• Introduction• AOGCM results and Eocene/PETM ocean circulation• Hydrate modelling• Conceptual model for hyperthermal orbital pacing

A mechanism for the orbital pacing of Eocene hyperthermals

Dan Lunt, Andy Ridgwell, Appy Sluijs, Jim ZachosStephen Hunter, Alan Haywood

Zachos et al, Nature, 2008

Orientation…

Palaeocene-Eocene Thermal Maximum

Dunkley Jones et al, 2010

Causes of PETM have been debated – implications for carbon input and therefore climate sensitivity

Bice and Marotzke, 2004

One hypothesis: release of methane hydrates due to enhanced warming:

Nunes and Norris, 2006

some evidence from ocean-only modelling.

…and “data”:

Topography

Vegetation

(1) Palaeogeography

So, what did we do? – Fully coupled HadCM3(L).

Zachos et al, Nature, 2008

(2) CO2

1680ppmv = 6*

1120ppmv = 4*

560ppmv = 2*

Runs at 1,2,4,6 * pre-industrial CO2. ~3500 years

Global mean temperature vs CO2 forcing

Normalised Surface Temperature, Eocene 2*-1*, 4*-1*, 6*-1*

‘Kink’ in HadCM3 climate sensitivity

Normalised 1km ocean Temperature, Eocene 2*-1*, 4*-1*, 6*-1*

Streamfunctions: 1*,2*,4*,6*

Mixed Layer depth, 1* 2*, 4*, 6*

DJF:

JJA:

Implications for PETM:

Bice and Marotzke, 2004

• Eocene model shows fairly linear surface temperature response to CO2, but…

• Deep ocean warms non-linearly, associated with reduction in deep water formation and switch in ocean circulation.

• Implications for hydrates: initial forcing (e.g. volcanic) could cause ‘run-away’ positive feedbacks.

• Supported by other modelling work (Winguth et al, 2010)

Lunt et al, Geology, 2010.

Conclusions (1)

Evidence for multiple hyperthermals…

And orbital pacing….

See also Westerhold et al.,

Lourens et al

“Hence, we suggest that the extreme seasonal contrast at both hemispheres during eccentricity maxima increased intermediate seawater temperatures, thereby triggering the release of oceanic methane hydrates.”

6 Additional simulations with orbital pertubations, 1000years in length:

Maximum seasonality: obliquity=0.43 rad, eccentricity=0.054

Minimum seasonality: obliquity=0.38 rad, eccentricity=0

Mixed layer depth, JJA

Minimum seasonality Maximum SH seasonality

1km Temperature difference,

Minimum seasonality minus Maximum SH seasonality

0

50

100

150

200

250

300

6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8

Time-dependent sediment hydrate model of Davie and Buffett (2001).

Simulates hydrate evolution, given a bottom-water temperature forcing, plus other (poorly constrained) variables.

Here: depth = 1500m, at ‘bullseye’ of orbitally-induced temperature anomaly

Forcing:Response:

Millions of years

CH

4 [

kg/m

2]

Conceptual ‘threshold’ model for combined CO2 and orbital impacts on circulation, hydrate, and temperature,

Informed by GCM and hydrate modelling

• GCM responds in a similar way to orbital forcing as to CO2 forcing.

• Conceptual model, informed by GCM and hydrate model results, produces results qualitatively in agreement with data.

• Therefore, we propose a possible mechanism for orbital pacing of hyperthermals, via hydrate destabilisation

Conclusions (2)

Warm Climates of the Past – a lesson for the future?10 - 11 October 2011The Royal Society, London

Speakers include: Jim Hansen, Bette Otto Bliesner, Jeff Kiehl, Paul Valdes

Possible reasons for circulation switch:

Comparison with data:

Temperature ANOMALIES, PETM – ‘backgound’ Eocene

Lunt et al, Geology, 2010

Heinemann et al, Climate of the Past, 2009

Winguth et al, Journal of Climate, 2010

Huber et al, PPP, 2006

Roberts et al, EPSL, 2009

Panchuk et al, Geology, 2008

Eocene Model Intercomparison Project (‘EoMIP’) results

280 560 1120 2240 4480 ppmv

What are the reasons for the differences between models…? And the differences between models and data…?