Warm and sensitive Paleocene-Eocene climate e

Transcription

1 Warm and sensitive Paleocene-Eocene climate num e ric a mo l del lin g Malte Heinemann 1),2) Johann H. Jungclaus1), Jochem Marotzke1) Max Planck Institute for Meteorology, Hamburg, Germany 2) IMPRS - Earth System Modelling 1)

2 late Paleocene to early Eocene (55 million years) homo sapiens (160 thousand years) dinosaurs extinct (60 million years) Pangaea breaks up (180 million years) cyanobacteria (photosynthesis) (3 billion years) earth forms (4.6 billion years) big bang (15 billion years) today Introduction

3 Introduction Titanoboa 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 Cenozoic (last 65 million years) (Zachos et al 2001) crocodiles & turtles near Arctic (Estes and Hutchinson 1980) giant snake in Columbia (Head et al. 2009, snake paleo-thermometry)

4 temperature proxy δ18o (benthic forams) Introduction PETM modified from Zachos et al. (2001) Paleocene Eocene time [million years ago] Antarctic glaciation 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)

5 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 latitudes imply a low equatorto-pole temperature gradient equable climate climate models could not reproduce the small equator-to-pole temperature gradient suggesting that models lacked highlatitude warming (or tropical cooling) mechanism (Barron 1987, Huber and Sloan 2001)

6 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: methane hydrates from marine sediments (Dickens et al. 1995) IFM-GEOMAR 2002 methane hydrate hypothesis requires a large climate sensitivity not previously simulated (Pagani et al. 2008) also: requires a trigger!

7 age [million years] Constraining pco2 increase during PETM carbon isotope excursion major carbon reservoirs (after Nunes & Norris 2006) (after Ridgwell & Edwards 2007) δ13c [o/oo] 1 amount of carbon release necessary to explain carbon isotope excursion depends on carbon source

8 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)

9 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)

10 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

11 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

12 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)

13 Paleocene-Eocene boundary conditions depth [m] height [m] topographic reconstruction from Bice and Marotzke (2001) rivers flow along height gradients, no lakes, no glaciers homogeneous soil and vegetation parameters

14 some parameters Paleocene/Eocene pre-industrial reference

15 Simulated annual mean surface temperature Paleocene-Eocene pre-industrial reference [K] Paleocene-Eocene simulation is on average 9.4K warmer than the pre-industrial reference Paleocene-Eocene high latitudes are sea-ice-free

16 zonal mean sea surface temperature [K] Comparison to temperature reconstructions 310 2) ) 4) crosses indicate reconstructed pre-petm sea surface temperatures 5) 6) 1) latitude [on] 90 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]

17 zonal mean PE-PR surface temperature difference [K] Warming relative to pre-industrial reference 40 warming is largest at high latitudes average warming of 9.4 K latitude [on] 90 Paleocene-Eocene simulation (PE) exhibits a smaller equator-to-pole temperature gradient than the pre-industrial reference (PR)

18 Analysis of the warming / radiative budget pre-industrial PE planetary albedo α = SWtop SWtop ε = LW LW longwave emissivity top s reduced albedo and reduced emissivity in the PaleoceneEocene run (PE) cause a warming

19 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 T 4 (y) y top incoming shortwave radiation (y: latitude) Stefan-Boltzmann constant

20 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 SW (y)[1 α(y)] + F(y) = ε(y) σ T 4 (y) y top incoming shortwave radiation Cv (y: latitude) Stefan-Boltzmann constant convergence of meridional energy transport

21 zonal mean PE-PR surface temperature difference [K] 1-D energy balance model results ECHAM5/MPI-OM energy balance model latitude [on] 90 apply ECHAM5/MPI-OM-diagnosed albedo, emissivity, and energy transport convergence to energy balance model energy balance model nicely fits ECHAM5/MPI-OM

22 zonal mean PE-PR surface temperature difference [K] Energy balance model results 40 black line: total warming blue line: effect of emissivity latitude [on] 90 2/3 of warming due to emissivity, 1/3 due to planetary albedo

23 zonal mean PE-PR surface temperature difference [K] Energy balance model results black line: total warming blue line: effect of emissivity red line: effect of emissivity and albedo black red: meridional energy transport latitude [on] 90 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

24 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

25 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

26 pco2 [ppm] CO2 sensitivity experiments x pre-industrial x pre-industrial x pre-industrial x pre-industrial simulated time [years] 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)

27 global mean surface temperature [K] Paleocene-Eocene climate sensitivity 1120ppm runaway climate K K simulated time [years] 840ppm 560ppm 280ppm 3200 our model is much more sensitive than previous PE models (Shellito et al found +2K per pco2 doubling) reduction of pco2 to 280ppm leads to cold climate, not appropriate as a pre-petm surrogate

28 Comparison to warming during the PETM sea surface temperature increase due to pco2 increase from 560 to 840ppm 5K 8K 4K 3K 3K 9K blue dots and numbers indicate paleo-locations and reconstructed warming [K] pco2 increase by 280ppm is enough to cause a warming comparable to that during the PETM

29 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,

30 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.

31 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

32 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?

33 Meridional overturning circulation (560ppm) depth [km] streamfunction global latitude [on] Pacific Atlantic latitude [on] latitude [on] [Sv] sinking in the North Atlantic and the Southern Ocean upwelling in the North Pacific

34 Horizontal velocity in the Atlantic (560ppm) 1700m magnitude [m/s] m magnitude [m/s] North Atlantic deep water forms western boundary current; Antarctic bottom water flows northward in eastern Atlantic zonal structure challenges deep water track reconstructions

35 depth [km] Sensitivity of meridional overturning to pco2 560ppm 840ppm streamfunction streamfunction global latitude [on] global latitude [on] [Sv] increasing pco2 leads to a weaker, shallower overturning

36 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.

37 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

38 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!