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Warm and sensitive Paleocene- climate num m eri o cal del ling Malte Heinemann1),2) Johann H. Jungclaus1), Jochem Marotzke1)

1) Max Planck Institute for Meteorology, Hamburg, Germany 2) IMPRS - Earth Modelling Introduction today late Paleocene to early Eocene (55 million ) homo sapiens (160 thousand years)

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big bang (15 billion years) Introduction cerrejonensis precloacal vertebrae compared to vertebrae of 3.4m Boa constrictor

artist's reconstruction / Jason Bourque (2008) late Paleocene to early Eocene was the warmest period during the (last 65 million years) (Zachos et al 2001) crocodiles & turtles near Arctic (Estes and Hutchinson 1980)

giant in Columbia (Head et al. 2009, snake paleo-thermometry) Introduction

) PETM

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short-lived global warming event known as the Paleocene/Eocene Thermal Maximum (PETM)

PETM associated with an increase of atmospheric greenhouse gas concentrations (e.g., Dickens et al. 1995) Research question (1)

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

the reconstructed warm high imply a low equator- to-pole temperature gradient — “equable” climate

climate models could not reproduce the small equator-to-pole temperature gradient — suggesting that models lacked high- warming (or tropical cooling) mechanism (Barron 1987, Huber and Sloan 2001) Research question (2)

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

magnitude of pCO2 increase not well constrained

one suggested CO2 source: hydrates from

marine sediments (Dickens et al. 1995) IFM-GEOMAR 2002

methane hydrate hypothesis requires a large not previously simulated (Pagani et al. 2008)

also: requires a trigger! Constraining pCO2 increase during PETM carbon isotope excursion major carbon reservoirs (after Nunes & Norris 2006) (after Ridgwell & Edwards 2007)

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55.4 -2 -1 0 1 13 o δ C [ /oo] amount of carbon release necessary to explain carbon isotope excursion depends on carbon source Research question (3)

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

present-day conveyor belt (W. Broecker, modified by E. Maier-Reimer) Research question (3)

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

the dissociation of methane hydrate requires a trigger

based on ocean modelling, Bice and Marotzke (2001) suggested that a large-scale ocean circulation change may have caused bottom water warming and methane hydrate melting paleo-reconstructions support the notion of an ocean circulation switch at the onset of the PETM (Nunes and Norris 2006) Outline

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

Summary Outline

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

Summary

(1) climate Numerical climate model coupled atmosphere – ocean – sea ice general circulation model COSMOS-AO consists of:

atmosphere: ECHAM 5.3 (T31 L19) developed from ECMWF operational forecast model; spectral dynamical core; parameterisation package developed in Hamburg (Roeckner et al. 2003)

ocean and sea-ice: MPI-OM 1.2 (144x87 L40) ocean model based on primitive equations for hydrostatic Boussinesq fluid; simple sea-ice dynamics and thermodynamics (Marsland et al. 2003, Jungclaus et al. 2006) Paleocene-Eocene boundary conditions

depth [m] height [m] 5000 2500 0 0 1500 3000

topographic reconstruction from Bice and Marotzke (2001)

rivers flow along height gradients, no lakes, no glaciers

homogeneous soil and vegetation parameters some parameters Paleocene/Eocene pre-industrial reference Simulated annual mean surface temperature

Paleocene-Eocene pre-industrial reference

[K] 210 240 260 270 280 285 290 295 300 305 310 315

Paleocene-Eocene simulation is on average 9.4K warmer than the pre-industrial reference

Paleocene-Eocene high latitudes are sea-ice-free Comparison to temperature reconstructions

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K 310 crosses indicate reconstructed

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e 270 s -90 -45 -30 -15 0 15 30 45 90 latitude [oN] most sea surface temperature reconstructions within seasonal variability simulated Arctic surface 11 to 13K colder than reconstructed

1) Thomas et al. (2002) [δ18O]; 2)+3) Tripati and Elderfield (2004) [Mg/Ca]; 4) Zachos et

al. (2003) [TEX86]; 5) Zachos et al. (2006) [TEX86]; 6) Sluijs et al. (2006) [TEX86] Warming relative to pre-industrial reference

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Paleocene-Eocene simulation (PE) exhibits a smaller equator-to-pole temperature gradient than the pre-industrial reference (PR) Analysis of the warming / radiative budget

pre-industrial PE

= ↑ ↓ planetary albedo α SWtop SWtop 0.318 0.292

= ↑ ↑ longwave emissivity ε LWtop LWs 0.585 0.541

reduced albedo and reduced emissivity in the Paleocene- Eocene run (PE) cause a warming 0-D energy balance model

compute surface temperature from model balancing incoming shortwave and outgoing longwave radiation

albedo emissivity surface temperature

↓ ∂ SW (y)[1− α(y)] + F(y) = ε(y)σ T4 (y) (y: latitude) top ∂y incoming Stefan-Boltzmann constant shortwave radiation 1-D energy balance model

compute zonal mean surface temperature from simple model balancing incoming shortwave radiation, outgoing longwave radiation and horizontal energy transport

albedo emissivity surface temperature

↓ ∂ 4 SW (y)[1− α(y)] + CvF(y) = ε(y)σ T (y) (y: latitude) top ∂y incoming Stefan-Boltzmann constant shortwave convergence of radiation meridional energy transport 1-D energy balance model — results

