Discussion of Abrupt Climate Change

1. Discussion of Abrupt Climate Change Paleo-Polar Joint Working Group Meeting June, 2006

2. Some history Cross working groups, PaleoWG, PCWG, & OWG CCSM Abrupt Change Task Team Some Science Issues What causes the abrupt warming - Dansgaard-Oeschger events? What is the response of the global climate system to abrupt coolings? Will there be something like abrupt change in the future?

3. 106 km2 15ºC Winter warming by 2050 in A1B scenario ensemble mean! Abrupt summer sea ice decay in ~2030 in one ensemble member! 8C 16C -2C Working Hypothesis (for now) for D-O events and future abrupt change: It’s the Sea Ice! Primary focus of the PCWG and OWG contingency Modest sea ice change can cause 10ºC warming and 50% more precipitation on Greenland in an LGM climate - Li et al 2005

4. Heinrich and other freshwater events in the North Atlantic Primary Focus of the PaleoWG contingency Understanding global response of climate system to freshwater forcing using CCSM and paleoclimate records • Mechanism and feedbacks in coupled system • Transmission of signal by ocean and atmosphere • Rates of change and recovery • Dependence of response on background climate state and associated forcings Heinrich Event 1 Drier Wetter 8.2 Event

5. Experiments of the PaleoWG •  Equilibrium simulation for 8.2 ka to provide control and initial conditions for paleo-hosing experiment. • Idealized paleo-hosing simulations to understand the response and recovery, spatially and temporally, depending on background climate state:  LGM, 100-year FW pulse of 1 Sv plus >100 years of recovery  8.2 ka, 100-year FW pulse of 1 Sv plus years of recovery »Deglacial, still in design stage • More in-depth study of 8.2 event, particularly exploring range of realistic freshwater estimates of amount of water and location of impulse

6. Idealized Freshwater Hosing Simulations (CMIP) 1 Sv / 100 years, North Atlantic 50 to 70°N 8.2 ka LGM 1 Sv (courtesy of Bette Otto-Bleisner)

7. Experiments of the PCWG/OWG • Idealized hosing experiments (as discussed by Cecilia Bitz)  Modern, instantaneous FW pulse =1/3 of Greenland ice cap plus recovery period • LGM, same FW pulse but 100 yr recovery • Ocean Flush - Initially weaken ocean stability with thermohaline off so it resumes abruptly • Simulations to diagnose mechanisms in IPCC future scenarios • Simulations to assess whether future scenario mechanisms may play a role in different control climates

8. 106 km2 Abrupt sea ice transitions in future scenarios • Three important factors driving abrupt transitions • An increasingly “vulnerable” (thinner) ice cover • A trigger - rapid OHT increases • Positive feedbacks - albedo and oht changes

9. Science Issues Discussion of what should be the next steps? Some Possibilities • Sensitivity studies to further explore interesting behavior in current simulations • Simulations of important freshwater events of the past (Meltwater Pulse 1A, Younger Dryas) • Understand impact of Greenland ice cap meltback - past and future • Explore sensitivities to climate state, location, amount and duration of freshwater forcing • Further exploration of “threshold” behavior in sea ice • Others?

10. Priorities/Needs • What should be the priorities for additional work on abrupt change? • Possible protocol abrupt change experiment? • What new model capabilities are needed? • Are other resources needed?