1. EGU 2007, CR140 Dan Lunt Introduction. GCM and ice sheet simulations. Conclusions. Other mechanisms for inception. Future plans. The closure of the Panama Seaway and the onset of Northern Hemisphere glaciation: Cause or Coincidence? D.J. Lunt, P.J. Valdes A. HaywoodI.C. Rutt

2. EGU 2007, CR140 Dan Lunt The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence? Zachos et al, 2001 CAUSE OR COINCIDENCE? Bartoli et al, 2005

3. EGU 2007, CR140 Dan Lunt Hypothesis to test Closure of the Seaway enhancement of Atlantic circulation warmer SSTs increased evaporation at high latitudes. increase in precipitation inception of Northern Hemisphere Glaciation + increase understanding of climate / ice sheet interactions for future climate change! Ruddiman, Earths Climate The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

4. EGU 2007, CR140 Dan Lunt ~ 370 m depth Closed Seaway (late Pliocene)Open Seaway (early Pliocene) The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence? 2 experiments with a fully-coupled atmosphere-ocean General Circulation Model (AOGCM), HadCM3.

5. EGU 2007, CR140 Dan Lunt Surface air temperature, closed Seaway – open Seaway Bi-polar response. Max 7 o C warming in North Atlantic following closure. Global average: +0.25 o C The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

6. EGU 2007, CR140 Dan Lunt Atlantic Circulation [Sv] Atlantic circulation, Pliocene closed SeawayAtlantic circulation, Pliocene open Seaway The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

7. EGU 2007, CR140 Dan Lunt Precipitation change, Closed-Open The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

8. EGU 2007, CR140 Dan Lunt Ice-sheet model – GLIMMER. Based on Payne (1999). Predicts evolution of ice-sheet geometry (extent, elevation), given a driving climate (temp and precip). Annual PDD scheme for surface mass-balance Temperature downscaled using high-resolution (40 km) orography and a constant lapse rate. The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

9. EGU 2007, CR140 Dan Lunt Modelled Pliocene (closed)Modelled Pliocene (open) Closed - open Modelled Pliocene (open) Modelled Pliocene (closed) Ice sheet volume [m of sea level] 1.161.21 The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence?

10. EGU 2007, CR140 Dan Lunt Conclusions Model results indicate that the closure of the Panama Seaway did not directly cause Northern Hemisphere Glaciation during the Pliocene. The process for the slight enhancement of glaciation following closure of the seaway is as proposed in the original hypothesis, i.e. mixing of Atlantic and Pacific waters leading to a massive enhancement of the Atlantic MOC, increased evaporation and precipitation over Greenland, which leads to enhanced glaciation. Climate Dynamics, in press. CAUSE OR COINCIDENCE? Given caveats!

11. EGU 2007, CR140 Dan Lunt Other mechanisms for inception Atmospheric composition changes (e.g. CO 2 ). Orbital fluctuations. Open Seaway CO 2 = 400ppmv Modern orbit Closed Seaway CO 2 = 400ppmv Modern orbit Closed Seaway CO 2 = 280ppmv Modern orbit Closed Seaway CO 2 = 280ppmv Glacial Inception orbit

12. EGU 2007, CR140 Dan Lunt Future work Validation with paleodata: Daniela Schmidt, Richard Pancost, Bristol Earth Sciences and Bristol Chemistry Other effects?? Rockies uplift…. Gavin Foster, Bristol Earth Sciences Effect of the closure on the Antarctic ice sheet Complete sensitivity studies e.g. Solely orbital forcing.

13. EGU 2007, CR140 Dan Lunt