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

CCSM Abrupt Change Task Team Some history CCSM Abrupt Change Task Team Cross working groups, PaleoWG, PCWG, & OWG DO events are the numbers associated with warmings. H events are the yellow bars which are defined by periods of ice rafted debris found in sediments. Some proxy data seems to show H events were coolings (coincident with IRD) and freshwater dumps (timing is bad as it appears to be often shortly after the IRD event). I recommend making the point that we don’t think future abrupt change will resemble past abrupt change too much. Mostly because the Laurentide ice sheet is gone, which made the climate fundamentally different. Cecilia’s focus is on DO events because other abrupt events like Heinrich events, while clearly abrupt changes in ocean sediment debris, I am not convinced of a consistent abrupt climate signal. There is no doubt about DO events though. However, I am only sure the DO event signal occurs on Greenland (Heinrich events do not have a abrupt signal on Greenland). There are abrupt signals elsewhere, but it is debatable if they are coincident with DO events and/or they may not be easily linked to climate change. 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?

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 15ºC Winter warming by 2050 in A1B scenario ensemble mean! 8C -2C 16C 106 km2 Abrupt summer sea ice decay in ~2030 in one ensemble member! In A LGM climate! Modest sea ice changes (eg magnitude of the amplitude of the modern seasonal cycle of sea ice) can produce a 10C warming and 50% increase in precip on Greenland.

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 Cariaco basin is a high temporal resolution marine core in the Caribbean off the Venezuala coast. Variations are primarily due to variations in summer rainfall and runoff. During both Heinrich event 1 and the 8.2 ka event, Cariaco basin shows reduced precipitation/runoff Heinrich 1 is a large fw event associated with a large discharge of iceberges into the N. Atl. A marine proxy in the subtropical north Atlantic indicated a rapid and almost complete shutdown of the THC during this event. The 8.2ka event is the release into Hudson Bay from an ice-dammed superlake. The 8.2 event reflects a reduction of precipitation that occurred in 30-40 yrs and lasted 200 years. Drier Wetter 8.2 Event

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

Idealized Freshwater Hosing Simulations (CMIP) 1 Sv / 100 years, North Atlantic 50 to 70°N 8.2 ka LGM The top two figures show the surface air temperature change in CCSM3 simulations induced by imposing 1 Sv of water into the North Atlantic for 100 years. The left is for an interglacial period, 8.2 kyrs BP, and the right is for glacial conditions, 21 kyrs BP. Both show similar patterns of cooling but with notable exceptions. Cooling is greatest in the North Atlantic at the location of the fw hosing but extends farther NE at 8.2 due to less initial sea ice. The LGM hosing simulation shows greater cooling in the SH. There are also significant differences in the response in the subtropical North Atlantic and North America associated with differences in the subtropical High response. Bottom shows scaled (to 50 years of control before fw forcing) of max of Atlantic MOC and annual NH sea ice area. Initial rates of change differ depending on climate state (8.2 vs LGM) but scaled change ends up similar for both. In neither case, does the THC collapse. LGM hosing simulation has not recovered yet, even after 100 years of no hosing. 1 Sv (courtesy of Bette Otto-Bleisner)

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

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 106 km2

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?

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?