Paleoclimatology: An introduction

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Paleoclimatology An introduction

• Daniel Kirk-Davidoff
• Department of Meteorology
• University of Maryland

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Problems in Paleoclimate

• The Faint Young Sun, Snowball Earth and long-term
  homeostasis
• Mass Extinctions Bolide Impacts, Volcanic
  Catastophes Global Anoxia
• Warm Climates
• Ice Ages Trends and Cycles
• Paleocalibration
• Odds and Ends

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The Faint Young Sun and Homeostasis
255 K
248 K
Blackbody Temperature
241 K
233 K
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Long-Term Carbon Cycle(Berner, 2003)
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Solution ?
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Energy-Balance Climate Models
(Due to Budyko, Sellars)
Where T(?) is the zonal mean temperature as a
function of latitude, S(?) is the insolation,
??????? is the albedo (expresed as a function of
temperature), A and B represent infrared cooling
of the earth as a function of surface temperature
and greenhouse gases, and D is a diffusivity
(this last term is sometimes replaced by a
Newtonian restoration to an equilibrium
temperature profile).
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Snowball Earth

• Basic idea something happens to reduce CO2
  fluxes into the atmosphere
• World ices over completely
• Weathering shuts down
• CO2 builds up to very high levels, until
• Ice melts back rapidly to nothing.
• Arguments
• How thick was the ice?
• How do you model weather processes in a global
  ice age?
• If there was so much ice, how did life survive in
  the ocean?

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Mass Extinctions
Deccan Traps One of the largest volcanic
provinces in the world, erupted circa 60-65 Ma
The Chicxulub Crater Due to an impact around 65
Ma
Which one killed the dinosaurs?
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Warm Climates

• Lots of interesting issues here
• What causes million-year scale climate changes
  anyway? Orthodoxy is that theyre forced by
  changes in mantle convection, which in turn force
  CO2, but our knowledge of past CO2 levels is
  highly leveraged, and probably highly uncertain.
  
• Even for more recent climates, warm climates are
  so warm, especially in polar regions, that
  theyve defeated models forced by greenhouse gas
  increases.

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Temperature proxies indicate that the Eocene
(55-35 MA) was a very warm period in Earths
history.
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• Eocene (and Cretaceous!) polar warmth is
  difficult to explain
• If it's due to enhanced greenhouse effect, why
  aren't the tropics warmer?
• Solutions involving enhanced dynamical fluxes
  have been proposed, but suffer from a basic
  difficulty if fluxes are in any sense
  diffusive, how can more heat be transported
  across a smaller gradient? Only by much stronger
  stirring!
• Emanuel (2001) proposes that hurricanes provide
  this stirring for the ocean hurricanes might
  occur over a much broader swath of the ocean in a
  climate with warmer poles but will this help to
  warm the continental interiors?

Perhaps we need a radiative solution!
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Can Polar Stratospheric Clouds provide the
solution? Sloan and Pollard (1998) inserted
optically thick PSCs in a GCM and showed a very
substantial warming of polar regions.
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1. Increasing GHG 1 Reduced ?T/?(latitude) ?Warmer
Tropical Tropopause
8. Moister Stratosphere more abundant, thicker
PSCs ? reduced ?T/?(latitude)
2. Reduced ?T/?(latitude) ? Reduced Generation of
Planetary and Gravity Waves
3. Reduced Wave Activity ? Reduced Propagation
of Waves into the Stratosphere
7. Moister Stratosphere Cooler Polar
Stratosphere ? More abundant, thicker PSCs
4. Reduced Propagation of Waves into the
Stratosphere ? Reduced Stratospheric Overturning
6. Warmer Tropical Tropopause ? Moister
Stratosphere
5. Reduced Stratospheric Overturning ? Warmer
Tropical Tropopause 1 Colder Polar Stratosphere
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a
430 K
390 K
Altitude
350 K
310 K
290 K
Equator
Pole
b
430 K
390 K
Altitude
350 K
310 K
290 K
Pole
Equator
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Ice Cycles Trends and Cycles

• Total global ice signal recorded in variations of
  oxygen isotopes of marine shelled creatures
  (foraminifera, others) relevant process is
  Rayleigh Distillation
• The recorded signal has oscillations that relate
  to the variations of earths orbital parameters.
  

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Tertiary Climate History
From Billups and Schrag (2003)
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Continental Drift
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Obliquity Variations
Note Martian obliquity variations are much
larger, due to the absence of a large moon.
This has interesting consequences for Martian
paleoclimatology
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CO2, Ice Volume, and Paleocalibration
Can we use paleoclimate evidence to predict the
sensitivity of the earth to changes in CO2? Wed
need to know all the other forcing changes, and
wed need to know how to separate forcing from
response. Then we can just take Change in
Temperature and divide it by Change in Forcing
?T/?Q
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Antarctic Record of CO2 and Temperature
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Why does CO2 change with Temperature?
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• Covey et al., 1996 assembled the data and came up
  with this graph
• Difficulties with paleocalibration
• Do we really know the error bars?
• What about the possibility of different responses
  to different forcing?
• What if only meridional fluxes are forced?

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Odds and Ends

• Messinian desiccation
• Climate and human evolution
• Paleo-ozone depletion
• Climate and human history

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Why I love Paleoclimatology, but dont totally
trust it.

• Theres so much to study! Almost anything that
  could have happened seems to have happened!
• Geologists are cool-they travel to interesting
  places, go camping a lot, will sometimes take you
  along, always need a hand.
• Because the time scales are so large, small,
  simple models are often required, and
  publishable!
• BUT. The story is always changing. Biases in
  estimates are impossible to know for sure. Big
  picture usually stays the same, but details
  change a lot.