Longterm climate variation

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Long-term climate variation

• Glaciations and orbital cycles

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Background

• Something is causing the symmetrical advance and
  retreat of Northern Hemisphere glaciers
• Marine sediments show that there have been at
  least 17 prior glaciations
• Evidence is from isotopes of oxygen stored in
  foraminifera

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Paleoclimate evidence

• Shows that temperatures were about 10 deg C
  cooler during glacial episodes
• CO2 concentration was about 200 ppm
• Cycles occurred about every 100ky for the last
  700ky

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What is causing these cycles?

• A definitive answer is still lacking, but most
  scientists believe that glacial cycles are
  associated with the Milankovitch Cycles
• After Milutin Milankovitch, 1879-1958, Serbian
  astrophysicist
• Natural variation in orbital parameters

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Theory

• Summertime insolation at high latitudes is
  thought to determine glaciations
• Low summer radiation glaciation
• Due to the eccentricity of Earths orbit,
  radiation varies by time of year
• Perihelion, closest to sun, occurs during our
  winter
• Aphelion, farthest from sun, occurs during our
  summer
• Affects the seasonal contrast less contrast,
  more tendency to glaciate

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Three features

• Precession
• Obliquity
• Eccentricity
• Though to be caused by the Earths somewhat
  non-spherical shape and the pull of gravity

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Precession

• The direction that the Earth is pointed
• Currently pointed to the star Polaris
• Not always the case
• Goes through one complete rotation every 25,700
  years

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But!

• The Earths major axis along its ellipse is also
  rotating, but in the opposite direction
• Consequently, there are two main periods for the
  spin axis precession cycle
• 23,000
• 19,000

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Consequences of precession

• Every half cycle, the Hemisphere with the
  greatest seasonal contrast switches from North to
  South
• In other words, Hemispheres will switch from
  having hot summers and cold winters to having
  mild summers and mild winters
• Maximal glaciations occur with mild seasonal
  contrast, as in todays world
• In other words, we should be heading to the next
  glaciation!

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Obliquity variation

• The tilt of the Earths axis varies from about
  22-24.5 degrees
• At the winter solstice for a latitude of 40N
• At 22 degrees, insolation is 643 W m-2
• At 24.5 degrees, insolation is 590 W m-2
• At summer
• 22 1303 W m-2
• 24.5 1320 W m-2
• Seasonal amplitude
• 22 660 W m-2
• 24.5 730 W m-2

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Consequences of obliquity

• Highest obliquity produces the greatest seasonal
  contrast
• Lowest obliquity produces the lowest seasonal
  contrast
• Associated with glaciations
• Does not change total radiation
• Cycles occur at about 41,000 years

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Eccentricity

• The degree to which the Earths orbit is
  eccentric
• (Note figure 11-6 in book)

E C/A
C
A
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Eccentricity continued

• Currently, Earth eccentricity is 0.017
• Very close to a circle!
• Calculate the radiation at aphelion and
  perihelion
• Varies from 0.0 to 0.06
• Does change total radiation

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Consequences of eccentricity

• Earth receives 0.2 more radiation at maximum
  eccentricity
• Not enough to directly influence glaciations
• At 0 eccentricity, there is no perihelion or
  aphelion
• At large eccentricity, aphelion at N. Hemisphere
  summer can have a big impact
• Periods of variation are 100ky and 400ky

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Summary

• Glaciations favored by mild N. Hemisphere summers
• Low obliquity
• High eccentricity
• Precession causing aphelion (low radiation) to
  occur at N. Hemisphere summers

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Relation to isotopes

• The combined orbital forcings of the Milankovitch
  cycles do not line up exactly with the O18
  isotope data
• Dominant periodicities do
• So the orbital periods are used to tune the
  benthic data
• SPECMAP project does this for global sea bottom
  cores

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Climate system feedbacks

• The Milankovitch cycles alone do not produce
  nearly enough variation in radiation to account
  for the glacial cycles
• Some other processes must be interacting with the
  orbital forcings

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Feedback 1

• Albedo and ice
• A small decrease in temperatures
• Leads to a slight increase in ice
• Which increases albedo
• Decreases absorbed radiation
• Positive feedback to glaciation
• Try constructing a feedback loop
• Include temperature, summer radiation, ice, and
  albedo

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Feedback 2

• CO2
• Evidence from the Vostok ice core shows a very
  strong relationship between CO2 and temperature
• Very likely that the long-term carbon cycle
  influences this process
• Coral reefs
• Ca2 2HCO3- gt CaCO3 CO2 H2O
• Production of corals releases CO2
• Glacial cycles, weathering of corals, consumes CO2

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Biological pump

• Oceans photosynthesis results in the settling of
  carbon-based material to the deep ocean
• High ocean productivity low atmospheric CO2
  (biological pump very active)
• Low ocean productivity high atmospheric CO2
  (biological pump inactive)

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Nutrient availability

• Activity of the pump relies on availability of
  nutrients
• Could be increased
• Exposure of the continental shelf during
  glaciations could increase ocean nutrients
• Iron fertilization, caused by increased aridity,
  winds, dust movement (supported by paleo
  evidence)

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Productivity feedbacks

• The removal of forests during glaciations
  (release of CO2) is a negative feedback
• Would tend to slow down glaciations

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Feedback 3

• Marine productivity and methane sulfonic acid
  (MSA)
• Fast biological pump in glaciations (removal of
  carbon)
• High marine production of MSA, which acts as an
  aerosol
• Increases albedo

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Conclusions

• Isotope evidence is tuned to the Milankovitch
  cycles
• Milankovitch cycles are most likely amplified
  through a series of feedbacks leading to glacial
  and interglacial cycles