Introduction. GCM and ice sheet simulations. Conclusions. Other mechanisms for inception.

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

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

3. The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence? 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 Ruddiman, Earth’s Climate + increase understanding of climate / ice sheet interactions for future climate change!

4. 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. ~ 370 m depth Closed Seaway (late Pliocene) Open Seaway (early Pliocene)

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

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

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

8. The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence? 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.

9. The closure of the Panama Seaway and the onset of Northern Hemisphere Glaciation: Cause or Coincidence? Closed - open Modelled Pliocene (open) Modelled Pliocene (closed)

10. 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. CAUSE OR COINCIDENCE? Given caveats! Climate Dynamics, in press.

11. Other mechanisms for inception • Atmospheric composition changes (e.g. CO2). • Orbital fluctuations. Open Seaway CO2 = 400ppmv Modern orbit Closed Seaway CO2 = 400ppmv Modern orbit Closed Seaway CO2 = 280ppmv Modern orbit Closed Seaway CO2 = 280ppmv Glacial Inception orbit

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