists needed to
  develop better tools
• Tools for determining the age of deposits
• Tools for determining climate.

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http//www.ngdc.noaa.gov/paleo/slides/images/base/
iceage13.gif
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Determining Absolute Age

• Radiometric dating
• Magneto-stratigraphy

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Radioactive Decay
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Cosmic radiation of Nitrogen 14
creates Carbon 14 via neutron capture
Living things take up carbon, including carbon 14
Upon death, uptake of Carbon 14 ceases, but decay
continues. Carbon 14 releases electron and
reverts to Nitrogen 14
e
Proton
Neutron
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Problems

• Carbon 14 allows us to look back only about
  50,000 years, because after that, 14C has decayed
  such that it cannot be accurately measured.
• Intermediate half-life radionuclides and
  magnetostratigraphy save the day.

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Uranium Decay Series

• Much was learned about uranium in the early 20th
  century.
• Uranium 234 with a half life of 245,000 years
  proved to useful for dating glacial-interglacial
  cycles.
• In particular corals take up Uranium 234 from sea
  water.

http//www.doh.wa.gov/ehp/rp/air/22020Rad8.gif
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Magnetostratigraphy

• Use reversals of the Earths magnetic field to
  constrain the age of rocks.
• Method extends deep into Phanerozoic history.

http//www.geo.arizona.edu/palynology/geos462/pmab
utlr.gif
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Magnetostratigraphy and ?18O
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Measuring Paleoclimate

• Glacial deposits
• Loess
• Till
• Sea level indicators
• Raised Terraces
• Oxygen isotopes
• Temperature indicators
• Plankton Species composition
• Oxygen isotopes

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Baoji Loess Outcrop

• Loess is windblown dust deposited in front of
  glaciers.
• Loess deposits like the one at right are
  cyclical.
• Deposits of glacial till are also cyclical.

http//www.ngdc.noaa.gov/paleo/slides/images/base/
iceage17.jpg
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Raised Reefs

• Around the globe, reefs are exposed above sea
  level.
• Some raised reefs are the product of uplift, but
  others have been exposed by falling sea level.
• Dating raised reefs reveals periods of globally
  high sea level associated with interglacial
  periods.

http//www.nd.edu/acasad/images/wedding/honeymoon
/857-999-0019.jpg
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Raised Reefs
Imbrie and Imbrie, 1979
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The Terrestrial Record

• The terrestrial record of glacial deposits and
  sea-level indicators was incomplete. Glaciers
  are good at rearranging Earths surface.
• Nonetheless, dated features pointed strongly to a
  100,000 year periodicity to glacial-interglacial
  oscillations.
• Milankovitch thought the 100,000 cycle of
  eccentricity was weak.
• Milankovitch theory was not widely accepted.

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Turning to the Deep Sea

• The deep sea sediment record is more complete
  than the terrestrial record.
• High resolution bio- and magnetostratigraphy
  could push record of change back through several
  glacial cycles.

http//www.ngdc.noaa.gov/paleo/slides/images/base/
iceage14.jpg
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What Forams Reveal

• Foraminifera (forams) fossils are useful metres
  of climate.
• Species assemblages respond to shifts in water
  temperature
• Shells contain record of ocean chemistry.

http//www.ngdc.noaa.gov/paleo/slides/images/base/
iceage16.jpg
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The 100,000 Year Cycle

• The species composition and isotopic signature of
  deep-sea forams both showed 100,000 year
  periodicity.
• Are the shorter-period oscillations of axial tilt
  and precession present?

Imbrie and Imbrie, 1979
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Enter Fourier Analysis

• Extracting higher frequency periods is
  challenging due to uncertainty in absolute ages.
• Hays et al in 1976 demonstrated that Fourier
  Analysis could be used to extract higher
  frequency signals.
• Fourier analysis is based on Fouriers theorem
  that any periodic function can be represented by
  a set of sine and cosine waves of various
  amplitude and frequency.

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Fourier Analysis
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Fourier Analysis
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Fourier Analysis
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Fourier Analysis
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Fourier Analysis
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Milankovitch Emerges!

• Fourier analysis produces a plot of amount of
  variance in signal accounted for by different
  frequencies.
• Hays et al (1976) analysis showed peaks at all of
  the dominant Milankovitch frequencies.

Imbrie and Imbrie, 1979
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But Wait

• Orbital forcing is relatively weak, so why is
  climate response so strong?
• Some positive feedback must exist in the system.

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Ice Cores
http//www.exploratorium.edu/climate/cryosphere/da
ta/vos-e4-2.gif
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Conclusions

• Ice ages are caused by changes in the Earths
  orbit about the sun.
• All major frequencies predicted by Milankovitch
  are important.
• The dominance of the 100,000 year cycle is not
  well understood.
• The carbon cycle plays a key, not yet understood
  role in amplifying the effects of orbital
  variability.

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Ceara Rise - Eq. Atlantic
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