Presentation on theme: "Introduction AOGCM results and Eocene/PETM ocean circulation Hydrate modelling Conceptual model for hyperthermal orbital pacing A mechanism for the orbital."— Presentation transcript:
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 Zachos Stephen 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) CO ppmv = 6* 1120ppmv = 4* 560ppmv = 2*
Runs at 1,2,4,6 * pre-industrial CO 2. ~3500 years Global mean temperature vs CO 2 forcing
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, 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 seasonalityMaximum SH seasonality 1km Temperature difference, Minimum seasonality minus 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 Forcing:Response: Millions of years CH 4 [kg/m2]
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 CO 2 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? 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
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
ppmv What are the reasons for the differences between models…? And the differences between models and data…?