A mechanism for the orbital pacing
of Eocene hyperthermals
Dan Lunt, Andy Ridgwell, Appy Sluijs, Jim Zachos
Stephen Hunter, Alan Haywood
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Introduction
AOGCM results and Eocene/PETM ocean circulation
Hydrate modelling
Conceptual model for hyperthermal orbital pacing
Orientation…
Palaeocene-Eocene
Thermal Maximum
Zachos et al, Nature,
2008
Causes of PETM have been
debated – implications for
carbon input and therefore
climate sensitivity
Dunkley Jones et al, 2010
One hypothesis: release of methane hydrates due to enhanced
warming:
…and “data”:
some evidence from ocean-only modelling.
Bice and Marotzke, 2004
Nunes and Norris,
2006
So, what did we do? – Fully coupled HadCM3(L).
(1) Palaeogeography
Topography
Vegetation
(2) CO2
1680ppmv = 6*
1120ppmv = 4*
560ppmv = 2*
Zachos et al, Nature,
2008
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
Streamfunctions: 1*,2*,4*,6*
Normalised 1km ocean Temperature, Eocene 2*-1*, 4*-1*, 6*-1*
Mixed Layer depth, 1* 2*, 4*, 6*
DJF:
JJA:
Implications for PETM:
Bice and Marotzke, 2004
Conclusions (1)
• 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.
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
1km Temperature difference,
Minimum seasonality minus
Maximum SH seasonality
Maximum SH seasonality
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
Response:
Forcing:
CH4 [kg/m2]
300
250
200
150
100
50
0
6
6.2
6.4
6.6
6.8
7
7.2
Millions of years
7.4
7.6
7.8
8
Conceptual ‘threshold’ model
for combined CO2 and orbital
impacts on circulation,
hydrate, and temperature,
Informed by GCM and
hydrate modelling
Conclusions (2)
• 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
Warm Climates of the Past
– a lesson for the future?
10 - 11 October 2011
The 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
Eocene Model Intercomparison Project (‘EoMIP’) results
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
280
560
1120
2240
4480 ppmv
What are the reasons for the differences between models…?
And the differences between models and data…?