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o t 0 z -90 -45 -30 -15 0 15 30 45 90 latitude [oN] 2/3 of warming due to emissivity, 1/3 due to planetary albedo Energy balance model — results

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o t 0 z -90 -45 -30 -15 0 15 30 45 90 latitude [oN] 2/3 of warming due to emissivity, 1/3 due to planetary albedo meridional energy transport changes have regional effects, but hardly influence the pole-to-equator temperature gradient Conclusions (1)

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

we get close

the simulated Arctic surface temperature is still too cold

reduction of PE equator-to-pole temperature gradient due to radiative effects, rather than due to meridional energy transport changes Outline

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

Summary

(2) climate sensitivity CO2 sensitivity experiments

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pCO2 increase necessitates 2 modifications of ECHAM5: 1- ensure positive definite optical thicknesses in longwave radiation scheme; else: ECHAM5 crashes (not shown) 2- adapt ozone climatology to increased tropopause height in warming climate; else: artificial warming (not shown) Paleocene-Eocene climate sensitivity

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s 292 2000 2400 2800 3200 simulated time [years] our model is much more sensitive than previous PE models

(Shellito et al. 2009 found +2K per pCO2 doubling)

reduction of pCO2 to 280ppm leads to cold climate, not appropriate as a pre-PETM surrogate Comparison to warming during the PETM

sea surface temperature increase due to

pCO2 increase from 560 to 840ppm

5K blue dots and numbers indicate 8K paleo-locations and 4K 3K reconstructed 3K warming 9K [K] 0 2 3 4 5 6 7 10

pCO2 increase by 280ppm is enough to cause a warming comparable to that during the PETM Implication for methane-hydrate hypothesis

dissociation of methane hydrates could have caused release of 1000 to 2000Pg C (Dickens et al. 1995)

methane oxidises to CO2 (quick compared to duration of the PETM)

2000Pg C are equivalent to about 960ppm pCO2

our results suggest that the PETM warming only requires

some 280ppm pCO2 — still leaving room for carbon uptake by the ocean, vegetation, … Conclusions (2)

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

According to our results, the Paleocene-Eocene was very

sensitive to a variation of pCO2.

The climate sensitivity is large enough to allow for the methane hydrate hypothesis. Outline

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

Summary

(3) ocean circulation switch Approach previous study (Bice and Marotzke 2001)

stronger hydrological forcing (ocean model) → switch from Southern Ocean to North Pacific deep water formation → bottom water warming (→ methane melting → PETM) we

vary atmospheric pCO2 (including all feedbacks) → ocean circulation switch? Meridional overturning circulation (560ppm)

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d 4 5 global Pacific -90 -60 -30 0 30 60 90 0 30 60 90 0 30 60 90 latitude [oN] latitude [oN] latitude [oN] [Sv] -40 -20-15-12-9 -6 -3 -1 0 1 3 6 9 12 15 20 40

sinking in the North Atlantic and the Southern Ocean in the North Pacific Horizontal velocity in the Atlantic (560ppm)

1700m magnitude 3070m magnitude [m/s] [m/s] 1 0.1 0.1 0.03 0.05 0.01 0.02 0.006 0.01 0.003 0.004 0.001 0.001 0.0006 0.0004 0.0003 0.0002 0.0001 0 0 North Atlantic deep water forms western boundary current; Antarctic bottom water flows northward in eastern Atlantic zonal structure challenges deep water track reconstructions Sensitivity of meridional overturning to pCO2 560ppm 840ppm streamfunction streamfunction 0

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d 4 5 global global -90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90 latitude [oN] latitude [oN] [Sv] -40 -20-15-12-9 -6 -3 -1 0 1 3 6 9 12 15 20 40

increasing pCO2 leads to a weaker, shallower overturning Conclusions (3)

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

No.

Increasing pCO2 leads to a generally shallower, weaker ocean circulation.

The zonal structure of the simulated horizontal velocities challenges deep water track reconstructions. Outline

(1) Can we reproduce the warm and equable Paleocene-Eocene climate using a state of the art climate model?

(2) What caused the Paleocene-Eocene Thermal Maximum?

How sensitive was the PE climate to pCO2?

(3) Can we confirm the hypothesis that an ocean circulation switch caused the methane hydrate melting — using a coupled model?

Summary

summary Summary

Warm climates in Earth’s past challenge our understanding of climate system processes and our ability to project them.

(1) We present the first coupled Paleocene/Eocene simulation

with moderate pCO2 that gets close to reconstructions.

(2) The Paleocene / Eocene Thermal Maximum (PETM) probably was not triggered by an ocean circulation switch.

(3) A relatively small input of carbon – possibly from methane hydrates – could have caused the PETM. Thanks!