paleoclimatology: an introduction daniel kirk-davidoff department of meteorology university of...

Paleoclimatology: An introduction

Daniel Kirk-DavidoffDepartment of Meteorology

University of Maryland

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

The Faint Young Sun and Homeostasis

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Long-Term Carbon Cycle(Berner, 2003)

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Solution ?

Energy-Balance Climate Models

(Due to Budyko, Sellars)

€

Cp(ϕ )dT (ϕ )

dt= (1− α(T (ϕ )))S(ϕ )+−(A(CO2)+ B(CO2))T (ϕ ))+

∂

∂ sin(ϕ )(1− sin2(ϕ ))D

∂T (ϕ )

∂ sin(ϕ )

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).

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?

Mass Extinctions

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The Chicxulub Crater:Due to an impact around 65 Ma

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Deccan Traps:One of the largest volcanic provinces in the world, erupted circa 60-65 Ma

Which one killed the dinosaurs?

Warm Climates

Lots of interesting issues here:

•What causes million-year scale climate changes anyway? Orthodoxy is that they’re 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 they’ve defeated models forced by greenhouse gas increases.

Temperature proxies indicate that the Eocene (~55-35 MA) was a very warm period in Earth’s history.

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!

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.

1. Increasing GHG1 Reduced T/∂(latitude)Warmer Tropical Tropopause

2. Reduced T/∂(latitude) Reduced Generation of Planetary and Gravity Waves

3. Reduced Wave Activity Reduced Propagation of Waves into the Stratosphere

4. Reduced Propagation of Waves into the Stratosphere Reduced Stratospheric Overturning

5. Reduced Stratospheric Overturning Warmer Tropical Tropopause Colder Polar Stratosphere

6. Warmer Tropical Tropopause Moister Stratosphere

7. Moister Stratosphere + Cooler Polar Stratosphere More abundant, thicker PSCs

8. Moister Stratosphere + more abundant, thicker PSCs reduced T/∂(latitude)

Equator PoleA

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390 K

430 K

290 K

Equator Pole

Alt

itud

e

310 K

350 K

390 K

430 K

290 K

b

a

€

dq =∂q

∂TtdTt

dTt =∂Tt∂ωdω +

∂TtTeqdTeq

€

dTeq =∂Teq∂CO2

dCO2

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dω =∂ω

∂A+∂ω

∂JdJ

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dA =∂A

∂(Teq −Tp )d(Teq −Tp )

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dTp =∂Tp∂CO2

dCO2

+∂Tp∂Fp

dFp

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dFp =∂Fp∂τ

∂τ

∂qdq+

∂τ

∂TstdTst

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⎠

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dJ =∂J∂CO2

dCO2€

dTst =∂Tst∂ωdω

+∂Tst∂CO2

dCO2

+∂Tst∂τdτ

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 earth’s orbital parameters.

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Tertiary Climate History

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From Billups and Schrag (2003)

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

CO2, Ice Volume, and Paleocalibration

Can we use paleoclimate evidence to predict the sensitivity of the earth to changes in CO2?

We’d need to know all the other forcing changes, and we’d 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

Antarctic Record of CO2 and Temperature

Why does CO2 change with Temperature?

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?

Odds and Ends

•Messinian desiccation

•Climate and human evolution

•Paleo-ozone depletion

•Climate and human history

Why I love Paleoclimatology, but don’t totally trust it.

•There’s 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.